Winemaking at Naggiar Vineyards

GuildSomm | Comprehensive Guide to Winemaking


  1. A Brief History
  2. Wine Composition & Chemistry
  3. Pick Decision & Harvest
  4. Fruit Processing
  5. Wine Microbiology
  6. Fermentation
  7. Élevage
  8. Stabilization & Bottling
  9. Bibliography

For any wine industry professional, an understanding of the winemaking process and the motivations driving winemaking decisions is powerful. It provides a cause-and-effect-based framework for tasting, fosters better communication among the trade, and empowers critical thinking for navigating information about wine. This guide offers insight into the winemaking principles and practices that affect the style and quality of wine in the glass.

When embarking on the study of winemaking, it is helpful to keep a few fundamental concepts in mind:

  • Grapes and wine are subject to variation and imprecision. Because fruit composition is a limiting factor, winemakers do not have complete control over outcomes. Rather, they adapt techniques to the grape variety, vineyard, and vintage at hand.
  • There are few universal truths in winemaking, and each decision depends on context. What works in one case (for a particular region, variety, or vintage) may not have the same result elsewhere—there are many caveats, exceptions, and stylistic considerations.
  • Plenty of unknowns remain. Many lessons are learned through experience and experimentation, and decisions often rely on intuition.

It is currently fashionable throughout the trade to diminish the role of the winemaker, suggesting that “wine is made in the vineyard” and glorifying “hands-off” winemaking. It is true that winemaking, in its most basic form, occurs naturally­­: fruit left alone in a tank will typically ferment. But removing people from the picture undermines the significant human endeavor required to shepherd wine from grapes to bottle. While many winemakers seek to minimize intervention, and the principles that underlie the science of winemaking are universal, all decisions—including the decision of whether or not to act—impact how the resulting wine will taste.

A Brief History

Whether for spiritual, practical, or hedonistic pursuits, people have been making wine for at least 8,000 years. For much of history, wine bore little resemblance to its modern incarnation. Wines were sweetened with honey and diluted with saltwater, and off-aromas necessitated the use of herbs, ash, and resin to make the wine more palatable. Though the principles underlying fermentation have remained the same over millennia, a better understanding of the science and impact of winemaking practices has enhanced the winemaker’s ability to craft grapes into a more delicious and stable product.

Most wines on the market, even the most traditional, are influenced by knowledge acquired recently. In the past 200 years, developments in microbiology have demystified the fundamentals of fermentation. While Antonie van Leeuwenhoek first observed yeast and bacteria in the 17th century, it wasn’t until the mid-1850s that Louis Pasteur discovered that yeast is the agent of fermentation. It was only in the early 1950s that several researchers—including Émile Peynaud in France, Brad Webb in California, and others in Portugal—all simultaneously isolated the first malolactic bacteria culture.

Innovations during the 19th century simplified many winery operations, including pressing, crushing, destemming, and wine transfer. The past century alone saw the widespread adoption of many tools considered fundamental to modern winemaking, including stainless steel, temperature control, inert gas, modern pumps and presses, and bottling lines. This series of innovations allowed for the production of some very different styles of wine, especially fresh and fruity styles, through aromatic preservation and minimization of oxygen influence.

While none of these modern tools are required to make great wine, their use has permitted a wider range of styles and helped raise the bar on wine consistency and quality overall—key for the success of an industry with a global production of roughly three billion cases per year.

Wine Composition & Chemistry

A wine’s composition is determined by the compounds present in the fruit, the microbial populations in the juice and wine, the techniques and additions used during winemaking, contributions from the fermentation or aging vessel, and the effects of oxidation and aging. Material other than grapes (MOG), especially stems that may be included intentionally, will contribute to a wine if included in the fermentation. Occasionally, smoke, taints from insects, or oils from nearby foliage can also have an effect.

While highly simplified, it is helpful to consider wine components in categories differentiated by source. Flavors and aromas present in the fruit are referred to as primary, compounds that arise from fermentation are secondary, and those resulting from aging and oxidation are referred to as tertiary. (These same terms are sometimes used to categorize aromas descriptively rather than by source—that is, fruit, non-fruit, and earth aromas, respectively.) The precise composition of a wine is constantly in flux, since many chemical reactions, especially the effects of oxygen exposure, occur slowly, resulting in the continual evolution of wine in a glass, bottle, or barrel.

Wine is a complex mixture of tens of thousands of chemical compounds. Dry wine is comprised mostly of water and ethanol; aroma, color, and flavor compounds represent only 3% of wine by weight. These minor components include glycerol (a “sugar alcohol”), organic acids, unfermentable sugars, proteins, fusel alcohols (larger alcohols), phenolic compounds such as pigment and tannins, and aroma compounds like esters, terpenes, and thiols. Many of the compounds responsible for the flavor of wine are present in minuscule, part-per-trillion concentrations.

Fruit is the main ingredient in wine, and most of the flavors and aromas in the finished product, with the exception of those that come from oak, arise from compounds in the grapes. Grape clusters are composed of skins, pulp, seeds, and stems, and the proportion of each component varies with grape variety and berry size. Grapes primarily contain water and sugar, along with organic acids, salts, phenolic compounds, proteins, and other flavor and aroma compounds. Pigment, tannin, and flavor molecules are stored in the skins. Seeds contribute tannin and other bitter compounds, and juice is comprised mostly of sugar, acid, and water.

Varieties used in wine production typically belong to the species Vitis vinifera, which was domesticated from wild grapevines for its high yields and sugar content and the ability to self-pollinate. Other relevant grape species that are used occasionally for winemaking, and more frequently as rootstock, include Vitis rupestris, Vitis riparia, Vitis berlandieri, Vitis labrusca, Vitis aestivalis, Muscardinia rotundifolia, and Vitis amurensis. Today, over 10,000 grape varieties are known, with roughly 1,400 in commercial production. This range of grapes demands a diverse set of winemaking practices—because fruit composition varies by variety, the techniques used to coax forth the best of what a grape can offer are variety dependent. And not all grapes are capable of making great wine on their own; some are better employed as a part of a blend.

While all aspects of the growing environment impact fruit composition, temperature and water availability are the most critical. Grapes grown at warmer temperatures will ripen sooner and can achieve higher potential alcohol concentrations and riper fruit flavors. In cooler climates, it may not be possible to achieve full ripeness every year, and fruit may have a lower initial sugar concentration and more savory flavors. Winemakers in warm climates may have the option to harvest grapes with a greater spectrum of flavor and ripeness levels than those in cool climates, where the timing of harvest is often more likely to be dictated by the end of the season. Excess water can result in delayed ripening, larger berries, and diluted fruit, whereas moderate water deficit increases concentration and shortens the ripening period. At the extremes, excessive heat (above 95 degrees Fahrenheit) and severe water stress can cause dehydration and delay ripening by inhibiting the vine. Frost, whether at the beginning or end of the growing season, can destroy green plant tissue and limit the vine’s ability to ripen fruit.

In the traditional sense, winemaking begins at harvest, but many important decisions have already been made by this time. The choices made in the vineyard during its establishment and throughout the growing season influence fruit composition and, ultimately, wine style. In many regions, wineries purchase fruit from vineyards they do not own, and the winemaker may not see the fruit until it has been harvested. The objectives of winemaker and grower are not always well aligned. It may not be in the best interest of growers to produce the highest quality fruit, for example, as yields and labor costs also motivate their decision-making. For this reason, many winemakers seek to influence the farming of their vineyards.

A wine’s style and quality are limited by the composition and condition of the fruit, and many downstream winemaking decisions depend on these factors. Even the best winemaking cannot transform bad grapes into great wine, and it is the responsibility of winemakers to preserve fruit quality throughout the winemaking process.

Wine Chemistry

In order to understand winemaking objectives, it’s essential to understand the major chemical components in fruit and wine, including sugar, acids, and phenolic compounds.


Wine grapes contain a high proportion of sugar, regularly exceeding 20% at harvest. Glucose and fructose are the main grape sugars. There are also small amounts of “unfermentable” sugars, or those that wine yeast will not convert to alcohol. Most unfermentable sugars are pentoses, which means that their chemical structure includes five-carbon atoms, while glucose and fructose are hexoses (six-carbon sugars). Throughout the world, a number of scales are used to estimate the sugar content of juice and fermenting grape must, including Brix, BaumĂ©, Oeschle, and Klosterneuburg (KMW or Babo).

Beginning at veraison, sugar begins accumulating in berries. It is generally believed that sugar accumulates until roughly 22 to 24 degrees Brix, and increases after this point are driven by dehydration. Grapes used for dry and sparkling wines are typically harvested in the range of 18 to 27 degrees Brix, or roughly 180 to 270 grams per liter sugar. Wine is typically deemed “dry” based on its level of residual sugar. Several thresholds are used depending on the context, but a common definition considers wine to be dry when it contains less than two grams per liter of residual sugar. These wines typically register between 0 and −3 degrees Brix, depending on their alcohol and dissolved solids content. Sweet wines may have as much as 150 grams per liter of sugar or more.

Wine made with grapes harvested later in the season and botrytized fruit may contain more fructose than glucose. Many wine yeasts preferentially consume glucose and struggle to metabolize fructose. As a result, it is in these types of wines where stuck fermentations, or fermentations that stop before all of the sugar has been consumed, are more likely to occur.

Sugar content is measured in the field using a small handheld device called a refractometer, which infers sugar concentration by measuring the refractive index of a liquid. In the winery, sugar content is measured with a hydrometer or densiometer. Because density depends on temperature, a correction is necessary if the temperature of the juice deviates from about 70 degrees Fahrenheit. Sugar is also analyzed chemically in the laboratory. Because juice density depends on the concentration of sugar as well as other dissolved solids, this is the most accurate method for inferring the potential alcohol of a wine.

A wine’s initial sugar concentration can be used to estimate potential alcohol (a prediction of the final alcohol if the wine is fermented to dryness). In the EU, potential alcohol is estimated using the official conversion ratio of 16.83 grams per liter sugar yielding 1% ABV. The actual conversion ratio depends on the efficiency of the yeast and typically ranges from 16.5 to 17.5. If all of the sugar was converted to ethanol, 15.7 grams per liter sugar would yield 1% ABV. In reality, yeast converts only 90 to 95% of sugar to alcohol, and the rest is converted to other biproducts of fermentation, including glycerol and fusel alcohols.

Measuring Sugar

Brix is a measurement, common in the US and other New World countries, of the total soluble solids in a juice, which includes sugar as well as other constituents. Brix is determined by measuring the density of a juice relative to a solution of sucrose in water, though Brix is actually a specific gravity measurement (a relative density). The concentration of sugar in the juice can be inferred from the Brix, where 1 degree Brix is equivalent to 1% sucrose by weight. (Once fermentation has begun, this relationship no longer holds since alcohol also affects density.) Because Brix is technically a measurement of all of the solids dissolved in juice, it slightly overstates the true proportion of sugar. The Klosterneuburg Must Weight (KMW) or Babo scale, used in Austria, Italy, and Eastern Europe, attempts to account for this overstatement by applying a factor of roughly 0.85 to the Brix scale, which assumes that 15% of the solids are non-sugar.

Baumé is another specific gravity measurement used in France, Spain, and Australia. It is analogous to Brix but uses a salt (sodium chloride or table salt) solution as opposed to sucrose as the reference. While salt is not an intuitive choice, it is convenient since Baumé is an estimate of the potential alcohol, where a juice at 14 degrees Baumé is likely to have a final alcohol concentration of about 14%. Baumé is converted to Brix by multiplying by a factor of 1.8.

Specific gravity (closely related to density) is perhaps the most fundamental scale used to estimate sugar content, and its use seems to be gaining in popularity. The Oechsle scale, used in Germany and Switzerland, is mathematically related to specific gravity.


Acidity affects not only the taste of a wine but also its color, ageability, and microbial stability. Tartaric is the primary organic acid that occurs naturally in grapes; others are malic and citric. Lactic, succinic, and acetic acids are formed during fermentation and are present in wine at low concentrations.

Acidity is measured using two different and equally important parameters: pH and titratable acidity. Both affect the wine’s taste. The perception of sourness is most determined by the wine’s titratable acidity—wine with a high titratable acidity (TA) tastes more sour. Wine’s perceived texture is affected by pH. High pH (low acid) wines may seem soapy, while low pH (high acid) wines are perceived as having “harder” tannins. Additionally, pH affects a wine’s hue and the efficacy of sulfur dioxide, with lower pH wines requiring less SO2 for microbial stability.

pH is a scale of acidity, and values range from 0 (very acidic) to 14 (very basic). Water is considered neutral with a pH of 7, while wine generally has a pH between 3 and 4. Technically, pH is a measure of the concentration of hydrogen ions (or protons) in a solution. The pH scale is logarithmic, so wine at a pH of 3 has 10 times the acidity of wine at a pH of 4.

Total acidity is a measure of the organic acids present in wine. In practice, total acidity is difficult to determine and it is instead approximated by measuring a wine’s titratable acidity. These terms are often used interchangeably in wine literature, but TA is a more precise description of what is actually being measured: the protons in the juice or wine (like pH), as well as those that can easily be removed from organic acids dissolved in the wine. Strictly speaking, TA is a measure of the amount of acid (protons or hydrogen ions) available to react with a strong base through titration to a defined endpoint. (Note that the standard endpoint for titration is pH of 7 in the EU and 8.2 in many New World winemaking countries. Thus, the numbers are not technically comparable.) The titratable acidity of grapes and finished wine is typically in the range of 4 to 9 grams per liter tartaric acid equivalents, though it would not be unusual for sparkling wine and other high-acid wines to exceed this range. TA is expressed in grams per liter of tartaric acid in the United States and grams per liter of sulfuric acid in France. Tartaric can be converted to sulfuric equivalents by dividing by a factor of 1.5.


Phenolics are an important class of compounds that lend color, flavor, and texture to wine. Tannins and anthocyanins are two examples. Phenolics share a common chemical structure that includes a phenol ring, and polyphenols are larger compounds with multiple rings. Phenolic compounds are important for ageability and play a key role in oxidation chemistry. While phenolic content varies by grape variety and growing conditions, it is commonly believed that the concentration of these compounds is a reliable predictor of red wine quality.

Many different phenolic compounds exist in wine, and they are often separated into groups with similar chemical structure and functionality. Phenolic compounds are typically divided into flavonoids, non-flavonoids, and tannins. Flavonoids are polyphenols that contain a very specific 3-ring chemical structure, while the non-flavonoids include an assortment of smaller phenolic compounds. Tannins are separated into condensed and hydrolysable tannins. Condensed tannins are polymers of flavonoids that are extracted from grapes. The term tannin, used generally, refers to these. Hydrolysable tannins are derived from oak and comprised of non-flavonoids.

Non-flavonoids are small, bitter compounds that can be further categorized into several subgroups including hydroxycinnamates, benzoic acids, and stilbenes. Flavonoids include anthocyanins, catechins, and other polyphenols that are located in grape skins, seeds, and stems. They are extracted through skin contact and maceration. The concentration of flavonoids is much higher in red wine than in white.

Anthocyanins refers to a family of pigmented compounds responsible for the vibrant color of young red wine. Their extraction from the skins of red grapes begins as soon as the berries are crushed, and they immediately start binding with tannins and other compounds to create more stable pigments sometimes referred to as polymeric pigments. During fermentation, they continue to be extracted from the skins and depleted by polymerization. After pressing, the concentration of anthocyanins decreases as they are converted to polymeric pigments. After a year or so, the color of red wine is driven by the concentration of polymeric pigments. Anecdotally, polymeric pigments are associated with midpalate fruit sweetness, and for this reason, a wine’s color intensity may be correlated with other flavor attributes (though this observation may be variety dependent).

Catechins are small polyphenols that are extracted mostly from seeds and stems (though also from skins) and are largely responsible for bitterness in wine. While the concentration of catechin in wine is low, they are significant in wine as they are a major constituent of tannin.

Tannins are large molecules that impart astringency and bitterness in wine. From a strict chemistry standpoint, they are characterized by their ability to bind with protein, which explains the astringency perception they induce—tannins react with proteins in the wine drinker’s mouth. Tannins are often regarded as a single component in wine but can be more accurately thought of as a cohort of distinct compounds of different lengths and configurations made of catechin “building blocks.” The structure of catechins and tannins favors reactions among each other, as well as with anthocyanins. Smaller tannin “units” polymerize, or bind together, forming longer chains. These bonds are also easily broken, so at the same time that bonds are forming, others are breaking apart. Tannins’ ephemeral behavior makes them difficult to measure or study in a meaningful way, and for this reason, knowledge about their behavior is evolving as scientists develop better tools to study them.

During red wine fermentation, winemaking techniques are used to extract phenolics from the grapes. Anthocyanins extract rapidly at the beginning of fermentation. Tannins and catechins are more soluble in alcohol than water, so their rate of extraction is faster toward the end of fermentation. Phenolic compounds can improve quality and ageability, but over-extraction of tannins and catechins results in wines that taster bitter, hard, and closed. In particular, high levels of catechin can lend an unpleasant bitterness.

