A Guide to Wine Phenolics

In a 1991 segment of 60 Minutes, America was introduced to the now well-known “French Paradox,” the observation that the French have low rates of heart disease despite their rich diet. At the end of the show, with glass in hand, host Morley Safer left the audience with an enticing message: “The answer to the riddle, the explanation of the paradox, may lie in this inviting glass.”

This short segment is credited with a 39% increase in red wine sales the following year and a steady increase in wine consumption in the United States that continued until recently. While Americans were busy drinking more wine, a number of studies attempted to link wine compounds to wellness and longevity. Polyphenols, a promising class of plant-derived antioxidants responsible for red wine color and mouthfeel, became the focus of much of this attention. Resveratrol, the celebrity wine phenol, is the most familiar of these. Outside of winemaking, discussion of polyphenols is often motivated by their purported health benefits. While resveratrol’s virtues are frequently overstated, interest in wine phenolics has grown into one of the most relevant and dynamic topics of research in winemaking today.

It is well accepted that the quality and the ageability of red wine are driven by its phenolic compounds. A number of studies have demonstrated that consumer preference, cost, and score are tied to the amount of phenolic material a wine possesses. From a winemaking perspective, better tannin and color management is generally considered the most impactful way to improve wine quality and longevity. Several companies, including WineXRay and Enologix, have formed specifically to advise winemakers on optimizing the phenolic profile of their wines.

The sensory experience of wine is significantly influenced by its phenolic composition, and polyphenols play an essential role in shaping a wine’s identity. Color density and hue, tannin level, and texture are all determined by phenolics. A wine’s phenolic signature is so central to its identity that some scientists have looked into using it to authenticate variety and origin. While this has been moderately successful for fresh grapes, accounting for differences that arise during winemaking and storage has made it difficult to apply the method to wine.

Over the last 25 years, deciphering the puzzling world of wine phenolics in the winery and in the vineyard has been a major objective of wine scientists. Our understanding is constantly evolving­­­—the information presented in this article, while current, is subject to change in the coming months and years. The study of phenolic compounds can be daunting, fraught with complicated nomenclature and a dizzying array of chemical reactions that challenge even seasoned chemists. But this is an exciting area of research, as knowledge is constantly expanding, and while the topic might seem niche, an understanding of key phenols helps elucidate their impact in the glass.

Phenolics 101

Polyphenols (phenols, phenolics) describes a diverse group of compounds that share a phenol ring incorporated into their chemical structure. Phenolic compounds are ubiquitous throughout the plant kingdom, where they serve many functions: they protect from predation, disease, and ultraviolet radiation; attract pollinators; and provide pigmentation.

A few important groups of phenols in wine are anthocyanins, flavan-3-ols, and their derivatives, which include tannins and most wine pigments. The majority of phenols in wine are grape derived, though small amounts may be contributed by oak or additives. Thanks to their unique chemical properties, in addition to giving wine its color and structure, phenolic compounds are crucial to a number of chemical reactions, like oxidation, that are important for a wine’s development.

At a high level, the phenols in grapes and wine are categorized into flavonoids and non-flavonoids. Flavonoids share a characteristic three-ring backbone structure and include anthocyanins, flavonols, and flavan-3-ols, and derivatives of these include tannin and wine pigments. Flavonoids come from the skins, seeds, and stems of grapes, so they are most relevant for red grapes. The teinturiers are a notable exception to this rule and contain anthocyanin in their red-colored flesh. The category of non-flavonoids is a catchall for smaller phenolic compounds that lack the three-ring flavonoid structure. The majority of non-flavonoids in wine are from the pulp of the grape, and while they are found in both red and white wine, they are the primary phenolic compounds in white wines.

While in blind tasting the term phenolic is often used to describe white wine, and tannic is preferred for red, it is useful to recognize that both of these sensations result from the same family of compounds. From a winemaking perspective, the discussion of phenolics tends to focus on red wines, which are produced through extraction from skins and seeds and contain more phenols than white wines. A typical red wine contains between one and four grams per liter of these compounds, while the content in white wines is roughly 10% of that.

Anthocyanins: The Color of Youth

Wine color is an important sensory cue; it makes a first impression and sets the expectation for what a wine will taste like. Studies have shown that wine color influences a taster’s impression of flavor. Deeper-colored wines are perceived as more flavorful, sweet, and fruity. The world of red wine encompasses a diverse range of hues, from red to purple when wines are young, with intensity ranging from pale to opaque. As a wine ages, its color becomes more garnet as orange and brown shades develop, and its color intensity diminishes.