Astringency decreases over the lifetime of a wine, but the mechanism for this is still not well understood, though several theories have been proposed. Contrary to popular belief, average tannin length appears to decrease throughout aging. At equal concentrations, longer tannins are believed to taste more astringent than shorter ones, yet it is unclear whether a longer tannin or its components are more astringent. The formation of polymeric pigments has also been speculated to reduce astringency as wine ages. Additionally, very large tannins can form and become insoluble and settle out during wine aging. This loss of phenolic matter may also contribute to the reduction in astringency as wine ages.

It is clear that phenolic compounds are incredibly important for red wine style and quality, but relatively little is known about their complex behavior. This is an area of some of the most exciting wine research today, both in the winery and vineyard. These compounds are difficult to study, and scientists have much to learn about how they interact with each other and impact the texture and flavor of wine.

Other Compounds

Most of the flavors and aromas in wine come from components that are found in relatively small concentrations, including esters, terpenes, pyrazines, norisoprenoids, and thiols. While winemakers rarely measure these compounds, some contribute important impact aromas. (Find more insights into the origins and characteristics of these compounds in the Compendium.)

Pick Decision & Harvest

Several key decisions have a major impact on the style and quality of a wine. Arguably the most important of these is the decision of when to harvest. The harvest date determines the ripeness level, chemistry, flavors, and condition of the fruit. Most winemakers rely on a variety of indicators to inform their harvest decision, including taste, chemistry, and physical characteristics of the fruit and vine. While winemakers seek to pick the fruit at optimum ripeness, environmental and practical constraints may override stylistic priorities.

Prior to the widespread use of modern viticultural techniques, achieving ripeness was challenging, and the best vineyards were those with the ability to consistently ripen fruit. This is evident as many historically acclaimed vineyard sites are mid-slope and south facing, with growing conditions that favor early ripening. Better viticultural practices and warmer temperatures have made it easier to attain adequate ripeness in many regions, and a more recent challenge is achieving flavor and tannin ripeness before sugars become too elevated and acid too diminished.

Ripeness is a spectrum, and the ideal time to pick is best considered a window, not a discrete point. Underripe fruit results in acidic but flavorless wine, while overripe fruit is jammy, alcoholic, and sometimes lacking in acidity. (Some wines, of course, exploit these characteristics to yield a unique style, including Champagne and Amarone.) Ideal ripeness lies somewhere in the middle of these two extremes but depends on stylistic intent and preference.

Historically, winemakers relied heavily on sugar concentration to make the pick decision, with fruit harvested once a certain Brix level was attained. Increasingly, other parameters such as acidity, flavor, and phenolic ripeness have become important. In an ideal world, each of these would achieve optimal ripeness simultaneously, but this is not always the case. (The synchrony of ripeness factors can, however, signal quality.) Part of choosing when to harvest is determining which components are most important. Often, a winemaker will favor certain parameters over others and, when necessary, adjust the chemistry in the winery.

Sugar & Acidity Levels

Even today, sugar concentration is frequently the metric used to determine the date of harvest since it determines the wine’s potential alcohol. A winemaker may choose to pick at higher Brix to allow tannins, color, and flavors to ripen further, but this may require adjustment in the cellar to create a balanced wine. The rate of sugar accumulation varies but is roughly 0.5 to 1.0 degrees Brix per week after veraison. Cloudy, windy, or very hot weather slows down and even halts sugar accumulation.

In addition to sugar, winemakers typically monitor the pH and TA of the fruit. During ripening, pH increases and TA decreases, and winemakers seek to harvest once they are in a range that will result in a balanced wine. The precise range depends on the grape variety, the intended wine style, and whether the wine will go through malolactic fermentation.

Winemakers may also consider the concentration of malic acid. If malolactic fermentation is intended, it gives an indication of how the acidity will change after this fermentation. Otherwise, winemakers may prefer to delay harvest until the malic acid concentration is below a particular threshold, since a high concentration of malic acid can lend an overt green apple character to the wine.

Phenolics, Flavor, & Physical Characteristics

Where modern viticulture has allowed for sugar and acidity to ripen predictably, the discussion of ripeness expands to physiological ripeness, or the maturity of color, tannins, and flavors. Unlike sugar and acidity, these “secondary metabolites” are not easily augmented in the cellar through additions. A winemaker may choose to delay harvest in order to achieve riper tannins or a specific flavor profile.

During ripening, the color of red grapes (or the anthocyanin concentration) becomes darker and more intense to a point and then begins degrading. The texture of the tannins evolves from more rustic, green, and hard to softer and finer grained. Winemakers may perform chemical analysis to track the major phenolic compounds, but these analyses do not give insight into important aspects like texture. While some have developed experience-based interpretations of the measurements, to assess the more subtle nature of tannin structure, tasting is key.

Many winemakers look for specific flavor markers when deciding when to harvest. Grapes follow a somewhat predictable variety- and site-specific flavor evolution trajectory. For example, as Chardonnay ripens, its flavor may evolve from lemon to green apple to yellow apple to red apple. Very ripe Chardonnay grapes may develop tropical or even caramelized flavors. Winemakers may know from previous experience when they prefer to harvest—such as within the yellow apple “window.” This requires experience and meticulous record-keeping, since the flavors in the grapes may or may not translate into the same flavors in the wine. Working with multiple vineyards can yield general knowledge about variety-specific ripening patterns, but multi-year experience with a single vineyard site is critical to developing a good understanding of how flavors in that fruit will translate into flavors in the resulting wine.

For certain grape varieties, winemakers may look for the disappearance of specific flavor markers. For example, in Cabernet Sauvignon, some winemakers will wait to pick until the methoxypyrazine (bell pepper) level falls below a predetermined threshold.

Besides chemical properties, there are physical indications that it is time to harvest. Seeds turn from green to brown and become crunchy. As the berries ripen, pulp separates from seeds more easily, the fruit seems juicier, and skins become softer and chewier. Berries soften and become susceptible to dehydration. The stems lignify, turning from green to brown, and berries become easier to remove.

Signs that the canopy is shutting down, including leaf senescence or defoliation, indicate the reduced capacity of the vine to ripen fruit further. Frost will destroy the canopy’s ability to photosynthesize, which is necessary for fruit ripening. When the vine is preparing for dormancy, harvest becomes more urgent; there is little ripeness to be gained, and the fruit may begin to decline.


Decisions made throughout the winemaking process rely on samples taken from fruit, must, or wine, whether for tasting or chemical analysis. While this sounds straightforward, obtaining a representative sample of a heterogenous natural product like grapes or must is challenging. The distribution of sugar and other chemical components is not even within a cluster, vine, or vineyard block. Many winemakers use specific sampling protocols aimed at minimizing bias. However, sampling is inherently imprecise.

Key sampling events occur prior to harvest and when making additions, pressing, blending, and fining. The decisions made at these steps are only as good as the samples they are based on, particularly during vineyard sampling. Precision in sampling leads to more predictable outcomes, which allows the winemaker to be less manipulative later in the process.

Vineyards are often divided into blocks, or smaller parcels that are managed individually. These blocks may be picked, fermented, and aged separately, so the selection of the block boundaries is essentially the first blending decision. By handling smaller parcels individually, lots are more homogenous, resulting in more accurate sampling and management tailored to the individual plot. Management techniques over the past 50 years have aimed at achieving more uniformity across parcels.

Practical Constraints

Ideally, the timing of harvest is dictated by the readiness of the fruit. However, in many regions, inclement weather at harvest time threatens to destroy the season’s work, and winemakers face the difficult decision of whether to harvest before the fruit has reached optimum ripeness. Rain near harvest can dilute fruit and induce rot (although a small amount of rain can be beneficial). When fruit is nearly ripe, it is particularly susceptible to dehydration, and heat spikes during this time can lend a dried fruit character to the wine.

Other practical considerations often influence the timing of harvest, including the availability of labor and equipment, winery capacity and tank space, and other environmental factors, including the risk of late summer fires.


Harvest is the busiest season for winemakers. The vineyard must be monitored, and the condition of the fruit changes rapidly, so timing is critical. In the winery, there may be tens of lots requiring attention, all at different stages of fermentation. Beyond winemaking decisions, there are logistical demands; each pick requires coordination of a harvest crew, harvesting equipment, and transportation. Once the grapes are picked, there must be capacity in the winery to process the fruit, as well as tank space and available staff to manage the fermentation. The threat of adverse weather adds an additional layer of complexity. To manage the demands of harvest, prioritization is essential, and pragmatism may be favored over perfection.

Fruit is harvested by hand or machine. Hand-harvested fruit is picked into small boxes, half-ton bins, or gondolas and transported to the winery (or press house) as whole clusters. Machine-harvesters straddle a vineyard row and remove fruit from the stems using technology similar to that of modern destemmers. Machine-harvested fruit is transported to the winery as grape must (a combination of juice and berries) and is ready to load directly into the tank or press with no additional processing.

While the decision to harvest by hand or machine has important stylistic and quality implications, it is often driven by practical factors including cost, labor availability, and vineyard terrain. Hand-harvesting is traditional and versatile, effective with varied terrains and trellis systems. It allows for sorting, both in the field and once the fruit arrives at the winery. Whole-cluster wine styles, including wines that will be whole-cluster pressed (such as Champagne) or undergo carbonic maceration, require hand-harvesting. On the other hand, harvesting by hand is slow, labor intensive, and expensive. For inexpensive wines, it is often cost prohibitive.

When labor is available, hand-harvesting is still the method preferred by most wineries that prioritize quality, although there are exceptions. For example, the iconic style of Sauvignon Blanc from New Zealand has been defined by machine-harvesting, as it enhances specific aromatic characteristics that are difficult to replicate through hand-harvesting. Further, because of the speed and responsiveness of the method, machine-harvested fruit may be better quality than hand-harvested fruit from a poorly managed pick.

Machine-harvesting is becoming increasingly popular, as many regions do not have a sufficient or reliable labor supply to harvest all of their fruit manually. Machine-harvesters can harvest 10 to 20 tons per hour, while one person can manually harvest 1 ton per hour if working very quickly. Speed is especially useful in times of impending weather, and mechanical harvesters allow for night harvesting in regions where it is not culturally acceptable for crews to work at night. Although the initial investment in a mechanical harvester is high, this method is ultimately cheaper than manual harvesting.

Forethought is required for machine-harvesting, since compatible vine spacing and trellis systems are necessary; for established vineyards, harvesting by machine is not always possible. In the vineyard, compromised fruit can be dropped prior to harvest, and the most cutting-edge mechanical harvesters are equipped with onboard optical sorters. Post-harvest sorting, however, is difficult with machine-harvested fruit, though basic sorting may be employed to remove leaves and other MOG. Stylistically, some skin contact is implied with machine-harvested fruit, which may or may not be desired, especially for white wine styles.

Machine-harvesters have been used for at least 50 years, and while the original models damaged the fruit and vine, newer models are much gentler and will only improve. Labor shortages will continue to make machine-harvesting increasingly important.

Once fruit is harvested, it is transported to the winery for processing, which may include sorting, removing the grapes from the stems, crushing the berries, and/or pressing the fruit. Since machine-harvested fruit has already been destemmed, it requires minimal processing.

Best Practices for Handling Fruit

Harvested fruit is particularly fragile and vulnerable to oxidation and spoilage. To minimize damage, growers have widely adopted practices such as gentle handling, keeping fruit cool and intact, and minimizing the time between harvest and encuvage.

Low temperature slows the rate of oxidation reactions and microbial growth. Cool fruit is easier to process as it is less prone to unintentional crushing. In regions with cool daytime temperatures, fruit is harvested in the morning, and in warm regions, harvesting at night has become standard. Less common, but becoming increasingly popular, wineries may store fruit in a refrigerated room or container overnight to reduce the fruit’s temperature prior to processing. This practice aids with cellar logistics, and because the fruit is processed cool, quality can be better despite increased storage time.

Once fruit is crushed (intentionally or unintentionally), compounds that are otherwise contained safely inside berry cells are released and become susceptible to oxidation by enzymes that occur naturally in the grapes. This is the same phenomenon responsible for the browning that occurs with a sliced apple. Sulfur dioxide is the most common tool used to denature these enzymes and may be added during fruit storage and processing to reduce oxidation.

Gentle handling of grapes reduces unwanted oxidation and extraction. Oxidation is responsible for juice browning and loss of aroma and flavor compounds. For this reason, crushing is minimized until just before fruit is put into the fermentation vessel. Gentle handling is particularly important with white grapes, whose flavors are driven by delicate aroma compounds, and for styles of wine where skin contact is not desired. Gentle handling avoids pulverizing stems or seeds that may contribute bitter and green flavors to the must.

Winemakers may take an oxidative or reductive approach to fruit processing and handling. Reductive handling aims at preserving aromatics and fruit flavors and preventing browning. With reductive handling, dry ice (carbon dioxide) and other inert gas and sulfur dioxide may be used generously during fruit processing to protect the must from oxidation. This technique preserves fruit and floral flavors and delicate aromatic compounds, including thiols.

For some white wine styles, intentional oxidation or hyper-oxidation of the juice or must is practiced. With hyper-oxidation, the juice turns brown initially but clarifies throughout fermentation. This allows the most susceptible compounds to be oxidized and discarded as lees, resulting in a finished wine that is potentially less fruity but more resilient against post-fermentation oxidation.

Fruit Processing


Sorting is intended to remove anything from the fruit that would confer unpleasant aromas or flavors. Most wineries do some sorting, whether to remove material other than grapes, rot, and/or raisins. Damaged fruit is typically removed, though in certain vintages, it is not practical to remove all of the rot (as this would result in miniscule yields), and the wine’s flavor may be impacted.

Sorting can be done by hand or machine, in the field during harvest or once the fruit is in the winery. An array of equipment has been developed to aid in the effort, including shaker tables, hand-sorting tables, and optical sorters. Cluster sorting removes MOG and compromised clusters. Berry sorting is performed on destemmed fruit to remove raisins, small green “shot” berries, pieces of stem, and insects. Sorting is more extensive for red grapes or fruit that will see skin contact, since fruit that is pressed right away has less time to extract bad flavors.

Implementing basic sorting is one of the easiest things a winery can do to improve quality. Approaches to sorting vary from basic to extensive, with some producers removing only leaves and other foreign objects and others removing everything except perfect berries. One metric that determines quality is which fruit ends up in the wine, and the approach will typically be less selective when making an inexpensive wine versus a more premium wine. While sorting is widely considered one of the more important improvements of modern winemaking, it is debatable whether extreme sorting (removing everything except perfect berries) substantially improves quality.


Fruit damaged mechanically by weather, machines, birds, or insects is susceptible to spoilage by the fungus Botrytis cinerea. While all grapes have naturally occurring enzymes that oxidize the fruit once berries are crushed, botrytis produces a particularly virulent oxidation enzyme called laccase. Laccase causes rapid oxidation that is not deterred by sulfur dioxide or alcohol, unlike other enzymes. Botrytis imparts a specific flavor profile (ginger and saffron, accompanied by oxidation) that is generally considered a flaw in dry wines, though some wines are defined stylistically by the presence of botrytis, including SavenniÚres, Austrian Smaragd styles, and some wines from Alsace. Under ideal, dry conditions, botrytis infection results in noble rot, a condition that defines some important sweet styles including Sauternes and Tokaji.

Crushing & Destemming

Before grapes are transferred to the tank or press, they may be destemmed or crushed. While some crushing during processing is unavoidable, intentional crushing is accomplished mechanically by passing fruit through a crusher machine, or more traditionally by foot-stomping. Modern destemmers gently remove berries from the stems, and berries emerge from the machine mostly intact. The choice to crush or destem has stylistic as well as practical implications.

White Winemaking

In white winemaking, juice is separated from stems and skins prior to fermentation and through pressing. Clusters can either be loaded directly into the press, referred to as whole-cluster press (not to be confused with whole-cluster fermentation), or destemmed prior to pressing. If some skin contact is desired, white grapes may be crushed prior to pressing.

Destemming allows more grapes to fit in the press, which reduces processing time and increases extraction from the skins. Whole-cluster pressing minimizes skin contact, resulting in clearer juice with fewer skin-derived compounds, including phenolics that may cause bitterness. The stems act as a press aid, improving juice yields and clarification. White wine that has been whole-cluster pressed is often considered to be higher quality, though this depends on stylistic intent. It tends to result in a clean, bright, and delicate style, while wines from destemmed and/or crushed fruit can be more textured. Whole-cluster pressing is required for many styles of sparkling wine, where skin contact is undesirable.

Red Winemaking

In red winemaking, there are several processing options, and each has practical and stylistic implications. Fruit may be:

  • Destemmed and crushed (traditional fermentation; most common)
  • Destemmed but not crushed (whole-berry fermentation)
  • Not destemmed and crushed (fermentation with stems)
  • Not destemmed or crushed (whole-cluster fermentation, carbonic maceration)

Crushing begins the extraction process sooner, and because sugar is not trapped inside the berries, it is more available to the yeast. This can result in a faster, warmer fermentation. When tank space is at a premium, minimizing the time in tank is logistically beneficial. Stylistically, traditional fermentation avoids flavors contributed by carbonic maceration or stem inclusion.

Carbonic Maceration

Carbonic maceration is an intracellular fermentation that occurs inside intact berries in the absence of oxygen. This fermentation is mediated by enzymes naturally present in the grapes and does not require yeast or bacteria. Once the alcohol level inside of the berries reaches 2%, the enzymes are denatured, and the fermentation stops.

Carbonic maceration lends a distinct flavor profile and a sense of freshness to wine. Elevated levels of esters, especially ethyl cinnamate and isoamyl acetate, contribute aromas of strawberry, kirsch, banana, and pink bubblegum and a sense of aromatic lift. Beaujolais Nouveau is a well-known example of the flavor impact of this technique. Depending on style objectives, different degrees of carbonic maceration may be desirable. Some find these aromas off-putting and seek to avoid them entirely.

In strict carbonic maceration, carbon dioxide is added to a tank of whole clusters to displace oxygen and ensure an anaerobic environment, and the tank is sealed. The clusters remain intact and enzymatic activity takes place inside the berries. While little color is extracted from the skins, color from the skins is transferred into the pulp. After one to three weeks, the grapes are pressed sweet, and the wine completes primary fermentation off skins via the action of yeast. The resulting wines are simple, light, fruity, and often low in tannin and color.

Whole-cluster and whole-berry fermentations encourage subtle flavor contributions from carbonic maceration. In general, the longer berries remain intact and the greater the percentage of intact berries, the greater the carbonic character in the resulting wine.

Semi-carbonic maceration, often used synonymously with whole-cluster fermentation, refers to the practice of including a percentage of whole clusters, ranging from a small amount to 100%, in the fermentation. Juice in the bottom of the tank begins fermenting traditionally and produces carbon dioxide that induces carbonic maceration inside the intact clusters. The clusters may be broken down through cap management or foot-treading throughout the fermentation. Including whole clusters yields a slower, cooler fermentation, since stems create air channels that allow heat to dissipate and because sugar is released and consumed by the yeast more slowly. Some sugar typically remains trapped inside the berries and is released at pressing. This practice is more common with specific varieties, including Pinot Noir and Syrah. While whole-cluster fermentation is often used to describe a single practice, this is an oversimplification. The results vary depending on the percentage of whole clusters used and how long into the fermentation the berries remain intact. Whole-berry fermentation, where fruit is destemmed but not crushed, is another variation. Here, some carbonic maceration occurs within the whole berries, which are broken down more quickly than whole clusters by enzymes and during cap management, resulting in a subtle carbonic flavor profile.

Including stems in the fermentation effectively adds another ingredient to the wine. Stems increase the concentration of phenolic compounds (especially catechins) and potassium. When stems are included, the resulting wine is often lighter colored and more tannic, with a higher pH and lower alcohol. There are different philosophies on stem inclusion; some winemakers are partial to stems, while others believe that they contribute green, herbaceous, or bitter flavors to the wine. For this reason, they may be either avoided or included only when they have certain characteristics—for example, brown stems but not green. The allure of stems seems to vary by variety and site. Stem inclusion is rare with Bordeaux and other tannic varieties, which often have sufficient tannin and where green flavors tend to be avoided.


The amount of time juice spends on the skins is the fundamental difference between red and white winemaking. White grapes are pressed prior to fermentation, while red grapes are fermented on their skins. White grapes handled as if for red winemaking yield orange wine, and red grapes may be handled like white grapes to make rosé.

Most wine grapes have clear juice and pulp, comprised primarily of sugar, water, and organic acids. (The exception is teinturier grapes, which have colored flesh.) Color, tannin, and many flavor and aroma compounds are contained in the skins and extracted through skin contact. While the quantity of these compounds is highly dependent on grape variety, the extent of extraction is dictated by the winemaking process, including such factors as amount of time on skins, temperature, and the physical contact of skins and juice.

Extraction occurs more quickly at higher temperature, and a longer period of time on skins results in greater extraction. Cap management techniques increase the surface area of contact between skins and juice, which increases the rate of extraction.

Pectolytic enzymes, which break down pectin, may be added to speed up the extraction process. These enzymes help break down the grape skins, facilitating the extraction of color, tannin, and flavor. This is particularly important when logistics limit the duration of time allowed on skins. Similarly, pectolytic enzymes may be added prior to pressing to facilitate extraction and increase yields.

Extraction is intentionally minimized for grapes with botrytis, bitter skins, underripe tannins, or other unwanted flavors.

White Winemaking

Extraction from the skins is limited and even avoided in white winemaking, as it can lend unpleasant bitter or green, leafy flavors, but there are instances where some skin contact is desirable. Skin contact can be used to increase the concentration of varietal aromas or phenolic extraction, resulting in a more textured wine. This can be accomplished through a short maceration on skins lasting from 2 to 48 hours.

For white wines that are intended to be light, fresh, and easy drinking, skin contact is typically avoided. A short maceration of a few hours may be used on Chardonnay to improve the wine’s structure and ageability. Aromatic grapes like Muscat and GewĂŒrztraminer are good candidates for longer skin contact, but they also have bitter skins, so a winemaker must be careful to avoid over-extraction.

Rosé Winemaking

Rosé is often made through the light extraction of red grapes using either the direct-press or maceration method. In the direct-press method, whole red grape clusters are pressed, and the juice is handled like white wine. This is essentially a red wine made with no skin contact. A second method for rosé winemaking, sometimes referred to as saignée, involves macerating on skins for a short time before bleeding juice off of the tank. This can be thought of as a red wine with a short skin contact and generally imparts a slightly darker color to the wine. A third option for rosé winemaking is to blend in a small portion of red wine or colorant with a white wine. In many regions, this method is not permitted, but blending is standard for making rosé Champagne. Red wine may also be used just prior to bottling to adjust the color of rosé made using other techniques.

Red Winemaking

Red winemaking involves a longer period of skin contact, typically 10 to 21 days, though some fermentations remain on the skins for only 3 or 4 days and others may macerate for several months, referred to as extended maceration. During fermentation, extraction is increased through cap management techniques.

Prior to fermentation, red wines may be held uninoculated at cold temperature for anywhere from a few days to a couple of weeks in a process known as a cold-soak. During this time, fruit enzymes break down the grape skins, beginning the extraction process, and the populations of native yeast (favored over Saccharomyces at cold temperature) build slowly. Some winemakers believe that cold-soaking increases color extraction, though this is debatable and depends on fruit composition. When the winery is busy, tanks may be held at cold temperature until there is capacity in the cellar to manage the fermentation.

Thermovinification and flash détente are niche techniques that accelerate the extraction of red grapes by exposing them to very high heat for a short period of time. These methods are convenient, since they require less tank space and management, but they are not generally accepted for quality winemaking. They are, however, useful for creating an acceptable product out of low-quality or compromised fruit. For fruit infected with botrytis, high temperatures will denature laccase, preventing excessive oxidation. Fruit with high levels of pyrazine or smoke may also be improved with these techniques, since those compounds require longer contact times to be fully extracted.

In thermovinification, must is heated to between 140 to 180 degrees Fahrenheit for a period of 30 minutes to 24 hours, with higher temperatures requiring less time. The must is often pressed directly after heating, and fermentation proceeds off of skins. With flash dĂ©tente, grape must is heated rapidly to near-boiling temperatures (185 degrees Fahrenheit), then cooled rapidly using a vacuum. This results in the complete destruction of the berries on a cellular level—fruit that has been subjected to flash dĂ©tente resembles jam. The treated must is usually settled overnight, drained, and pressed, and fermentation takes place off of the skins. The wine produced using these techniques is fruity and accessible, with jammy flavors. Very little tannin is extracted with either method, and tannin is systematically added prior to or after heat treatment.


Pressing separates the juice (or wine, in the case of reds) from the skins and seeds and marks the end of the process of extraction from the skins.

White Winemaking

Pressing itself is a form of extraction, where higher pressure yields more extraction from the skins and distinct quality levels are produced depending on the pressure applied. The light press is obtained through pressing at low pressure. This fraction is more aromatic and acidic than the heavy press, obtained at higher pressure. The heavy-press fraction is darker colored and more oxidized, with lower acidity and more pectin, phenolics, and vegetal flavors.

A winemaker’s press cut dictates when the juice coming out of the press will be diverted from light press to heavy press. This is a key decision in white winemaking, as the nature of the wine depends not only on what is extracted but also what is not. Winemakers seek to maximize the volume of the light-press juice, while avoiding unpleasant attributes that would lower its quality. They typically taste the juice coming out of the press, looking for a change in aromatics, acidity, and the level of oxidation. Some winemakers measure the pH, and the shift in this metric helps to inform their decision. Often, winemakers have certain yield targets in mind that also help guide the press cut.

The two press portions may be kept separate throughout fermentation and can be recombined later, if desired. Heavy-press wine is generally regarded as lower quality and is frequently fined or filtered to remove undesirable characteristics prior to fermentation. It can be an interesting blending component.

White grape must may be first placed in the press or tank and the liquid portion drawn off prior to pressing. This creates a third press fraction known as free run, juice liberated without the application of pressure. Free-run juice may be kept separate or mixed with the light press. This practice is most common in industrial-scale winemaking but may also be used for wines made with skin contact. In practice, many winemakers use free run and light press interchangeably when referring to white wines.

Red Winemaking

For red wines, when the winemaker is satisfied with the level of extraction from the skins, free-run wine is drained from the tank, halting extraction. Once a tank has been drained, the juicy skins that are left behind are pressed to retrieve the press wine. With reds, most of the wine is free run, with press wine representing less than 20% of total volume.

The decision of when to drain and press a tank is generally based on taste, though many winemakers routinely press once fermentation is complete or after a certain amount of time on skins. Infrequently, a chemical analysis of the phenolic components in a wine may be performed to assess the level of extraction. When possible, winemakers prefer to wait until fermentation is complete to press, to avoid a stuck fermentation. Most of the yeast population is adsorbed on the skins, so if the wine is drained mid-fermentation, some yeast is lost. The temperature often drops rapidly when the wine is removed from the skins, further shocking the yeast.

As with white wine, there is a quality difference between free-run and press wine. With reds, press wine is more tannic, which may be addressed by egg white fining. Press wine might be aged separately from the free run and added back during blending or declassified. When quality is high, it is sometimes added directly to the free run after pressing.

Press Types

The type of press and the way it’s used have implications for the quality, amount of extraction, and oxidation level of the juice or wine.

Presses fall into two broad categories: batch and continuous. As the name suggests, with a batch press, the press is loaded, the grapes are pressed, and the pomace is emptied out of the press. In continuous pressing, grapes are loaded into the press and pomace is expelled continuously. While continuous presses are sometimes used in high-volume winemaking, batch presses are preferred for quality.

Several types of batch presses exist. A basket press is a traditional style of vertical press that has been used since the Middle Ages. These presses were an adaptation of the screw press, which has an even more ancient history. Grapes are placed inside a cylindrical basket with a lid. Pressure is applied to the lid, and the grapes are compressed slowly, releasing juice. Today, basket presses are used more often for reds than whites, since the pressure applied is uneven and results in low yields when used on unfermented berries. Basket presses are relatively small compared with pneumatic presses, which are available in a wide range of sizes.

Pneumatic presses are the most common type of press. They are gentle and provide good quality and high yields. Pneumatic presses are more time consuming to load and clean than basket presses but often have a larger capacity and are less time intensive overall. For this reason, they are often preferred by larger producers. There are several types of pneumatic presses, but all have a horizontal cylindrical tank with perforated screens or internal drains that allow juice to escape.

  • A bladder press has an inflatable cylindrical bladder in the center of the press that expands radially, compressing the grapes symmetrically against the tank's sides.
  • A membrane press is similar, but the inflatable bladder is located along one side of the tank and grapes are compressed against the other side.
  • A tank press is a fully enclosed membrane press that allows the winemaker to exclude oxygen for very reductive winemaking. This may be preferred for bright and clean white wine styles with delicate aromas, like popular styles of Sauvignon Blanc.

In batch presses, pressure is applied slowly and gradually increased to extract more juice. The press program used for pneumatic presses may include stages where the pressure is alternatingly applied and then released at gradually increasing pressures. The pressure ranges from about 0.2 to 2.0 bars or more. If a pneumatic press is used, the press may be “rolled” or “rocked” to break up the press cake (compressed pomace) and allow for more extraction, though this will result in higher amounts of grape solids and is typically only utilized when high yields are the primary goal.

A typical white press cycle takes two or more hours, while a red press cycle is shorter. White wine yields around 120 to 170 gallons per ton of juice, with the heavy press wine representing less than 20% of the total. Extraction rates may be dictated by law.

Pressing is an inherently oxidative process. However, oxidation can be minimized by using dry ice and, with juice, sulfur dioxide for protection.


Juice that has been pressed contains a lot of solids, including small pieces of skin, tartrates, and microbes, though better quality pressing yields clearer juice. Prior to fermentation, white and rosé juice is often clarified to remove these solids, as they can impart bitter flavors. There are several options for clarification. Most often, juice is clarified by débourbage, or settling overnight at cold temperature, followed by racking or decanting the juice off of the solids that have settled to the bottom of the tank. Larger, more process-oriented wineries may remove solids through filtration or centrifugation. Another method of clarification is flotation, in which gas is pulsed through the juice, and the solids float to the top of the liquid. The solids may be skimmed off or the tank may be drained, leaving the solids behind.

Bentonite may be added after pressing to help clarify juice prior to racking. Here, bentonite acts as a settling aid, helping to remove grape solids, yeast, bacteria, pectin, and proteins. A naturally occurring clay, bentonite attracts proteins through electrostatic forces. It is not soluble in the juice and will be removed during racking, Settling enzymes may also be added just after pressing to facilitate faster settling.

Solids inclusion appears to increase viscosity and the concentration of volatile thiols in the finished wine, including those responsible for the flinty character of wines exhibiting so-called positive reduction. Some choose not to clarify the juice prior to fermentation or add back some of the lees after racking. Winemakers may measure the turbidity, or cloudiness, of the juice to determine the amount of solids to include. Solids contain yeast nutrients, and their inclusion may help the fermentation go more smoothly and increase yeast’s production of esters.

Must Adjustments

Philosophies on making additions vary. On one extreme, winemakers adjust the chemistry of the must to fit a formula, and on another, they avoid additions even when they would improve the wine. Highly commercial winemaking operations may benefit from using formulaic protocols, which help maintain consistency and simplify fermentation management.

Many winemakers seek a minimalist approach when it comes to making additions. When presented with unbalanced fruit, there are several options. Blending a low-acid lot with a high-acid lot can create balance. Traditionally, this was achieved through field blends (interplanting complementary blends in the vineyard) and co-fermentation. Alternately, a winemaker can adjust the chemistry of the wine by treating either a portion of the wine or the entire lot. Simple must adjustments are common, with many in warmer regions adding acid and in cooler regions adding sugar. Whenever possible, it is preferable to make any necessary additions prior to fermentation, so that the components can integrate more completely. (Yeast nutrients, a very common addition, will be discussed in the section on fermentation.)

Potential Alcohol

Since sugar is converted to alcohol during fermentation, if the sugar levels in the must are high, the resulting wine, if fermented to dryness, will be high in alcohol. High potential alcohol may cause yeast to struggle to finish a fermentation. It can also result in an unpleasant, “hot” sensation in the wine, though the perception of alcohol depends on more than just the percent ABV. Some wines hide their alcohol well, and others seem hot despite modest alcohol levels.

Sugar levels at harvest can also be too low, resulting from unripe fruit, rain at harvest time, or virused vines. A low potential alcohol may be augmented by adding a sugar source either through chaptalization, where sucrose is added to a fermentation, or by adding grape concentrate, a solution of grape sugar, acid, and color compounds prepared through reverse osmosis. Reverse osmosis can also be used to remove water from grape must in order to increase concentration or potential alcohol.

Chaptalization with sugar is not allowed in some winemaking areas, including California (where must may be enriched through the addition of grape concentrate). That said, winemakers in all regions are prone to bending the rules.

Higher sugar levels at harvest are typically associated with warmer climates. Modern viticultural practices have increased the efficiency of sugar accumulation. Similarly, many commercial yeast strains were selected for efficient conversion of sugar to alcohol. These factors have contributed to an increase in potential alcohols throughout many winegrowing regions, including regions that previously relied on chaptalization.