Red wine color is mostly determined by anthocyanins and their derivatives. Young wine contains “free” anthocyanins, which are relatively unstable. As wine ages, the anthocyanins bind with tannins and other compounds in wine, forming stable “bound” anthocyanins, known as polymeric pigments.

The word anthocyanin was coined by German pharmacist Ludwig Clamor Marquart and comes from the Greek words antho and kyánōs, meaning “blue flower.” In nature, anthocyanins are responsible for the array of red, purple, blue, and black pigments of many fruits, vegetables, and flowers. They function as enticers, attracting pollinators and seed dispersers to ensure proliferation.

Six anthocyanins—malvidin, cyanidin, delphinidin, petunidin, peonidin, and (rarely) pelargonidin—and their analogues are found in wine. Each of the individual anthocyanin species has a slightly different hue, with some more red and others more purple. Both the total amount of anthocyanin and the proportion of each species play a role in determining wine color, and they vary by grape variety and provenance. Malvidin-3-glucoside is the most abundant species, often accounting for at least half of the anthocyanin content in wine.

Several features of anthocyanins lead to interesting and measurable differences among grape varieties. Anthocyanins are glucosides, which means their chemical structure includes a sugar molecule. Non-vinifera grape species contain anthocyanin diglucosides, or anthocyanins with two sugars attached. Because diglucosides are absent in Vitis vinifera, regulating bodies monitor their levels to ensure that hybrid grapes have not been illegally included in a blend.

“Acylated” anthocyanins have an additional compound, such as acetic acid, which is joined to the sugar molecule. They are more color-stable and soluble than their non-acylated analogues, and they can confer deeper color intensity to the wine. The amount of acylated anthocyanins in a wine is influenced by grape variety and growing conditions. Pinot Noir lacks acylated anthocyanins, while other light-colored grape varieties, including Gamay, Sangiovese, Nebbiolo, and Grenache, contain only a small proportion.

While wine professionals tend to think about anthocyanins as red in color, an approximate 10 to 15% of the anthocyanins in wine are actually pigmented. Free anthocyanin binds with bisulfite, the major form of sulfur dioxide in wine, to form a colorless compound, a phenomenon called bisulfite bleaching. Anyone who has ever added SO2 to grape must can attest to the dramatic bleaching effect the addition has on the color of the juice. While colorless, the anthocyanins are still there, and as SO2 dissipates, the color returns.

Anthocyanin takes different forms depending on the pH, so its color is pH dependent. At low pH (more acidic), anthocyanins are red; at moderate pH, they appear colorless; and at high pH (more basic), they are blue. While the majority of anthocyanins are in their colorless form at wine pH, at a pH of 3.2, wine appears redder, while at a pH of 4.0, it is more purple. The addition of water to a glass containing a small amount of wine reveals the blue color of anthocyanin as pH is increased, demonstrating this behavior. Nonetheless, hue is not a reliable indicator of a wine’s pH, and it should not be relied upon to infer a wine’s level of acidity in blind tasting. For example, when young, Catena Zapata's high-altitude Adrianna Vineyard River Stones Malbec 2016 has a deep purple color, despite its modest pH of 3.55.

The Curious Nature of Hyperchromicity

The nature of wine color is complex. While wine pH and anthocyanin composition play an important role in determining the color of young red wine, color is also influenced by a behavior known as copigmentation or hyperchromicity.

Scientists observed that when they diluted a sample of young red wine, the loss in color intensity was disproportionate, and the diluted wine appeared paler than they had anticipated. This was due to copigmentation, a synergistic effect between anthocyanin and other compounds in the wine that enhances color intensity. Upwards of 30% of the color of young red wine is attributed to copigmentation.

In copigmentation, anthocyanin forms complexes with other phenolic compounds called cofactors. The phenol rings of anthocyanins and their cofactors stack like pancakes and are held together by electrostatic forces. This arrangement is believed to enhance the color of anthocyanin by trapping it in its colored form. Additionally, it has been suggested that tying up some of the anthocyanin into these complexes allows more color to be extracted from the skins.

For copigment complexes to form, a sufficient concentration of anthocyanins and cofactors is required, since the compounds must be physically close enough to attract one another. As wine ages, the pool of free anthocyanins diminishes, and copigmentation complexes break down. After a year or two, copigmentation contributes little to a wine’s color, and the wine becomes redder and less intensely colored. However, some studies have suggested that wines with a high degree of copigmentation in their youth may result in more intensely colored mature wine.

Traditionally, the co-fermentation of red and white grapes has been used to enhance color stability, possibly the result of enhanced copigmentation. Classic examples of this include Syrah and Viognier, as well as Sangiovese with Trebbiano and Malvasia.