The most common method for reducing potential alcohol is adding water to the must. Typically, this is done in tank prior to fermentation. In many regions, the addition of water for the purpose of reducing alcohol content is not legal. However, it is typically permissible to use a “reasonable” volume of water for making additions, which gives winemakers a loophole to adjust the must. Alternatively, vines may be irrigated just prior to harvest, which is effectively a water addition, albeit less precise than adding water to the tank.

Post-fermentation water additions are typically frowned upon, as additions made at this time are generally considered less integrated and more manipulative. With red wine, water added prior to fermentation increases the capacity for extraction from the skins, whereas water added after fermentation is just a dilution.


Warm climates and certain grape varieties, rootstocks, and soil types can produce fruit that is naturally low in acidity, potentially resulting in unbalanced, flabby wines. Grapes with insufficient acidity may be augmented to adjust the pH, TA, and perception of freshness in the resulting wine. Acid is one of the most common wine additions, considered standard practice in many regions.

Tartaric acid is typically used to adjust acidity since it is stable (that is, it cannot be broken down during fermentation) and found naturally in grapes. Malic, citric, and sulfuric acids are also used occasionally. Because lactic acid bacteria convert citric acid to diacetyl, reminiscent of popcorn butter, an addition of citric acid can enhance the buttery character of a wine. If this flavor is not desired, citric acid should be avoided. Sulfuric acid addition is not legal in many regions.

A wine’s pH and TA change throughout fermentation (especially for red wines) and aging, which makes it difficult to predict the ideal addition prior to fermentation, though this is learned through experience. Winemakers often make an addition prior to fermentation and might make a smaller adjustment after malolactic fermentation is complete.

If acidity is too high, a wine can also be deacidified. There are several methods, all involving adding salts that react with tartaric acid to form tartrate salts that settle out of the wine. (However, the presence of tartrate salts in a wine does not imply that the wine has been deacidified.) Malolactic fermentation will also reduce a wine’s acidity, but low pH can inhibit primary and malolactic fermentation.


Tannin can be added to grape must to improve deficiencies, stabilize color, and improve a wine’s tannin structure. Tannin addition is often used to ameliorate fruit with undesirable flavors. It reduces the oxidative impact of botrytis, as well as the sensory impact of pyrazine and other off-aromas, including smoke taint. Tannins that are added to wine are referred to as enological or exogenous tannins and come from a variety of sources, including grapes.

While there are many options when it comes to wine additives (including those added to finished wine, discussed later in this guide), it is often the case that relatively few are used on a given wine. Generally, better quality fruit and thoughtful decision-making reduce the need for additives.

Wine Microbiology

Yeast and bacteria are fundamental to the winemaking process. In addition to converting sugar into alcohol and malic acid into lactic acid, they transform many chemical precursors found in grapes into the flavors associated with wine. Understanding the ecology of wine microbes helps winemakers ensure a healthy fermentation and guard against spoilage.

Grape juice and wine are inhospitable media, and few microorganisms are well adapted to the high levels of sugar, alcohol, and acid present. Wine microflora includes yeast, lactic acid bacteria, and acetic acid bacteria. The most important of these are Saccharomyces cerevisiae and Oenococcus oeni. Saccharomyces cerevisiae is an alcohol-tolerant yeast that dominates most alcoholic fermentations, and Oenococcus oeni is a lactic acid bacteria frequently responsible for malolactic fermentation. Beyond these, many other yeasts and bacteria are present throughout the winemaking process, and some participate in fermentation. Roughly 90 species of yeast and 30 species of bacteria have been identified in wine. Some molds are found on grapes, but few persist under alcoholic conditions. Fortunately, no known pathogens exist in wine, and wine spoilage is a matter of taste, not illness.

Microorganisms are ubiquitous, and the species found in wine originate from several sources. Some species of yeast and bacteria are present on grapes, while others reside habitually in biofilms on winery surfaces and equipment. Insects also play an important role in transporting yeast throughout the winery and vineyard. Inoculation is frequently used to introduce a preferred strain of yeast or bacteria, and these strains often establish populations within the winery and appear in fermentations even when they are not added intentionally. There is no shortage of inoculum in the winery setting, and when the conditions are favorable, microbial populations will grow.

Each wine microbe has different adaptations, and the species that is best suited to the environment at a given time will thrive, increasing in number and gaining competitive advantage. As conditions change throughout the course of fermentation, the microbial populations in the grape must evolve, with some actors present at high concentrations on the fruit, the population of others increasing during fermentation, and still others appearing during the aging process.

While microbial diversity lends complexity to a wine, some yeasts and bacteria are generally undesirable as they tend to produce unwanted flavors and aromas. Through fermentation management, winemakers seek to create conditions where desirable microbes will thrive and growth of spoilage organisms will be limited.

Cleaning & Sanitation

Effective cleaning—knowing where undesirable microbes are likely to hide and how to remove them—is a fundamental aspect of cellar craft. A common quip is that much of winemaking is actually just cleaning. It’s the first job of many young winemakers, and for good reason: some level of cleanliness is a basic condition for wine quality.

In many cellars, cleaning and sanitation help control microbial growth and avoid contamination. Equipment and winery surfaces are first cleaned to physically remove juice, biofilms, tartrates, and other potential contaminants. Then, they can be sanitized using heat or chemicals to kill any remaining microbes. This is especially important for surfaces that the wine will touch. Another source of contamination is the wine itself, so care is taken when topping and moving between lots to ensure that tainted lots don’t accidentally inoculate clean ones. Cleaning will never remove all potential spoilage microorganisms from the winery, but effective standard operating procedures (SOPs) and common sense regarding opportunities for contamination can greatly reduce the risk of microbial spoilage.

Approaches to sanitation in the winery range from rigorous to relaxed. Ultimately, cleaning style is an important element of winemaking style. Some believe that the best way to guard against spoilage is to encourage healthy microbial ecosystems within the winery. This works very well for some producers and can add an interesting complexity to the wine; for others, it is less successful.


Yeasts are single-celled eukaryotic fungi. They require a carbon source, like sugar, for energy and a nitrogen source, like ammonia or amino acids, for growth and metabolism.

Yeasts are organized taxonomically by genus, species, and strain. Prior to the availability of DNA testing, yeasts were characterized by their physical characteristics and behavior, or phenotype. As a result, yeast nomenclature is confusing, as some species are referred to by multiple names. For example, Kloeckera apiculata and Hanseniaspora uvarum are frequently mentioned in discussions of indigenous yeast, and despite their very different names, they are identical. Colloquially, winemakers often refer to yeast by genus only, hence Saccharomyces, Brettanomyces, and Kloeckera. (For more information on yeast species commonly found in wine, refer to the Compendium.)

From a winemaking perspective, yeasts are frequently segregated into Saccharomyces and non-Saccharomyces yeast. The Saccharomyces genus includes several species of fermentative yeast traditionally used in winemaking, brewing, and baking. Saccharomyces are specialists, which means they are particular about their energy source and have evolved to prefer six-carbon sugars including glucose and fructose. They are found in high-sugar environments including fruit, wineries, and the digestive tracts of insects that visit fruit. Saccharomyces cerevisiae or, less often, Saccharomyces bayanus are found in nearly all fermentations. If a winemaker chooses to inoculate, it will almost always be with one of these Saccharomyces yeasts.

Saccharomyces are well adapted to the conditions of fermentation; they are alcohol tolerant and capable of finishing a fermentation, with some strains able to withstand alcohol concentrations of 16% to 17%— a unique characteristic. To ensure that fermentation finishes, it is important that a strong population of Saccharomyces is established early in the course of fermentation. Stress induced by low levels of nutrients can cause the yeast to produce high levels of reductive aromas or result in a stuck fermentation.

Despite being found in nearly undetectable numbers on grapes, Saccharomyces often appears “spontaneously” in a fermentation. There are several possible origins. Many wineries have a resident strain of Saccharomyces that tends to show up in fermentations, likely from contamination by winery equipment. Beginning at harvest, insects transport Saccharomyces into and throughout the winery. Studies have shown that minuscule, undetectable populations on the fruit are able to build up during the beginning of fermentation, when conditions are favorable, and ultimately become the dominant species. Even with inoculation, occasionally one of these ambient yeast strains is responsible for fermentation.

Yeast strains differ in their tolerance of alcohol, temperature, and acidity, as well as their tendency to make biproducts like volatile acidity (VA) and hydrogen sulfide. Many commercial yeast strains have been isolated from nature, and others were bred within the laboratory. Yeast stains are selected for a number of traits that are important for a healthy fermentation, including the following:

  • Tolerance to environmental conditions: Yeast should be well adapted to the conditions of fermentation, including high concentrations of sugar or alcohol, low pH, low nutrient availability, and a wide range of temperatures.
  • Ability to finish fermentation: Yeast must be capable of fermenting a wine to dryness.
  • Positive sensory characteristics: Yeasts that produce high levels of desirable aroma and flavor compounds, including glycerol and esters, may be favored.
  • Lack of off-aromas: Yeasts that produce high levels of volatile acidity (vinegar), hydrogen sulfide (rotten eggs), or other unpleasant flavors and aromas are typically avoided.
  • Practical considerations: Yeast that are easier to work with from a practical standpoint may be favored in certain applications. For example, yeasts that produce low amounts of foam and flocculate (settle) well are desirable for traditional method sparkling wine.

While all fermentative yeasts must satisfy these conditions, each strain differs slightly in its tolerances and characteristics, including the production of flavors and aromas during fermentation.

Non-Saccharomyces Yeast

Large populations of some non-Saccharomyces yeasts are found on grapes, while others are typically not found in appreciable numbers until fermentation is complete. The term native or indigenous is often used to refer to this category of yeasts, but this is not necessarily accurate, as Saccharomyces may also be native (as opposed to arising from inoculation). Some of these yeasts contribute positively to a wine’s character, while others produce unpleasant flavors and aromas. In either case, most non-Saccharomyces yeasts are not capable of completing fermentation.

Many yeast species are found on grapes, and their relative populations depend on the condition of the fruit as well as management practices. Early in the season, Cryptococcus and Rhodotorula are abundant. As the fruit ripens, some of the more important species in terms of wine character appear, including species from the genera Hanseniaspora (Kloeckera), Candida (Metschnikowia), and Torulaspora. These yeasts are often present at the beginning of fermentation, but most are not very alcohol tolerant, and their populations decline once alcohol begins accumulating. Exceptions include at least two species of Candida that are capable of finishing the fermentation. Damaged and botrytized grapes have larger microbial populations than intact berries, and spoilage yeasts like Pichia, which produces large levels of volatile acidity (vinegar), are more numerous in these conditions. Non-Saccharomyces yeasts, including Kloeckera and Candida, are more sensitive to sulfur dioxide (SO2) than Saccharomyces, so sulfur additions during fruit receival limit competition and favor Saccharomyces.

Kloeckera apiculata is often the dominant yeast species at the beginning of the fermentation. Kloeckera and Candida are more cold-tolerant than Saccharomyces, so the practice of cold-soaking favors them. Kloeckera produces ethyl acetate (commonly used in nail polish remover), which can lend a pleasant, heady, aromatic lift to a wine at low concentrations. The aroma of ethyl acetate can appear quite strong prior to the onset of fermentation, but this typically blows off by the end. Occasionally, unpleasant levels persist in the finished wine.

Once fermentation is complete, the high-alcohol environment favors other yeast species, considered to be spoilage yeasts, including Zygosaccharomyces, Saccharomycodes, and the notorious Brettanomyces bruxellensis (Dekkera bruxellensis). Brettanomyces (affectionately referred to as Brett) is responsible for a number of off-aromas and flavors in wine, as well as some pleasant ones. Brettanomyces can metabolize sugars that Saccharomyces is not able to, and for this reason, even finished wine is vulnerable to colonization.

Most notably, Brettanomyces produces 4-ethylphenol and 4-ethylguaiacol, compounds associated with medicinal, barnyard, and smoky aromas. However, at least 40 other aroma compounds associated with Brettanomyces have been identified, including some that contribute more agreeable floral and spicy aromatics. The specific aromas produced depend on the yeast strain as well as the composition of the wine. Opinions regarding its effect on a wine run the gamut from appreciation to disgust and are often proportionate to the level of sensory impact, which is difficult to control. For most winemakers, the presence of Brettanomyces in the winery is disconcerting, since uncontrolled, it can easily overpower a wine. For many producers, Brettanomyces infection necessitates filtration or other means of microbial control prior to bottling. (For more information on the compounds associated with Brettanomyces and other wine faults, refer to the Compendium.)


Bacteria, which are smaller than yeast, also play an important role in winemaking. Two categories are relevant in wine: lactic acid bacteria and acetic acid bacteria. Lactic acid bacteria (LAB) exist in a diverse variety of nutrient-rich environments including plants and fruits, dairy products, pickled and fermented foods, sourdough, and human and animal digestive tracts. These bacteria are categorized by the tendency to convert glucose into lactic acid. Lactic acid bacteria are key in winemaking because they are responsible for malolactic fermentation.

Many LAB are inhibited at alcohol concentrations above 8%, so only a few species persist after fermentation is complete. Roughly 20 species from five genera have been isolated from wine including Oenococcus, Lactobacillus, Pediococcus, Leuconostoc, and Weissella. Most of these are capable of completing malolactic fermentation, though Oenococcus oeni is the most desirable, since it is relatively alcohol- and low pH-tolerant and less likely to produce high levels of volatile acidity and other wine taints.

Lactic acid bacteria are found on grapes and on winery surfaces, and they can also be added through inoculation. While LAB are prevalent on grapes and in must, their populations are generally static or diminished throughout primary fermentation. Populations of LAB typically rebound toward the end of alcoholic fermentation, when nutrients released through yeast autolysis (the decomposition of dead yeast cells) stimulate their growth. LAB participate in autolysis by producing enzymes that break down dead yeast cells.

While lactic acid bacteria are necessary for malolactic fermentation, some species are associated with a veritable laundry list of taints that can ruin wine. Some of the most off-putting of these includes biogenic amines (with names like putrescine and cadaverine), pyridines that cause a taint referred to as mousiness, acrolein (an incredibly bitter tasting compound), and a condition known as ropiness, which causes a viscous slime-like substance made of polysaccharides to form in the wine. Many of these taints are rare and easily avoided, since sulfur is highly effective at controlling bacterial populations. However, with the prevalence of natural and unsulfured wines, there are more commercially available wines displaying these characteristics.

Acetic acid bacteria (AAB) convert alcohol to acetic acid (vinegar), and while they are useful in vinegar production, in wine, they are universally considered spoilage organisms. Several genera of AAB including Acetobacter, Gluconobacter, and Gluconacetobacter reside on grapes, and only alcohol-tolerant Acetobacter persists in wine.

Oxygen is required for the growth and activity of acetic acid bacteria. AAB exist in large numbers on fruit, especially in damaged and botrytized grapes, but their populations decline in the reductive environment of fermentation. They reappear once fermentation is complete, particularly if wine is exposed to oxygen, as with wine stored under ullage (not topped). Protecting the wine from oxygen exposure through regular topping and maintaining reasonable levels of SO2 during aging keeps their populations under control.

Flavor Impact

Through fermentation, yeast and bacteria are responsible for many of the aroma and flavor compounds found in wine. There are several mechanisms whereby yeast and bacteria contribute to wine flavor chemistry, and the extent to which these compounds are produced depends on the particular strains of yeast and bacteria present. In addition to alcohol and lactic acid, wine microbes produce esters, aldehydes, and sulfur-containing compounds. They “release” aromatic impact compounds including thiols, terpenes, and norisoprenoids that are bound to sugars and other compounds in the must.


Just as yeast is added to dough when making bread, a winemaker may choose to initiate fermentation by inoculating with a commercial yeast strain. Alternatively, ambient yeast populations will generally initiate a fermentation “spontaneously.”

Inoculation allows a winemaker to select a specific yeast strain that is well adapted to the fermentation conditions at hand and that tends to produce a flavor profile compatible with stylistic intentions. Tailored yeast strains are available for reds versus whites, aromatic versus non-aromatic varieties, and a range of environmental conditions including temperature, alcohol, and nutrient availability. Typically, commercial yeast is freeze-dried and must be rehydrated, a process in which dried yeast is added to warm water, and, over an hour or more, fresh must is added slowly to acclimatize the yeast to the temperature and osmotic pressure of the grape juice. The rate of addition is on the order of 10 million cells per milliliter, which is significantly higher than the native populations. Some winemakers omit this step and add dried yeast directly to the tank, and others develop a more elaborate inoculation protocol. Inoculation can be an art, and nurturing the yeast early on to create the strongest possible population saves time and effort later.

The alternative to inoculation is so-called spontaneous fermentation, also known as indigenous or native fermentation. With native fermentation, yeasts present in the grape must initiate fermentation without inoculation. As described, there is no shortage of yeast in wineries. In most instances, a tank left on its own will ferment spontaneously, saving the winemaker the expense and labor of inoculating a tank with commercial yeast.