Flavonols are a class of flavonoids, and along with many non-flavonoids, they are important cofactors involved in copigmentation. Flavonols are potent antioxidants and are generally considered an important marker of wine quality. Flavonols act as sunscreen against UVA and UVB light, prompting greater production of flavonols in berries exposed to more intense sunlight, especially pre-veraison. Six flavonols are found in red grapes, and interestingly, three of these are absent in white grapes. Quercetin and myricetin are the major flavonols found in red varieties. Quercetin is not very soluble in wine, and as wine ages, it can form a haze or crystalline sediment, an issue sometimes observed in Sangiovese wines.

The Bitter Truth

For most consumers, few qualities are more off-putting in a wine than overly aggressive astringency or bitterness. Many flavor compounds are both bitter and astringent, so people tend to confuse these two sensations. While bitterness is a taste, astringency is a feeling or textural sensation, which has earned it the description of mouthfeel. While the mechanism of astringency is not perfectly understood, it seems that it is the result of tannins binding with mouth proteins, causing a lack of lubrication or dryness resulting in friction. For winemakers, it is useful to understanding the nature of astringency and bitterness in wine, how these properties change over time, and how to manage them to improve wine quality.

A wine’s astringency is determined by the quantity and quality of its tannins. Two types of tannins exist in wine: condensed and hydrolysable. Condensed tannins, also called proanthocyanins, are the most abundant class of phenols in wine, accounting for about 25 to 50%. Red wines typically contain 0.3 to 2.0 grams per liter condensed tannins, while hydrolysable tannin content rarely exceeds 20 milligrams per liter. (Note that comparison of tannin values is difficult, as several methods exist to measure these compounds, and there is not a standard reference.) Because of this disparity, the term tannin is often used to refer to condensed tannins.

Condensed tannins are found in all plants, and their unpleasant flavor functions as a deterrent to animals and insects that try to eat foliage or unripe fruit. The skins, seeds, and stems of grapes are sources of tannin. Grape varieties vary in their overall amounts of tannin, especially seed tannin, and environmental factors also play a role. White grapes have a similar amount of tannin in their skins as red grapes, but because skin contact is minimized, white wines contain very little tannin. Orange wines are made from the maceration of white grapes on their skins, and so they contain elevated levels of tannin.

The word tannin originates from the practice of tanning leather. In traditional vegetable tanning, tannin-rich bark is applied to leather for fortification and to make it water repellent. Many believe it is best to harvest the bark during the spring months, when the concentration of tannin is greatest. Tannin carries a negative charge and binds with positively-charged proteins, a property that is exploited in fining, where a protein such as egg white is used to remove some tannin from a wine.

Tannins are polymers of smaller polyphenols called flavan-3-ols (also known as flavanols—spelled with an a and distinct from flavonols). Tannins can be imagined as a chain composed of flavan-3-ol links. Flavan-3-ols, including catechin, are the major source of bitterness in red wines and are found primarily in seeds. Since seed extraction is generally minimized during winemaking, levels of catechin are often relatively low compared with other polyphenols. Red wines typically contain from 10 to 400 milligrams per liter.

Astringency is influenced by a wine’s overall amount of tannin as well as the nature and origin of those tannins. While wines that contain more tannins will be perceived as more astringent, other factors including tannin length and structure also impact the perception of astringency. Tannins are associated with a wide range of textures, including fine-grained, powdery, grippy, coarse, and drying. Other components in the wine can also influence the impact of tannin. For example, alcohol reduces the perception of astringency, while acidity enhance it.

Wine contains a mixture of tannins with different lengths, ranging from 2 to more than 30 flavan-3-ol units. A collection of tannins is sometimes described by the average length of its constituents, referred to as the mean degree of polymerization (mDP). Larger tannins are more astringent, likely since they have more binding sites to interact with proteins and are thus more grippy than shorter tannins. Smaller tannins and flavan-3-ols are more bitter.

Flavan-3-ols include several analogues that seem to impart different sensory characteristics. They can be galloylated, which means they have a gallic acid unit attached to them. An example of this is catechin gallate. Galloylated flavan-3-ols have been described as more hard, coarse, and drying compared to catechin. Additionally, flavan-3-ols are subdivided into catechins and gallocatechins, where gallocatechins are described as more viscous and velvety.

So, whether a wine has more gallocatechin (velvety, viscous, ripe) or catechin gallate (rough, coarse, unripe) incorporated into its tannins seems to make a big difference in terms of perception. While these definitions sound complex, tasting experience provides a clear example. Skin tannins are longer than seed tannins and are perceived as riper and more astringent, while seed tannins are more bitter. And indeed, seed tannins have more galloylated (coarse, drying) subunits while skin tannins contain more gallocatechins (velvety, viscous). In winemaking, skin tannins have long been preferred to seed tannins, since the former are considered to be more pleasant, while seed tannins are harsher. Ripe fruit from warmer regions and in warmer vintages is often driven by skin tannins, while fruit from cooler regions and in cooler vintages may be more driven by seed tannins.

Evolution During Aging

As a red wine matures, its color intensity fades, and the hue shifts from purple to orange. The wine’s astringency softens, and tannins seem to become finer and more integrated. Research suggests that anthocyanins are playing a critical role here as well. Beginning during fermentation, and continuing throughout wine aging, anthocyanins bind with tannins, creating polymeric pigments (also called pigmented tannins). These are the predominant pigments responsible for wine color after 6 to 24 months of aging and result in a brick red color. Not all of the polymeric pigments are actually pigmented. Some color loss occurs, but it is less significant than for free anthocyanin, and polymeric pigments are not subject to sulfite bleaching.

A wine’s phenolic profile is influenced by two important chemical reactions. The first of these is hydrolysis, which refers to the breaking of bonds that hold subunits together. The other is condensation, which refers to the formation of new bonds. Within a tannin chain, condensation and hydrolysis reactions occur frequently, and bonds are regularly breaking, with new bonds reforming. Tannins may bind to anthocyanins, or they may bind to other tannins. As a wine ages, this leads to a huge diversity of compounds.

When anthocyanin binds to tannin, it essentially makes a cap on one end of the chain that prevents further polymerization. As more anthocyanins are incorporated with tannins, this has the overall effect of reducing tannin length. The formation of polymeric pigments is believed to be responsible for a wine’s mouthfeel and tannin structure improving as a wine ages. Wine with a large concentration of pigment is associated with a chalky mouthfeel, softer tannins, and a sense of fullness on the midpalate.

Anthocyanin also binds other compounds in wine. Notably, it binds with acetaldehyde and pyruvate, compounds created by yeast during fermentation, to form a special class of polymeric pigments called pyranoanthocyanins. Because malolactic fermentation removes acetaldehyde and pyruvate, some winemakers intentionally delay malolactic fermentation to encourage these stable color compounds to form. Pyranoanthocyanins are not subject to bleaching and range in color from red-orange to yellow to blue. While wine contains relatively low concentrations of these compounds, they are believed to be important contributors to the color of aged wine.

Other noteworthy changes occur during aging. As anthocyanins are incorporated into tannins, copigmentation complexes break down, reducing the pool of purple pigments. There is a slow reduction in the overall amount of tannin as some settles out of the wine. Many believe that tannin helps a wine age, yet this can be overdone and result in unpalatable wines that don’t resolve over time. While the source of ageability is multifaceted, it is likely that the amount of tannin and anthocyanin—and perhaps more importantly, the amount of anthocyanin-rich polymeric pigment—plays a key role.


Though the discussion of wine phenolics is focused on flavonoids, non-flavonoids also play a supporting role. As their name suggests, non-flavonoids include most of the small phenolic compounds in wine that are not flavonoids, such as hydroxycinnamates, hydroxybenzoic acids, and stilbenes.

The hydroxycinnamates are found in all plants and include caftaric, coutaric, and fertaric acid. They’re found in grape pulp and are the most abundant non-flavonoids in wine. Hydroxycinnamates act as cofactors in copigmentation and participate in oxidation and juice browning. These are also the most important phenolic compounds in white wines.

Hydroxycinnamates are converted by Brettanomyces and other microorganisms to potent aroma compounds known as volatile phenols. This includes 4-ethylphenol (4-EP, from caffeic acid) and 4-ethylguaiacol (4-EG, from ferulic acid), chemicals responsible for the characteristic Brett-associated aromas of Band-Aid, barnyard, smoke, and spice. Similarly, smoky and spicy compounds from barrels and smoke taint are also included in this category.

The hydroxybenzoic acids include phenolic compounds extracted from barrels as well as fragments formed by the breakdown of larger phenolics during wine aging, including gallic acid (from galloylated seed tannins).

Oak tannins, or hydrolysable tannins, are polymers of non-flavonoid compounds. Barrel aging and the use of oak staves, chips, or powders contribute oak tannins (which otherwise are not present) to wine. A wine aged in new, light-toast French oak barrels contains about 20 milligrams per liter of oak tannins after one year in barrel. New oak contributes roughly twice as much tannin as one-year-old oak, while older oak contributes less. French oak supplies over twice as much tannin as American oak. And, contrary to what some might assume, untoasted and light-toasted oak contribute more tannins than heavier-toasted oak.