Few winemaking techniques generate as much fervor as inoculation. While there are pros and cons of both techniques, the decision of whether to inoculate or not depends on the winemaker’s philosophy, risk aversion, and stylistic intent, and on the condition of the fruit. Certain wine styles are likely to benefit from native fermentation, while with others, it is less successful. One could argue that the standard for top-quality Pinot Noir is native fermentation and for Champagne inoculation, though clearly there are exceptions. Native fermentation is not appropriate for damaged grapes, which harbor large populations of spoilage microorganisms.

In quality winemaking, producers often look to native yeast fermentations for complexity and nuance. This is largely attributed to the fact that a greater diversity of species usually participates in the fermentation. On the other hand, for both inoculated and native fermentations, yeast can sometimes impart a strong signature which may or may not be desirable.

Inoculated fermentations are more predictable and reliable. While predictability is admittedly not sexy, it is valuable in commercial-scale winemaking. Many winemakers cannot risk the prospect of remediating tens of thousands of gallons of wine from a ferment that has gone awry. With inoculation, a large population of yeast is added directly to the tank, and fermentation typically begins more quickly than with native fermentation. This can be important when tank space is limited. Inoculation is widely believed to reduce the risk of a stuck fermentation, which may occur if a strong fermentative yeast does not establish a viable population due to competition. Another school of thought views microbial diversity as insurance, since at least one strain is likely to be well adapted to the specific environmental conditions and complete the fermentation. In general, the risks of native fermentation are probably overstated; they often proceed successfully. However, when problems arise, the consequences can be serious.

Inoculation can occur upon fruit receival, or, if the must is undergoing cold-soak, a winemaker may wait for up to two weeks before inoculating, which gives native yeast populations ample time to impact the wine. Some winemakers split the difference through co-inoculation with a non-Saccharomyces yeast. This attempts to foster microbial diversity while retaining the predictability that comes along with inoculation. Many wineries cultivate a particular yeast strain in their cellar. A pied de cuve, or a portion of yeast-rich, already fermenting grape must, may be used to inoculate a fermentation—as with using a sourdough starter for baking. It could be said that old Burgundy cellars use “selected” yeast, since a dominant house strain is likely to be responsible for fermentation. In fact, many commercial yeasts were isolated from such cellars.

In practice, the decision of whether to inoculate is not binary. The timing and way in which yeast is added, as well as practices such as cold-soak and use of SO2, have an effect on the microbial species present and the extent to which they participate in the fermentation.


Fermentation is the anaerobic conversion of carbohydrates into energy by enzymes. During alcoholic (primary) fermentation, yeasts consume glucose and produce ethanol, carbon dioxide, and heat. Fermentation is often presented as a simple chemical reaction, as shown below. However, fermentation is actually a chain reaction with 12 individual steps, and with plenty of opportunity for the formation of side products including glycerol, acetic acid, and fusel alcohols. Yeasts convert roughly 90% of glucose to ethanol and the rest into other compounds.

The first 10 steps of fermentation are known as glycolysis, the process whereby yeast converts sugar into energy. The remaining steps regenerate some components necessary for glycolysis and produce alcohol and carbon dioxide.

The evolution of carbon dioxide is one of the first signs of fermentation. The juice becomes cloudy and carbonated, and when approaching the top of the tank, the gas burns the nose. A very large quantity of CO2 is evolved, or given off, during fermentation, equivalent to 60 times the volume of the must. Carbon dioxide escapes into the air out of the top of the tank, which must be vented to prevent pressure from building up. CO2 displaces oxygen in the fermentation vessel, rendering fermentation a very reductive process. As long as the must is protected by a cover of CO2 gas, there is no risk of chemical oxidation. The displacement of oxygen by CO2 poses a significant risk for workers, and care must be taken to ensure good ventilation in the winery to avoid asphyxiation.

Fermentation Dynamics

In a healthy fermentation, dry wines generally begin in the range of 18 to 25 degrees Brix. Toward the end of fermentation, the readings become negative, since ethanol is less dense than water. At dryness, the Brix reading may register around −2 degrees Brix. At the same time, the temperature increases throughout most of fermentation and may begin to decrease toward the end.

Brix and temperature are monitored throughout fermentation, and at specific points, the winemaker may adjust the temperature, change the cap management protocol, or make additions for the yeast. Understanding the course of fermentation is important to diagnosing and preventing problems like stuck fermentations.

For the yeast, fermentation is comprised of three distinct stages: lag phase, exponential phase, and stationary phase. During lag phase, the yeast adapts to the high-sugar environment and little population growth is observed. Yeast reproduces during the exponential phase, building up a critical population mass on the order of 10 million to 100 million cells per milliliter. Once the population reaches critical mass, the yeast begins fermenting. Yeasts maintain this population throughout the fermentation, and cell counts decrease once fermentation is finished as the yeast begins to settle out, forming lees.

Yeasts require nitrogen-based nutrients and oxygen during the first third of fermentation in order to build a healthy population. They use oxygen to build healthy cell walls—necessary for survival in a high-alcohol environment. Oxygen may be added through splashing, open pumpover, or direct injection of air or oxygen. During fermentation, there is no threat of chemical oxidation as the yeast consumes the oxygen before it has time to react with the wine.

Low levels of nitrogen in the must are associated with yeast’s production of hydrogen sulfide (H2S), a reductive thiol that smells like rotten eggs, and stuck fermentations. Winemakers measure the yeast-assimable nitrogen (YAN), or the sum of ammonia and amino acids present in the juice, to assess whether the natural levels are sufficient for the yeast. The recommended levels have changed over the years, but YANs of 200 or more are generally considered ideal for yeast health. Because many vineyard soils are naturally low in nitrogen (as excessive fertilization is discouraged for wine quality), low YANs are not uncommon. Winemakers can augment nitrogen-deficient musts by adding diammonium phosphate (DAP), an easy-to-metabolize form of nitrogen, or complex nutrients such as amino acids that the yeast must break down in order to access the nitrogen. More “complex” nutrients may result in a steadier fermentation, whereas DAP can result in a rapid boost to the rate of fermentation. In practice, most winemakers add a combination of the two. Note that nutrient addition is not just a function of New World winemaking; it is also used in the Old World, and at wineries ranging from large to small. Must nitrogen levels can also be improved in the vineyard through fertilization, yet this has other consequences for fruit composition that may reduce wine quality.

Even when there is a sufficient supply of nutrients, the stress of fermentation may trigger yeast to produce H2S. Sulfide production depends on yeast strain, grape variety, and composition of the must. Most will blow off during fermentation, but if not managed properly, H2S will react to create other sulfides, including methane and ethane thiol, with aromas described as putrefaction and as skunk, onion, and rubber, respectively. These compounds, along with H2S, are often referred to as mercaptans in the wine industry, to differentiate these thiols from pleasant-smelling varietal thiols. (Note that the term mercaptan is synonymous with thiol.) An example of the latter is 3-mercaptohexanol, a compound which commonly lends grapefruit and passionfruit aromas to Sauvignon Blanc.

When fermentations become smelly or “reductive,” winemakers may add additional nutrients or oxygen to bolster the yeast, which often improves the wine’s aroma. Aerative cap management can also help volatilize and blow off H2S. If the smell persists, copper fining (described later in this guide) can be used to remove H2S and other mercaptans. Humans are extremely sensitive to these unpleasant thiols, which have thresholds of one or two parts per billion. Over time, mercaptans can react to create more stable, but less odor-active, disulfides (which smell of garlic and onions). Disulfides are difficult to remove from wine, since copper fining is relatively ineffective, so winemakers often seek to remove high levels of sulfides before disulfides are allowed to form.

Reduction in Wine

In a wine context, reduction, reductive, and reduced describe wine aromas that arise from volatile sulfur compounds (including H2S and other sulfides) produced by yeast during fermentation and lees aging. At low concentration, these compounds can be responsible for “positive reduction” (matchstick, flint) and can add complexity to wine, while higher levels cause unpleasant reductive aromas (eggs, skunk, rubber, cabbage, garlic, sewage). All yeasts produce some H2S during fermentation, but some strains produce more than others. Low levels of nutrients, high temperature, and other stressful fermentation conditions will favor the production of sulfides. Thus, some wines contain a higher concentration of sulfides than others.

Reductive is also used to describe winemaking techniques or a storage environment that minimize a wine’s exposure to oxygen. Reductive winemaking is employed to preserve fresh and fruity aromatics and avoid oxidation and the flavor markers that are indicative of it (nutty, honey, acetaldehyde). Use of stainless steel instead of oak, aging on lees, fewer transfers, and using inert gas or dry ice are reductive winemaking practices. During aging, most wines are handled reductively, but to differing degrees. But reductive winemaking does not create the sulfides responsible for reductive aromas—yeast do.

A wine that lacks sulfides after fermentation may be aged reductively without much risk of the wine becoming reductive. It is also possible for a wine to simultaneously contain both reductive aromas produced by yeast during fermentation and oxidized aromas acquired through oxygen exposure during aging.

There is some overlap between these two notions of reduction in wine, however, since reductive storage conditions may enhance reductive aromas. Yeast deprived of oxygen in the early stages of fermentation is likely to produce more H2S. Additionally, reductive storage during aging favors the transformation of some sulfides into their smellier mercaptan form. Conversely, mild oxidative conditions favor the less odor active forms of some sulfides, and oxygen exposure, for example through racking or decanting, may help mask certain reductive aromas.


Fermentation is an exothermic reaction, producing heat. The yeast strain, presence of whole clusters, and size and material of the fermentation vessel impact the fermentation temperature. Traditionally, cold ambient cellar conditions helped regulate the rate of fermentation. Today, most winemakers use some form of temperature control, either through jacketed tanks or another means of heat exchange, to influence the rate of fermentation.

Fermentation temperature has a big impact on wine style and yeast health, and the ability to influence temperature is an important winemaking tool. Yeast requires a temperature range of 45 to 95 degrees Fahrenheit, below which they will be inactive, and above which they will die. They are most active at temperatures in the mid-70s to mid-80s Fahrenheit. Yeast strains have different temperature tolerances, and an appropriate yeast strain may be selected for the desired conditions. This is important at the extremes, especially for wines made at very cold temperature.

The maximum temperature attained during fermentation is an important parameter for understanding wine style. Higher temperature drives off delicate aromas and increases the rate of all chemical reactions, including fermentation. Yeast-derived aromas are also temperature dependent.

White wines are generally fermented at lower temperature than red wines, as no extraction from the skins is necessary, and in order to preserve volatile aromas. White wine fermentations typically range from the mid-40s to mid-60s degrees Fahrenheit, and colder fermentation temperatures result in crisp styles, while warmer temperatures favor riper, richer fruit and floral-driven aromas. With temperature control, it is easy to regulate the temperature of tank fermentations; however, barrel fermentations are more difficult to control and often hit maximum temperatures of 80 degrees Fahrenheit.

Red wine fermentations are warmer in order to facilitate extraction from the skins, with temperatures ranging from the mid-70s to the low 90s degrees Fahrenheit. Fermentation temperature is important for both the extraction and flavor profile of red wines. Higher temperatures extract more tannin and phenolic compounds, resulting in darker, fuller bodied wines. Though variety dependent, lower fermentation temperatures typically favor more fruity and bright wines with fresh flavors, while higher temperatures favor darker, riper fruit expressions.


Winemakers have many options when it comes to choosing the container for fermentation. Wines may be fermented in tank or barrel, and there are a range of shapes and sizes. Fermentation vessels may be made from stainless steel, wood, concrete or other materials, and tanks may be open or closed top. While there are some key differences, most notably between barrel and tank, the impact of the fermentation vessel on the final wine is largely overemphasized.

While many tank materials are considered neutral, oak—especially new oak—contributes a number of compounds that react during fermentation (discussed in more depth later in this guide). Yeast modifies certain flavor components found in oak barrels, and oak tannins may react with anthocyanins, helping to create stable color compounds. For this reason, wines that are fermented in barrel are often said to have better oak integration than those that are only aged in barrel. Porous materials like wood and unlined concrete are more likely to contribute microbial complexity to the fermentation than stainless steel, since it is not possible to sterilize them.

Larger vessels can be more difficult to homogenize, resulting in areas of high heat, cooler areas, and uneven extraction. While larger fermentations warm themselves and often need cooling, very small fermentations may struggle to reach desired peak fermentation temperatures on their own. Wood, concrete, and stainless steel have different heat-holding and exchange capacities, which influence the temperature profile during fermentation. Much of the difference between these materials comes down to fermentation temperature, which can be managed through temperature control.

Well-designed vessels promote convective mixing, and tanks that provide a larger surface area of contact, either between the cap and the juice, or the wine and the lees, should result in faster extraction. A number of technical tanks have been designed to optimize these conditions. While some differences may exist, other factors such as fermentation temperature and cap management practices seem to overshadow them.

Red Wine Extraction

Red wine extraction is accomplished through cap management. During fermentation, carbon dioxide gas pushes the grape skins to the top of the tank, separating the tank into a liquid portion at the bottom and a skins portion, or cap, on top. Cap management homogenizes the contents of the tank and increases skin contact and extraction. This affects the concentration of flavor and tannin in the finished wine. Cap management serves other purposes, too. It helps to regulate the fermentation temperature by breaking up hot and cold pockets, keeps the cap from drying out, and discourages the growth of acetic acid bacteria. It can also be used to introduce oxygen into the fermentation for yeast health (described further below). Common cap management techniques include pumpovers (remontage), punchdowns (pigéage), and rack and return (délestage).

In a pumpover, juice is pumped from the bottom of the tank over the top. Pumpovers can be more or less extractive depending on duration, pump speed, and the attachment at the end of the pump. The liquid may be returned to the tank using an irrigator, or sprinkler, which provides a gentle but thorough wetting of the cap, or by a more extractive technique called fire-hosing, or directing all of the liquid in a concentrated stream toward the cap.

Punchdowns use a plunger or large foot to mix the tank by pushing the cap into the liquid portion. They may be done manually or using an automated pneumatic tool. There are many ways to do a punchdown, and they can be gentle or very extractive; it depends on the technique. During the height of fermentation, a lot of force is required to break through the cap. Punchdowns do a better job of breaking berries apart and, in this way, can be more extractive than pumpovers.

In a rack and return, the entire liquid portion of the must is drained into another tank, leaving only skins behind. Afterward, the liquid portion is pumped back over the top of the original tank. This method breaks up the cap, lowers the temperature of the fermentation, and provides the most complete mixing possible. Because the tank is fully homogenized, extraction is more efficient.

Submerged cap fermentation is less common but popular in certain regions, including Piedmont. Here, the cap is intentionally submerged throughout the fermentation, ensuring that the skins stay in contact with the liquid and resulting in a greater rate of extraction.

The choice of technique typically depends on a winemaker’s preference and tradition; punchdowns are more common for Pinot Noir and Syrah, while pumpovers tend to be typical for Bordeaux varieties. From a cap management standpoint, a wine’s concentration is determined by the frequency of cap management, the effectiveness of mixing, and the physical extraction of the fruit. Some winemakers claim that either punchdowns or pumpovers are more extractive. This is a case where it’s not possible to generalize, and the result depends on the details of the technique.

More frequent cap management results in more extraction. During fermentation, tanks are commonly mixed one or two times per day, though three to six times per day is not uncommon if a bigger, more extracted wine style is desired. A winemaker may vary the frequency or technique used over the course of the fermentation to achieve the desired level of extraction. Extraction depends on solubility, and some compounds are more soluble in water and others in alcohol. Anthocyanins are more water soluble and reach a maximum concentration early in the fermentation. Conversely, tannin is more soluble in alcohol and extracts quickly toward the end of fermentation. Winemakers can exploit this property by adjusting the cap management protocol during certain points in the fermentation.

Extended maceration is the practice of leaving the wine on its skins for several weeks to months after primary fermentation is complete. This practice typically extracts more seed tannin, though it also appears to increase the rate of phenolic polymerization, which may result in more sweetness on the midpalate. Keeping the wine on its skins also changes the flavor profile, but the results are fruit and technique specific.

End of Fermentation

Many white wine styles, including “dry” white wines, contain some residual sugar. Wines may stop fermenting naturally, or they may be stopped intentionally. Arresting a fermentation is achieved either by temperature reduction and sulfur addition, which inhibits the yeast, or through filtration or centrifugation, which remove the yeast. Fortification with a high-proof spirit is another niche option to arrest fermentation. The momentum of fermentation is difficult to stop, and achieving a precise level of sweetness that results in a balanced wine is an art. Wines that contain residual sugar are vulnerable to refermentation and microbial spoilage, and it is especially important to maintain adequate sulfur levels throughout aging.

Some “dry” red wines also contain a small amount of residual sugar, but it is uncommon to intentionally arrest a red fermentation. In red wines, residual sugar is often the consequence of an addition of grape concentrate prior to bottling, or it can arise from a fermentation that stops naturally prior to dryness. When residual sugar was not the intention, these fermentations are referred to as stuck.

Stuck fermentations occur when the yeast becomes unduly stressed, due to fermentation conditions, competition, or a sudden change in temperature. Once a winemaker identifies that a fermentation is likely to stick, indicated by the shape of the Brix curve, there are several corrective measures available. Warming and frequent mixing (or stirring) of the tank can help keep yeast in suspension, giving them more access to available sugar. Slow fermentations may continue ticking away for several months. Allowing the wine to sit unsulfured with residual sugar is risky, however, since these wines are most susceptible to microbial spoilage. On a small scale, stuck fermentations can be blended away. There are several methods to “restart” a fermentation, but they are often unsuccessful, and even when they succeed, the wine quality is typically compromised.

Once primary fermentation is complete, red wines are drained and pressed. Both red and white wines may be settled and racked off of their gross lees (a combination of live and dead yeast that settle out) prior to transfer to their aging vessel, or they may be transferred directly with lees retained, to create a more reductive aging environment. The wine may be allowed to go through a “secondary” malolactic fermentation, or this may be blocked through the addition of sulfur dioxide.

Malolactic Fermentation

During malolactic fermentation (ML), malic acid is converted to lactic acid by lactic acid bacteria. Prior to initiating malolactic fermentation, the dominant bacteria species establishes a critical population mass on the order of 100 million to 1 billion cells per milliliter.

Malolactic fermentation lowers a wine’s acidity, since malic acid is more acidic than lactic acid, and stabilizes the wine. Malic acid is an energy source for a number of microorganisms, and its presence can induce microbial spoilage.

Lactic acid bacteria are inhibited by low pH (below 3.2), high alcohol (above 14.5%), low temperature, and sulfites; as a result, some wines struggle to complete malolactic fermentation. Secondary fermentation will proceed at temperatures in the range of 60 to 85 degrees Fahrenheit, with 70 degrees Fahrenheit being ideal. Very low temperature will prevent bacterial growth and activity. Bacteria are sensitive to SO2, and even a small amount can inhibit their activity.

Winemakers track the progress of malolactic fermentation by measuring the amount of malic acid in the wine. When none remains, fermentation is considered complete. As with primary fermentation, malolactic fermentation involves many chemical reactions, and side products are formed during the process. LAB produce wine aroma and flavor compounds, including acetic acid, acetaldehyde, diacetyl, and others. Diacetyl is one of the most recognizable products of malolactic fermentation. It is an intermediate side product of malolactic fermentation, and malolactic bacteria will continue to break it and other intermediates down after malic acid is depleted, so long as they are not inhibited by SO2. Sulfuring immediately after malolactic fermentation finishes results in more retention of diacetyl and other aromas derived from malolactic fermentation, while delaying sulfur addition until these aromas are no longer perceptible greatly reduces or eliminates their impact. For this reason, wines that complete secondary fermentation may or may not exhibit the aromas commonly associated with it.

Malolactic fermentation may be prevented, or a wine may be allowed to go through partial or complete malolactic fermentation. With white wines, this is a stylistic decision. Wines that are malo-dry have softer acidity and a different flavor profile. While not compulsory, most red wines go through malolactic fermentation for stability reasons. Occasionally, winemakers inhibit malolactic fermentation on red wine in order to retain acidity. It can be prevented by the addition of sulfur dioxide, lysozyme (an enzyme that destroys LAB), or filtration. Where malolactic fermentation is not intended, the wine is sulfured as soon as possible following primary fermentation and is almost always sterile-filtered prior to bottling to prevent refermentation in bottle.

While it is possible to inoculate for malolactic fermentation, it is often unnecessary, as naturally occurring lactic acid bacteria will typically initiate the fermentation spontaneously. Winemakers are less likely to inoculate for secondary fermentation than primary, but as with yeast, inoculation allows the winemaker to select a strain that is better adapted to conditions or more likely to produce desirable sensory characteristics.

The timing of malolactic fermentation has some interesting implications. It often proceeds slowly, taking place over a few weeks or months. Winemakers prefer primary fermentation to complete before malolactic fermentation initiates. The reason for this is two-fold: it avoids competition between yeast and bacteria, which could lead to a stuck fermentation, and it reduces the risk of high volatile acidity, since some lactic acid bacteria convert sugar to VA. Some winemakers prefer malolactic fermentation to begin immediately after primary fermentation and finish as quickly as possible, since a wine is vulnerable to spoilage and oxidation before it has been sulfured. These winemakers may be more inclined to inoculate for secondary fermentation.

However, for some wine styles, there are benefits to delaying malolactic fermentation. Many cite that this allows wine to be stored unsulfured for longer, resulting in less total sulfur use (though during this time, the wine is not protected from microbial infection). Delaying malolactic fermentation also has implications for wine color. Between alcoholic and malolactic fermentation, several compounds (specifically, acetaldehyde and pyruvate) react with anthocyanins to create stable pigments. Lactic acid bacteria consume these compounds during secondary fermentation, at which point these color-stabilizing reactions no longer occur. For light-colored varieties such as Pinot Noir, delaying malolactic fermentation can help to maximize a finished wine’s color intensity.


While active winery work slows down once fermentation is complete and the wine has been transferred to its aging vessel, important changes continue in the wine. Chemical reactions initiated during fermentation keep evolving slowly. Tannins mature, becoming less astringent, and the color of red wine becomes less vibrant, gradually shifting from red-purple to red-brown. Fruity and floral flavors that are prominent in young wine decrease and are replaced by more savory and oxidized flavors.

During aging, exposure to oxygen and flavor addition from oak and lees—or lack thereof—play an important role in shaping a wine’s style. These factors are modulated through the winemaker’s choice of aging vessel and cellar practices such as stirring, topping, racking, and sulfur addition.

Some wine styles require little to no aging and are bottled young to retain fruity and fresh characteristics. This is common for many styles of white and rosé wines, which often see a short élevage of three to six months. Many of these wines are intended to be consumed quickly, since the flavors that drive their styles fade over time. Other wine styles benefit from longer aging, and wines may be stored in tank or barrel for a period of several months to several years to mature prior to bottling. For wines aged in wood or other porous materials, the length of élevage increases oxygen exposure, and wines aged longer display greater signs of oxidation. The oxidative impact of extended élevage can be observed in the gran reservas of Spain and riservas of Italy.

The environmental conditions of storage should be stable and within a specified range of temperature and humidity. After primary and malolactic fermentation, wine is stored at cool temperatures typically ranging from 45 to 65 degrees Fahrenheit. White wines, especially fresher styles, are often stored on the cooler end of this spectrum, while reds may be stored slightly warmer, to encourage color and tannin maturation. Temperature stability is important, since temperature swings can loosen bungs, exposing the wine to more oxygen. For wines stored in wood, the humidity of the cave or barrel storage room must be maintained to prevent excessive evaporative loss and the wood from drying out.

Oxidation Chemistry

The extent to which a wine has been exposed to oxygen plays a key role in defining the wine’s style. Oxidation reactions reduce astringency and varietal and fruity aromas, and they create flavors expected in aged wine. Some of these reactions are considered beneficial, while others are generally regarded as detrimental.

Oxygen’s interaction with a wine depends on the timing of exposure. Prior to fermentation, grape enzymes mediate oxidation, resulting in a faster rate of reaction. This can result in temporary browning and loss of aroma components and may be reduced through the use of SO2 or colder temperature, which inhibits the enzymes. Microbial populations also consume oxygen prior to fermentation. Oxygen is important for yeast health, but acetic acid bacteria (which create volatile acidity) also thrive in oxygen-rich, high-sugar environments.

The fermentation environment is highly reductive. Yeast requires oxygen at the beginning of fermentation, and cellar practices are used to aerate the wine. Yeasts rapidly consume any oxygen in the must, preventing chemical oxidation of the wine. As primary fermentation slows down, less CO2 is available to protect the wine. For many wine styles, exposure to oxygen after fermentation is complete is minimized, and the wine is protected through measures such as SO2 addition, topping, and use of inert gases. Aging on lees, which continue to consume oxygen, will also help protect a wine. However, during élevage, some slow oxygen exposure is beneficial for certain styles, especially for tannic red wines. During aging, oxidation reactions help stabilize wine color and soften tannins by facilitating the polymerization of phenolic compounds.

Wine can be imagined as a reservoir of oxidizable components, including tannins, pigments, sulfites, aroma compounds, and alcohol. When wine is exposed to air, oxygen reacts with phenolic components in the wine and begins a cascading chain of reactions that terminates in the oxidation of compounds from the reservoir. Many of these reactions are slow. The first step of oxidation, where oxygen reacts with phenolic compounds in the wine, can take hours or days, depending on the phenolic content of the wine. Because red wine contains more phenolic compounds, oxygen is consumed more quickly in reds than in whites.

Once oxidation is initiated, a number of reactions are possible, and the outcome depends on the composition of the wine. Sulfites and phenolics “protect” the wine since they are easily oxidized, which prevents the oxidation of aroma and flavor compounds. Once these protective compounds have been used up, however, any additional oxygen exposure will ultimately result in the oxidation of ethanol to acetaldehyde, and the wine’s quality will decline. Because red wine has a greater concentration of pigments and tannin (antioxidants), it is able to withstand more exposure to oxygen before negative effects are observed. White wine has a smaller reservoir and a greater likelihood of damage due to oxidation.

Aging Vessel

A number of vessels are used to store aging wine, including barrels, large oak casks, stainless steel, concrete, clay, and glass. The impact of the aging vessel depends on whether any flavors or aromas are added to the wine (as with oak) and how much oxygen exposure occurs. As with fermentation vessel, the most obvious differences are observed between barrels, large porous tanks, and inert vessels like stainless steel. Many winemakers utilize a combination of different aging vessels to create more blending components and add complexity to their wines.

Aging in oak (or other wood) contributes flavor and aroma compounds and tannin. These flavor contributions are most significant for new barrels and reduce with each subsequent fill. Additionally, porous materials like wood and concrete are difficult to clean and thus more likely than stainless steel to contribute microbially derived flavors. Porosity permits the slow ingress of oxygen through the walls of the vessel. As a rule, smaller vessels are more impacted by oak flavors and oxygen exposure than larger ones, since their surface area-to-volume ratio is larger. Much of the air that enters the aging vessel is from opening the vessel, and the oxidative impact of this is greater for smaller vessels.

Barrel aging is beneficial for wine styles that are improved through slow oxygen exposure, and whose flavors are complemented by those derived from wood. Many red grape varieties, especially tannic ones, and certain white wines are improved through maturation in barrel, but it does not benefit all wine styles. Storing wine in barrel is more labor and space intensive, and more expensive, than aging in a larger tank.

Larger porous tanks made of wood or concrete have an impact on style between that of smaller barrels and storage in stainless steel. Concrete (when lined) and older wooden tanks are often considered neutral, though wood may still contribute tannin and microbial flavors. Both allow a similar rate of oxygen ingress, though the precise rate depends on a number of factors, including size and material thickness. Large tanks are less labor intensive than barrels for many cellar operations; however, wood and concrete tanks are difficult to clean and store.

Stainless steel tanks are the aging vessel of choice for wine styles that do not benefit from oak flavors or oxygen exposure, particularly wines that emphasize fruit and freshness. Stainless steel is neutral, impermeable to oxygen, easy to clean and sanitize, and inexpensive. Value-priced red wines are generally aged in stainless steel to reduce production costs. Winemakers sometimes simulate the effects of oak aging by adding oak staves or adjuncts to the tank (which add oak-derived flavors and tannins) and through micro-oxygenation (which imitates slow oxygen ingress). Other non-porous vessels made of mild steel and epoxy are also used as cheaper alternatives.


Barrels have been used since antiquity for storage and transport, and their use evolved alongside winemaking. Today, certain wine styles are expected to include the flavor profile imparted by barrel aging. The benefits of oak aging include the addition of flavor and tannin, concentration through evaporation, and slow oxidation, which gives the wine a softer, more mature profile.

White oak possesses several qualities that make it ideal for barrel making: it is a lightweight, watertight, malleable hardwood. Oak contributes desirable flavors and tannin and lacks unpleasant, sappy flavors that would taint the wine. Chestnut, acacia, redwood, eucalyptus, and other woods also have niche uses.

Barrel Production

Barrel production is a craft. The oak used for barrels comes from over-80-year-old trees from managed forests, and one mature oak yields between one and four barrels. American oak is more watertight and can be sawn into planks, whereas French oak must be split against the grain to prevent leakage, and only 20% of the tree can be used. Fresh oak is astringent and “planky” tasting, and in a process known as seasoning, the wood is dry-aged outside for as little as one year (for lower quality barrels) and three or more years (for higher quality barrels), leaching the harshest tannins and aroma compounds from the wood. Seasoning may be accelerated by using a drying kiln.

After seasoning, the staves are carefully selected and trimmed so that once bent, they fit together perfectly. Oak is a natural product with a lot of tree-to-tree variation, so coopers rely on a blend of staves from different trees to improve consistency. The staves are positioned to form the circumference of the barrel and secured on one side using a temporary iron hoop. Using fire and water, the staves are slowly bent by machine until they form the body of the barrel and can be fastened with another metal hoop.

Barrel toasting, typically performed by a master cooper, requires skill. The cooper toasts the barrel by positioning it over a small fire for a precise amount of time, then moving it through a series of fires, each with a different temperature. The temperature and time spent over each fire determine the toast level. Once the barrel is toasted and allowed to cool, the barrel heads are put in place, the barrel is sanded, the bunghole is cut, and the temporary hoops are replaced with permanent ones.

The cost of a new barrel starts around $450 for American oak, $600 for Hungarian oak, and $900 for French oak. Specialty French oak barrels can cost over $2,000 a piece.

The extent of oak’s impact on wine depends on factors including oak species and provenance, toast level and cooperage, and barrel age and size. French oak barrels are typically a blend of two oak species: Quercus robur and Quercus petraea (or Quercus sessilis), while American oak is typically Quercus alba. Oak species is significant, as Quercus robur is coarser grained and more tannic, while Quercus alba is denser, with less tannin and a higher concentration of oak lactones, resulting in more vanilla and coconut flavors. Quercus petraea lies somewhere in the middle in terms of tannin and lactones but also has elevated levels of triterpenoids, which contribute sweetness. Hungarian oak accounts for less than 5% of worldwide barrel production, and similar to French oak, it is a combination of Quercus robur or Quercus petraea.

In addition to oak species, provenance also has an impact, and single-forest barrels are sometimes produced for a premium. Most people cannot distinguish oak by forest, though differences in growing conditions and the ratio of Quercus species present do result in some trends. Trees that grow more slowly—due to cooler climate or less fertile soil—are tighter grained and may result in slower oxidation, more aroma compounds, and lower tannin, while looser grained wood yields the inverse. Anecdotally, oak from the forests of Tronçais, Allier, and Jupilles are tighter grained, Nevers and Bertranges are medium grained, and Vosges is looser grained. Oak from Limousin is Q. robur-dominant, coarse grained, and often preferred for spirits over wine. Oak from the ZemplĂ©n Hills forest in Hungary is 95 to 100% Q. petraea and tight grained.

Toast level is a function of both time and temperature, and toast is often described as light, medium, medium-plus, and heavy. In general, higher toast oak has less oak lactone and tannin and more grilled, smoky, toasty aromas. The concentration of volatile phenols and other aroma compounds increases with toast, though heavy toasting can drive off these aromas. Barrels may also be customized with toasted heads to give more toasty flavors and less tannin. Because each cooperage has its own toasting process and standards, cooperages typically have a house style.

New oak is the most impactful in terms of flavor and tannin, and by the second fill, only about half as many oak-associated compounds are extracted. By the third to fifth fill and beyond, the barrel is typically considered neutral, but neutral is a spectrum, and less extraction occurs after each subsequent fill. There is one caveat: oak lactones are indefinitely released from the wood in an acidic environment (albeit at reduced rates for older wood). By using a combination of new and neutral barrels, winemakers can dial in their optimal flavor contributions. The barrel’s size also has an impact. Bordeaux barrels (225 liters), also called barriques, and Burgundy barrels (228 liters) are standard, but many other sizes are available. Vessels with a larger capacity have a smaller surface area-to-volume ratio, resulting in less concentration of oak flavors and less evolution due to oxidation.

Oak Chemistry

Oak is made of cellulose (40–45%), hemicellulose (20–25%), lignin (25–35%), and tannin (5–10%), and the seasoning and toasting processes break these compounds down into important flavor compounds. While toasting increases the impact of certain flavor compounds, it drives off others. (A summary of oak-derived flavor compounds can be found in the Compendium.)

Oak tannins are called hydrolysable tannins or ellagitannins and are distinct from the tannins that come from grapes. French oak has more than twice the tannin content of American oak and may contribute some additional bitterness and astringency. Trans-2-nonenol is a tannin that imparts a green sawdust or cardboard aroma that decreases through seasoning and toasting.

Cis- and trans-oak lactone (known as the “whiskey lactone”) are important oak-derived compounds that contribute to wine aroma. They offer oaky, vanilla, coconut, sweet, and cocoa aromas to the wine. The cis isomer is present at twice the concentration of trans and is 10 times more aromatically potent, so it is typically considered more important for wine aroma. American oak has higher lactone concentration than French oak, and this is often cited as the primary difference between the aromatics contributed by French versus American oak.

Lignins are not soluble in wine, but when they are heated during the toasting process, they break down into volatile phenols. Vanillin, the primary compound in vanilla, and eugenol (clove, spicy) are present in raw oak, and their concentrations are enhanced through toasting. Eugenol levels are believed to increase during seasoning. While isolated vanillin smells like vanilla, in wine it is more associated with cinnamon, coffee, smoky, or chocolate aromas. Fermentation in barrel reduces the impact of vanillin, which is converted to vanillic acid by yeast. Guaiacol and 4-methylguaiacol (smoke, spice) are formed during toasting. These compounds are also present at elevated levels in smoke-tainted wines. Syringol (smoky) and cresols (leather) are other products of lignin decomposition. Though these compounds all have distinctive aromas on their own, their impact on wine aroma depends on the wine’s composition.

Cellulose and hemicellulose are made of carbohydrates that break down during toasting into aldehydes that impart caramel and toasty aromas. Examples are furfural and 5-methylfurfural, compounds that increase the perception of oakiness, almond, butterscotch, and caramel.

Used barrels, if not cleaned properly, are a common source of microbial contamination, especially from Brettanomyces, which may give the wine extra complexity at low levels or may completely overwhelm the wine. Empty barrels are often treated with SO2 gas to discourage microbial growth.

Barrels are expensive and labor intensive, and their use is not appropriate for all wines, especially those at a lower price point. Oak alternatives, including chips, staves, oak dust, and oak powder, flavor wine at a fraction of the cost. These do not give the slow oxidation effect of barrel aging, however, and micro-oxygenation may be substituted.

Many winemakers experiment with cooperages, toasts, and other customizations to determine which barrels are the best compliment for their fruit. Choices around oak, and wood in general, depend on the fruit and intended wine style and are learned through experience. Bigger, more tannic red wines typically can handle more new oak, but it is difficult to generalize. A particular barrel may work well for one wine and detract from another. Ultimately, winemakers may use a blend of barrels from different cooperages and at different toast levels, along with a combination of new and old barrels, to create a more complex and balanced wine.

Topping & Sulfuring

During aging, wine stored in barrel is concentrated slowly through the evaporation of water and alcohol through the barrel. Depending on the humidity level in the cave or barrel hall, volume losses can be as high as 10% per year.

The proportion of ethanol can change significantly during aging. Wine aged in a cave with a relative humidity of around 70% will lose alcohol and water in the same proportion. If the relative humidity is higher than 70%, the alcohol level will decrease through aging. Alternately, if the relative humidity is less than 70%, alcohol will increase.

The effect of concentration also increases acidity and all of the other components dissolved in wine. This concentration can cause certain compounds in wine to become insoluble. For example, tartrate crystals may become insoluble and settle out during aging.

Topping, or ouillage, is generally performed every two to six weeks to replace the volume of wine lost to the atmosphere. Wine stored under ullage is more susceptible to oxidation and the growth of acetic acid bacteria, and the greater the surface area in contact with air, the more oxidation occurs. Recent studies have indicated that most of the oxygen exposure during barrel aging comes from air entering through the bunghole, either from loosely bunged barrels or when the barrels are opened, as with topping. Some wineries choose not to top and store the barrels on their sides to minimize the potential for oxidation or microbial growth. Otherwise, barrels are opened infrequently to minimize oxygen exposure, and topping is a good time to sample the wine and make any necessary sulfur additions. During élevage, free sulfur is often maintained at 20 to 50 parts per million.

Topping is a notorious point of potential contamination, and the wine used for topping must be clean. Infected topping wine can rapidly inoculate an entire lot or cellar. Winemakers taste topping wine to ensure that it is free of defects, and some will send a sample to the lab. Though it may seem like a small detail, this can have a large impact on wine quality.

A number of wines are stylistically defined by oxidative aging, including Vin Santo and Oloroso Sherry. In this case, the barrels or aging vessel will be stored under ullage, and oxygen will be allowed to go in the barrel. This type of aging results in darker, more brown hues and nutty aromas. Oxidative aging requires time for the wine to develop; the same wine would not be achieved through hyper-oxidation.

Other wines are aged biologically, where the barrel is not topped and flor yeast—typically Saccharomyces yeast that forms a thick white film on the surface of the wine—is permitted to grow. The flor protects the wine from exposure to the air, but the yeast produces acetaldehyde, a flavor commonly associated with oxidation. Fino Sherry and Vin Jaune are classic examples of biological aging.

Sulfur Dioxide

Sulfur dioxide is an almost-universally used wine additive that has been employed for centuries. Owing to its dual ability to reduce the impact of oxidation and limit microbial growth, it is widely used for food preservation and can also be found in beer, fruit juices, dried fruits and meats, and other packaged foods. (Sulfur dioxide should not be confused with elemental sulfur, which is used in the vineyard as a fungicide.) While some sans soufre wines with no added sulfites are commercially available, yeast produces SO2 during fermentation, so even these may contain a small amount of sulfites. Notably, in the United States, USDA-certified organic wine may not contain added sulfites, which is generally not required by other organic and biodynamic certifications. (This should not be confused with USDA-certified wine made from organic grapes, which can contain up to 100 ppm SO2.)

Sulfur dioxide has an impressive synergy with wine, and there are multiple benefits to its use. SO2 is anti-oxidasic and can be used to denature oxidation enzymes in must. It is anti-microbial and highly effective at inhibiting bacteria and some non-Saccharomyces yeast, though Saccharomyces and Brettanomyces have higher tolerances and may not be disabled completely by SO2.

SO2 reduces the impact of oxidation through several mechanisms. While it does not react with oxygen directly (a common misconception within the industry), free sulfur binds with the products of the initial steps of the oxidation reaction. This prevents further oxidation of components in the wine and depletes the amount of free sulfur in the process (where 1 mg of O2 consumes 4 mg free SO2). Additionally, SO2 binds with acetaldehyde (bruised apple), a characteristic flavor marker in oxidized wine, and consequently masks the effect of oxidation.

Winemakers often differentiate between free and bound SO2. Free SO2 is the portion of SO2 that has been added and is available to protect the wine from oxidation and microbial spoilage. SO2 is highly reactive and binds to many components in wine, including acetaldehyde, sugar, and tannin. While free SO2 is important to winemakers, legal restrictions apply to the total SO2, which is the sum of free and bound. Wines with a high level of residual sugar or solids (like lees) require larger SO2 additions to achieve the same level of free SO2, since a greater portion becomes bound.

The chemical form of SO2 depends on pH, and two forms of free sulfur exist in wine: molecular and bisulfite. (A third form of SO2 exists, but its concentration is negligible at wine pH.) Most of the sulfur in wine is in the bisulfite form, which is responsible for combating oxidation. The molecular form of sulfur is the anti-microbial species and favored at low pH. Because of this, the efficacy of SO2 against microbial populations depends on the pH of the wine, with lower pH wines more protected for an equivalent rate of free sulfur. More specifically, a wine at pH 4.0 requires 10 times the amount of sulfur as a wine at pH 3.0 to be equally protected against microbial spoilage. This is yet another reason that pH is important in winemaking. Some adjust their SO2 additions based on wine pH, though this is not practiced universally. A molecular SO2 of 0.5 or 0.8 parts per million is recommended for red and white wines, respectively, to protect against microbial spoilage.

There are several stages during the winemaking process where SO2 is routinely added, though winemakers may choose to omit additions at any of these steps:

  • Prior to fermentation, SO2 denatures oxidation enzymes in the fruit. It inhibits bacteria and slows the growth of many yeasts. Additions of 25 to 100 parts per million or higher are common, where the rate depends on the type of wine and the health of the fruit, with white grapes and damaged fruit receiving more. With healthy fruit, some may choose to skip this addition. SO2 added at this point binds quickly and will be consumed during fermentation. The total SO2 is generally less than 10 parts per million at the end of fermentation.
  • During fermentation, SO2 is not desired as it will inhibit the yeast. However, it may be added to arrest a fermentation before dryness, if residual sugar is desired. Similarly, SO2 may be added just after primary fermentation to prevent malolactic fermentation. An initial post-fermentation addition of 50 to 100 parts per million total SO2, which results in 20 to 50 parts per million free SO2, is typical. For wines that are not sugar- or malo-dry, it is essential to maintain SO2 levels throughout aging to prevent fermentation from initiating spontaneously. Malolactic fermentation may also be delayed with a small (5–10 ppm) addition after primary fermentation.
  • For all other wines, the initial post-fermentation addition is generally made after malolactic fermentation is complete. Delaying SO2 until diacetyl and other biproducts of malolactic fermentation are no longer perceptible can help minimize those characters. During aging, winemakers may make periodic additions to maintain SO2 levels, as they are depleted over time.
  • SO2 levels at bottling are important since they help counteract the oxygen pickup that occurs during bottling and protects the wine over its lifetime. Most adjust free sulfur levels to 25 to 50 parts per million.

Several forms of sulfur are added to wine. Sulfur dioxide is a gas under ambient conditions, and the gaseous form is sometimes used to sanitize barrels during storage. For wine additions, a liquid form of SO2 is used to facilitate handling and measuring. Potassium metabisulfite (KMBS) is a salt form that releases SO2 when dissolved in water. KMBS is easier to handle and less noxious than the other forms and has grown in popularity. However, it increases the amount of potassium in the wine, which can result in higher levels of tartrates.

The legal limit for total SO2 allowed in dry wine is between 150 to 350 parts per million in most countries. In practice, most dry wines have less than 100 parts per million total sulfur, with 80 typical for red wine. Sweet wines require higher levels of SO2 and have higher legal limits, since a greater proportion of the total becomes bound (to sugar).

Understanding how SO2 behaves is key to minimizing its use. Healthy fruit, low pH and residual sugar, clean cellar practices, and minimizing oxygen exposure allow the winemaker to use less. Additionally, aging the wine on its lees or delaying malolactic fermentation pushes back the initial addition of SO2, which can reduce the overall amount added to the wine during the course of its life.


Racking, or soutirage, is a cellar operation that clarifies and aerates wine. During racking, the wine is transferred, and the solids that have settled to the bottom of the tank or barrel are left behind. These solids, called lees, include yeast, grape solids, and tartrate crystals. Transferring wine is inherently an oxidative process, but the winemaker may choose to limit or promote oxygen exposure during racking based on the condition of the wine and winemaking goals.

Desired level of clarity, lees contact, and oxygen exposure are considerations for the frequency of racking. Unless another method of clarification is used, wine will be racked at least once prior to bottling to remove the fermentation lees, and some are racked as many as six or more times for better clarity and tannin maturation and to avoid reduction. Alternately, “reductive” wine styles may be racked minimally and protected with inert gas during the process. Racking may occur during the following stages of the winemaking process. In general, winemakers seek to minimize wine transfer, and many of these steps are combined:

  • In white winemaking, just after pressing and prior to fermentation to remove solids. If solids are desired, a portion may be added back to increase turbidity, or the wine may be racked “dirty,” with some solids included during racking.
  • After primary fermentation to remove the gross lees, or the large volume of lees generated during fermentation.
  • After malolactic fermentation to remove lees prior to the addition of SO2.
  • Periodically throughout aging for clarification and aeration.
  • During blending.
  • After fining to remove fining agents and/or prior to filtration.
  • After cold stabilization to remove tartrate crystals.

During racking, wine is pumped or siphoned through a racking wand or racking arm, similar to a large straw, which is placed just above the layer of solids. Wine is drawn off, leaving the lees behind. When racking barrels, care must be taken to avoid moving the barrels too much and stirring up the lees. Depending on winemaking goals, some lees may be included in the transfer, or the wine may be racked “clean” to avoid transferring any solids.

Whereas oxygen exposure may benefit tannic or reductive red wines, for white and fruit-forward, light red wines, oxygen exposure is typically minimized to retain freshness and preserve fruit flavors. When the goal is to limit oxidation, inert gas may be used to displace oxygen from transfer hoses and the receiving vessel. If aeration is desired, inert gas is not used, and the wine may be splashed into the top of the receiving vessel to increase oxygen pickup. To counteract the effects of incidental oxidation, SO2 is often added during racking.

Racking may occur between barrels, barrel to tank, tank to barrel, or tank to tank. Often, a lot, or set, of barrels is racked to a tank, blended and homogenized, and returned to barrel to ensure uniformity across the lot.

A traditional method of racking still employed in France called soutirage à l’esquives involves draining the wine through a valve on the face of the barrel. Then, a winch inverts the barrel, decanting the wine off the lees until sediment appears. A small, shallow cup called a tastevin was traditionally used to taste and assess the clarity of the wine during racking. This technique is said to result in slower, more precise racking but appears to be more oxidative than its modern counterpart.


In winemaking, the term blending, or assemblage, refers to both the determination of the proportion of each lot to be used in a finished wine and the cellar operation of physically combining the wines. Wines made from different grape varieties, clones, vintages, or vineyards, or made using distinct techniques, may be blended. Most wines have been blended to some degree.

During a blending session, samples of each component wine are prepared and tasted. Blending is artistic and, depending on goals, can be an important opportunity for the winemaker to interpret the vintage and vineyard site. Determining a blend follows an iterative process of combining the wines in different proportions and determining the best blend by tasting. This is rarely a combination of all of the best lots. Great blenders understand how the structural components of each wine can knit together and create something better than the sum of their parts. An addition of as little as 1% can materially change the character of a wine.

But blending is not only an artistic endeavor; there are goals and constraints that limit and simplify the process. Blending can be used to achieve stylistic intentions or increase the wine’s complexity. A particular house style may be desired, as with many non-vintage Champagnes, and blending can be used to maintain consistency. Wine chemistry is often an important consideration, as there may be targets for alcohol, acidity, and residual sugar in the final blend. Blending can also be used to create a balanced wine from wines that are not optimally balanced, or to minimize a wine fault. For example, a wine with an unacceptably high level of volatile acidity may be blended until the VA is undetectable.

From a commercial perspective, each wine SKU may have a target volume, bottle price, and flavor profile that must be considered when blending. Financially, it is ideal to include all of the wine that was produced in a finished wine, though wineries sometimes bulk out wines that do not fit into their programs.

There are also legal considerations. Varietal, vintage, and appellation designations require minimum proportions of wines in a given blend. (For details, refer to the Expert Guide to Wine Law).

The timing of blending varies. Some winemakers prefer to blend as early as possible, to give the wine the maximum amount of time to integrate prior to bottling and to minimize the number of lots that must be managed in the cellar. Others prefer to wait to blend until closer to bottling to have a better sense of the character of each component wine. This is particularly useful for understanding newer vineyards, but the downside is that it requires managing more individual lots during aging.

While blending in the literal sense refers to a specific cellar operation, smaller, everyday blending decisions are made throughout the winemaking process. These include decisions regarding where to draw vineyard block boundaries, the extent of sorting, co-fermentation, whether to combine press wine with free run, and what to use for topping wine.

Stabilization & Bottling

After blending and before bottling, several winemaking steps can ensure that a wine is sound. A winemaker may want to remove excess fine solids that are suspended in the wine after racking. These include yeast and bacteria cells, small pieces of grape particulate, and colloids (large complexes of polysaccharides, phenolic compounds, tartrates, and proteins). While more savvy wine drinkers may not be deterred by light turbidity, clarity is important to many consumers, particularly at lower price points. Aside from cosmetic concerns, clarification can attenuate flavors, giving a greater sense of purity. Alternatively, these fine solids can protect the wine from oxidation and add flavor and complexity. Some winemakers choose to bottle wine with slight turbidity, particularly at higher price points, where consumers may be more accepting of “character.” Some techniques for clarification include fining, filtration, and centrifugation. Centrifugation is also used as an alternative to racking, reducing labor and wine loss. However, centrifuges are very expensive and thus only appropriate for very large, process-oriented winemaking facilities.


Fining selectively removes undesirable components in order to make a wine more pleasurable or stable. A fining agent binds with targeted compounds, either chemically or electrostatically, and forms complexes that settle out of the wine. These reactions occur rapidly, and the wine is racked to remove any solids that settle out within a few days or weeks. In wine literature, fining often refers to using egg whites to smooth out rough tannin, but there are other types of fining, too.

While fining agents target specific components in the wine, some unintended removal of other components will occur. For this reason, winemakers seek to minimize the amount of fining agent. Bench scale fining trials in the lab help winemakers determine the appropriate dose prior to treatment.

Tannin fining with protein can smooth out aggressive tannins in red wines and reduce bitterness caused by small phenolic compounds or pigments from skin contact or oxidation in white wine. Proteins bind with phenolic compounds electrostatically, forming insoluble complexes that settle out of the wine. A number of naturally occurring proteins are used to remove tannins and pigments, including casein (milk), albumin (egg whites), isinglass (fish swim bladder), and gelatin (tendons and muscles). Alternatively, PVPP and nylon are synthetic, vegan fining agents used to remove small phenolic compounds that cause bitterness and browning in white wine. Using ripe fruit and gentle extraction helps avoid the need for fining. Fining is more prevalent in cooler regions, where fruit is harvested less ripe and tannins may be underripe or rustic.

Modest increases in temperature during bottle aging decrease the solubility of proteins in wine, which can make wine hazy. Bentonite fining is a technique used to ensure that the wine is heat stable. (See below for more discussion on wine stabilization.) Bentonite is a naturally occurring, negatively charged Montmorillonite clay used to remove large, positively charged compounds, including proteins and polysaccharides, in juice and wine. As it is not soluble in wine, it will settle out entirely after a few days. Bentonite fining can be used to remove proteins from finished white wine. Protein instability is a white wine issue because polyphenols in red wine increase the solubility of proteins. There are lab tests that help the winemaker determine whether this treatment is necessary.

Bentonite is also used in white winemaking at the juice stage, just after pressing, to remove grape solids and clarify the juice prior to fermentation. Winemakers’ opinions differ regarding solids inclusion during fermentation, as it seems to benefit some styles and not others. Gentler pressing techniques naturally reduce the amount of proteins and other grape solids in the juice, and settling enzymes are also used to help clarify wine.

Copper fining removes unpleasant thiols, or mercaptans, that cause reductive aromas (rotten eggs, onion, garlic, skunk) in wine. As discussed previously, these aromas originate from unhealthy yeast during fermentation and aging. Historically, wines were exposed to cellar equipment made of copper that would reduce these aromas. Wines exhibiting reductive thiol character may be fined using copper sulfate, which reacts chemically with thiols to form insoluble compounds that settle out of the wine. Many countries have a legal limit for the amount of copper retained in wine (0.5 and 1 milligrams per liter in the United States and EU, respectively). Copper residue from vineyard treatments or contact with copper equipment in the winery also contributes to a wine’s copper content. Excess copper can increase the rate of oxidation and reduce varietal thiols, and high levels can cause a haze or red-brown precipitate in bottled white wines known as a copper casse.


Filtration clarifies wine or prevents microbial spoilage by physically removing yeast and bacteria (referred to as sterile filtration). While filtration is not necessary for all wines, when used properly, it can improve taste and stability. Filtration is one of the more technical aspects of winemaking, and handled incorrectly, it can damage the wine. Most wines with residual sugar or malic acid are sterile filtered to prevent refermentation in bottle.

The many types of filtration used in winemaking can be broadly classified as depth or surface filtration methods. In depth filtration, the wine is passed through a relatively thick filter media often made of diatomaceous earth (DE or Kieselguhr) and/or cellulose (paper). The filter media forms a sort of maze, and as the wine passes through, larger particles are trapped. Depth filtration is a nominal method, which means that the filter rating describes the filter’s average pore size. Some larger particles will also make their way through. Depth filtration is useful for wine that contains a high amount of solids, as it is less prone to clogging than surface filtration methods. Both surface and depth filter media are available in a range of pore sizes, from 0.2 to 10 microns.

While conceptually similar, there are several depth filtration configurations:

  • Pad filtration: Wine is filtered through preformed paper pads that may contain DE. This is easy to operate and easy to scale, since pads may be added or subtracted. It can leave a paper taste in the wine if pads are not properly rinsed prior to use.
  • Lenticular: Similar to pad filtration, but the wine is passed through a preformed paper and DE cylindrical cartridge. This method is easy to operate, and the equipment has a small footprint, but it can be slow and expensive to scale.
  • Plate and frame: A support screen is coated with DE, and wine that has been mixed with DE is passed through the screen. Because the filter media is constantly replenished, this is a good technique for wines with a large amount of solids. The initial setup is expensive, and it is messy to operate. This method is typically used by large wineries.
  • Rotary drum: Similar to plate and frame, but the support screen is cylindrical. The process can be very oxidative. It is typically used on heavy press wine or lees.

In surface filtration, wine is passed through a perforated plastic membrane with a uniform pore size. Surface filtration techniques are considered absolute, which means that they filter out absolutely all of the particles larger than the filter rating. Absolute filters clog easily, and pre-filtration using a depth method is typical to avoid rapidly plugging the filter. Surface filtration is often used to treat a wine on the bottling line. Common ratings are 0.45 microns (sterile) and 10 microns (light filtration or “bug catcher”). A pore size of 0.45 microns or smaller is used in sterile filtration to ensure that all yeast and bacteria are removed. Sterile filtration requires an absolute filtration method, since nominal methods still allow some larger particles through.

With conventional filtration, the flow of the wine runs perpendicular to the filter surface, which can result in rapid clogging. Crossflow is a unique type of surface filtration in which the flow of the wine runs parallel to the filter surface, making it less likely to clog. That said, the wine is still often pre-filtered to avoid clogging. Crossflow is the most technical and expensive of the filtration techniques described, and many small wineries outsource the task to an expert. While it is often used for sterile filtration, crossflow is technically a nominal filtration method.

Many winemakers have filtration preferences, but there is little consensus on any particular technique being better than another. The choice of filter is often based on the goal of filtration (clarification versus sterile) and practical considerations such as throughput, cost, frequency of use, ease of setup, and waste minimization.

Reverse osmosis is a niche filtration technique that removes a number of components including water, alcohol, and volatile acidity. Essentially, wine is passed through a very selective filter, which only allows through the smallest molecules, including water, alcohol, and some organic acids. Compounds pass across the filter through osmosis, or following a concentration gradient. Most of the wine does not pass through the filter, and the liquid that does is called permeate. The permeate may be discarded, or it may be treated to remove only a single chemical (such as ethanol) and then added back to the wine. Often, a small portion of wine is treated to a greater extent and added back to the main portion of wine to avoid overtreatment.


Stabilization is the process of ensuring that a wine won’t change unpredictably once in bottle. Winemakers seek to avoid faults that would cause the wine to become turbid or carbonated or to develop off-flavors or sediment. By removing the cause of potential problems prior to bottling, the winemaker ensures the soundness and longevity of the wine. Physical instability occurs when changes in temperature, pH, and alcohol content reduce the solubility of compounds in the wine. Microbial instability refers to the reactivation of yeast or bacteria once in bottle. Wine is stabilized after blending and just prior to bottling. While not the most glamorous topic, stabilization is an important aspect of commercial-scale winemaking, as the process provides assurance for the winemaker and pleasure for the consumer.

Cold Stabilization

When wine is chilled, the solubility of tartrate salts in the wine decreases, forcing them to precipitate out of solution. In many wines, potassium bitartrate and, to a lesser extent, calcium bitartrate, known colloquially as tartrates, are important examples of this phenomenon. Often, a wine is saturated, or at maximum capacity, with these tartrate salts. When the temperature is reduced, the tartrates become less soluble and form crystals.

Cold stability is often considered necessary since wine is frequently chilled in the refrigerator to below cellar temperature. While these salts are completely harmless, they are considered a cosmetic defect by some consumers. Less-experienced consumers may confuse the salts for glass, so this is particularly relevant for lower price point wines. Cold stabilization reduces the likelihood of crystals forming in the bottle. While it is standard for most white wines, red wines are less likely to be cold-stabilized, because phenolic compounds increase the solubility of tartrates.

Several techniques are used. The easiest of these is refrigeration, which involves holding the wine at very low temperature for several weeks. Afterward, the wine is racked, leaving the tartrates behind. Potassium bitartrate crystals may be added to “seed” or hasten the crystallization process. (Crystals form more quickly on a crystalline surface.) Cold stabilization is a simple process, but inefficient from an energy standpoint. Energy-efficient methods include ion exchange and electrodialysis, which remove potassium and tartrate salts, respectively, from the wine. There are also wine additives (including gum arabic, metatartaric acid, and mannoproteins) aimed at inhibiting crystallization.

Interestingly, cold stabilization using refrigeration alters the acidity of a wine. The removal of tartrate salts necessarily decreases a wine’s titratable acidity. The effect on pH depends on the starting value: for wines with an initial pH above 3.8, the pH increases, and vice versa for wine with pH below 3.8.

Microbial Stability

In wine, yeast and bacteria can be identified and quantified using microscopy, plating, or genetic testing (which uses PCR, or the polymerase chain reaction). The standard test used in the American wine industry to identify Brettanomyces or spoilage bacteria, as well as other wine microbes, is referred to as a Scorpion.

Most techniques used to make a wine microbially stable do not attempt to sterilize the wine but simply remove most of the microbial load. Beyond filtration, several products can be used to prevent microbial spoilage. SO2 is the most fundamental anti-microbial wine additive, but there are others, particularly for winemakers who prefer to limit their use of SO2. It’s important to note that none of these have the antioxidant powers of sulfur dioxide.

  • Lysozyme is an enzyme derived from egg whites that can be added to wine to prevent malolactic fermentation or to destroy lactic acid bacteria. Lysozyme will not protect against yeast or acetic acid bacteria.
  • Chitosan (No Brett Inside) is a positively charged fining agent derived from the exoskeleton of crustaceans that is used to remove yeast, including Brettanomyces.
  • Sorbic acid inhibits the growth of yeast, but lactic acid bacteria can convert it to a floral-scented compound responsible for so-called geranium taint.
  • Dimethyl decarbonate (DMDC), commonly known as Velcorin, kills yeasts and bacteria very effectively and can be added to wine on the bottling line as an alternative to filtration or higher levels of SO2. DMDC is very toxic to humans, but once it has been added to wine, it breaks down rapidly into harmless compounds. DMDC is used in sports drinks, juices, and other packaged beverages, but its use is controversial within the wine industry.
  • Pasteurization, or heating to high temperature, is another option for wine sterilization, but high heat has other deleterious impacts on wine and is not suitable for use in quality wine production. Pasteurization and other high heat methods occur at bottling.

While a number of products are available to avoid microbial spoilage, in practice, most wineries use SO2 and often filtration to stabilize their wines prior to bottling.

Closure & Packaging

Bottling requires dry goods, including glass, closures, capsules, and labels. A bottle’s closure impacts the wine’s taste and how it ages, and for this reason, the choice of closure is an important winemaking decision. The major closures used in wine production include traditional, technical (such as DIAM), and synthetic (such as Nomacorc) corks and screwcaps. There are several other novel closures, but none are commercially significant. When evaluating the options for closure, a few critical differences emerge. For winemakers, the major considerations when choosing a closure include flavor addition and subtraction, oxygen addition, and bottle-to-bottle consistency.

Cork is a piece of tree bark, and much like oak barrels add flavors to wine, cork contributes flavors. The most notorious of these is trichloroanisole (TCA), which is produced by certain molds in the presence of chlorine-containing compounds. A high incidence of cork taint, once estimated as high as 10%, led the industry to seek alternative closures. TCA-free natural corks are now available for a premium, and in time, much of cork production will likely include this processing step.

In the search for a perfect closure, new observations were made regarding a closure’s impact on the wine inside the bottle. Similar to barrel aging, slow oxidation results from oxygen that permeates a closure. The rate of oxygen ingress (also called OTR, or oxygen transmission rate) helps determine how a wine will age in bottle, where cork is the standard for aging expectations. Initially, screw caps were tin lined and had a very low OTR relative to cork. This caused some issues with increased incidence of reduction in bottle, largely remedied once winemakers adjusted their pre-bottling protocols. Today, screwcaps and synthetic corks are available in a variety of different custom OTRs, including ones that approximate the OTR estimated for an average cork.

The relationship between closure and oxygen exposure is a bit more complex than OTR alone. There are several mechanisms whereby oxygen is added to wine that are closure dependent. At bottling, there is some oxygen in the headspace above the wine. Because screwcaps have a larger headspace and because of differences at bottling, they contribute more oxygen than the other closures. During bottling, a cork is compressed in order to insert it into the neck of the bottle. Over the first few years of a wine’s life in bottle, oxygen from within the compressed cork is forced into the wine. This same phenomenon is absent for screwcaps. Over time, oxygen ingress is dependent only on the permeability of the closure or OTR.

Recently, researchers have suggested that oxygen does not actually pass through the cork during wine aging, as was once believed. After a period of two or three years of bottle aging under cork, little additional oxygen appears to enter the bottle, provided that it is stored horizontally to prevent the cork from drying out. This is an interesting discovery that defies a long-held assumption and indicates an interesting behavior that may be difficult to replicate with another type of closure for wines intended for long aging.

Consistency between bottles is another important consideration. Cork is prone to variation, as it is a piece of bark, and for wines where consistency is desired, it may not be ideal. In general, screwcaps and synthetic closures result in greater bottle-to-bottle consistency. However, bottling under screwcap is much more technical than the alternatives, and operator error can cause screwcaps to be even more prone to variation than cork.

Since the 1990s, much has been learned about the nature of closures, and the quality and consistency for all closures is better than ever before. The discussion around alternative closures began with TCA, and new discussions are circling back to flavors contributed or removed by the closure. Just as cork can contribute TCA, it may also add pleasant woody flavors to a wine. This may be a welcome contribution to certain red wines, but for some light-bodied reds and many white wines, it might detract from the wine. Similarly, synthetic corks have been said to add plastic flavors to wine, while both synthetics and screwcaps are said to scalp, or absorb, some flavors. The industry still has more to learn about the interaction between closures and the compounds in wine.

While closure is an important winemaking decision, marketing factors are equally compelling. Closure choice should consider wine style, price, and intended aging. It may also be promulgated by law. Champagne, for example, must be closed under cork. There is not one perfect closure, and the best choice for a given wine depends on winemaking considerations and the audience.

Glass bottles are an almost ideal storage vessel for wine. They are inert and suitable for decades of aging, but they are relatively expensive and, because of their weight, less environmentally friendly than some alternatives. Marketing and cost considerations might prompt other packaging, including canning, bag-in-box, tetra paks, and even plastic bottles. Most plastics have higher oxygen transmission and lead to flavor loss and addition. Canned wine can be prone to oxidation and reduction. The shelf life for these alternatives is typically less than a year. While they have important markets, from a wine quality perspective, none of them compare to glass.


While many other steps in the winemaking process don’t require modern technology, modern bottling truly preserves wine quality. Bottling is one of the most technical aspects of the winemaking operation, and bottling lines are expensive to own and operate. Many small wineries outsource their bottling to trucks or facilities that specialize in maintaining the intricacies of the process and operating the bottling line. Others prefer to bottle in-house to ensure that they have full control over the timing and process.

Just prior to bottling, there are a number of adjustments that a winemaker might make to ensure that wine goes to bottle in the best possible condition. The wine should be kept cool, but if it is too cold, condensation will form on the bottles and labels will not stick. The levels of gases dissolved in the wine may be adjusted. Oxygen is minimized, and carbon dioxide is adjusted to a desired level. Many red wines contain low levels of CO2, as the wine should not feel carbonated, but slightly elevated levels can increase the sensation of freshness or acidity in white and light red wines. 

Wine is vulnerable during bottling. A leaky hose can cause the wine to go to bottle with a higher level of oxygen than desired, causing unwanted oxidation. Sanitation is supremely important, since contamination at this point can cause major issues. Once a wine is in bottle, the winemaker has very little control, and trying to fix problems that arise at that point is tremendously difficult and can result in significant economic loss.

Bottling lines consist of a bottle rinser, a bottle filler, a corker, and a labeler. Modern bottling lines displace oxygen from the bottle prior to filling and have a mechanism for removing oxygen from the headspace prior to applying the closure. Bottling lines operate at high speed, and the tolerances are very small. Even a slight deviation in the shape of a bottle can cause it to shatter on the bottling line. Successful bottling relies on tracking many details simultaneously. It’s common, for example, to monitor the dissolved oxygen in the wine, fill heights, and label placement.

Most wines shut down aromatically just after bottling, which is known as bottle shock. Wineries may bottle age their wines for a period of time to avoid sending them to market before they are ready. Bottles may be labeled on bottling day or stored as “shiners” and labeled just prior to fulfillment. In the US, wines are generally labeled on the same line where they are bottled. In many European regions, however, it is typical for wines to be bottled, aged as shiners, and labeled prior to sale.

One of the most rewarding aspects of winemaking, bottling represents the cumulation of many months of work. Once the wine is in bottle, the job of the winemaker is finished.


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Special thanks to Caroline Hoogenboom of Tonnellerie Cooperage.

Compiled by Jennifer Angelosante (December 2019)

Edited by Stacy Ladenburger