For years, scientists believed that the tannin contributed by oak was at too low a level to impact a wine’s mouthfeel or astringency. While many still share this sentiment, recent studies have indicated that oak tannins may increase the perception of astringency, bitterness, and roundness in wine. Oak tannins can polymerize with anthocyanins and condensed tannins to form polymeric pigments during aging, and this may impact a wine’s sensory characteristics.

The stilbenes are developed in the skins of grapes to help combat mildew and other fungal diseases. The most recognizable of these is resveratrol. Though it does appear to have health benefits, these have been exaggerated—one would need to consume over 40 bottles of wine per day in order to get the recommended daily dose!

The Dynamic Processes of Ripening & Fermentation

To most effectively manage a wine’s development, winemakers must understand how tannin and anthocyanin develop and evolve throughout ripening and fermentation. Since the details are often fruit specific, phenolic measurements taken at critical points throughout the production process offer insight and are becoming more commonplace.

One of the most important decisions in winemaking is the timing of harvest. Along with sugar and acidity, the phenolic composition of the fruit changes during ripening. Color accumulates, and the fruit becomes less bitter and astringent in order to attract predators that will eat the fruit and spread its seeds. The term phenolic ripeness has been widely adopted to describe these changes.

During ripening, grapes' polyphenol content increases. Tannins, flavonols, and hydroxycinnamates are synthesized in the berry from fruit set until veraison, which marks an important shift in the plant’s phenolic production. The same assembly line that produced tannins and flavonols earlier in the season begins to produce anthocyanins, which increase throughout ripening. As anthocyanin accumulates, the grapes change color from green to purple. Occasionally, a slight decrease in the concentration of anthocyanin is observed at the end of fruit maturation.

The behavior of tannin during ripening is not as straightforward. There are some general trends, but they are not consistent across all varieties and vineyards. Seed tannin becomes less extractable throughout ripening, while skin tannin becomes more extractable at the end of ripening. As a result, overall tannin concentration decreases during the beginning of the ripening period and then increases again near harvest. Riper fruit often results in more tannic wines, with a higher proportion of tannin derived from skins rather than seeds, and longer tannins on average. The seeds become less bitter as extractable catechin decreases throughout ripening. There is a point of diminishing returns where longer hangtimes sacrifice wine balance without an improvement to color or tannin.

Over the last 20 years, phenolic ripeness has become an increasingly important factor for deciding harvest timing. Today, a growing number of winemakers track its progress analytically. Anthocyanin, tannin, catechin, and polymeric pigments are measured, and winemakers develop their own philosophies regarding the phenolic ripeness parameters that best suit their desired wine style. As an example, to make a ripe and concentrated style of Napa Cabernet Sauvignon, a producer may aim to harvest when anthocyanins are near their maximum concentration, but after catechin has dropped below a predetermined threshold. However, many winemakers continue to monitor ripeness the old-fashioned way. Through tasting, they assess extractable tannin and catechin levels from the texture, bitterness, and astringency of grape skins and seeds. Some look at the color of their spit after chewing on the skins as a means of measuring color.

During fermentation, anthocyanins begin extracting right away, and their concentration typically reaches a maximum after three to five days on the skins. Afterward, the concentration of free anthocyanin declines steadily for the remainder of the life of the wine as it’s converted to polymeric pigment. Skin tannin begins extracting immediately during fermentation, while seed tannins don’t extract significantly until the latter half of fermentation. For this reason, a longer skin maceration often results in a greater influence of seed-derived tannin. Tannin has an affinity for the proteins and polysaccharides in the berry skins, and as tannin is extracted, some is re-absorbed onto the skins.

Beginning almost immediately after crushing, anthocyanins bind with skin tannin, forming polymeric pigments, and by the end of fermentation, 25 to 50% of the anthocyanin may already be incorporated in polymeric pigments. As tannin binds with anthocyanin, more is extracted from the skins, so elevated levels of color can actually increase the amount of tannin in a wine.

The concentration of phenolics is highest at pressing and declines slowly as the wine ages. Press wine is rich in polyphenols. Winemakers seek to drain the wine off of its skins when they have reached their desired level of tannin. Because this may occur before fermentation is complete, sugar can mask the level of tannin. This is an illustrative example of the utility of phenolic measurements, since tasting can be deceptive.

The Role of the Winemaker

While an increased focus on the phenolic compounds has sometimes been implicated in encouraging riper fruit and heavier extraction, understanding these dynamics can help a winemaker achieve a desired wine style, whether delicate or concentrated. While some tannin is considered necessary for ageability, wine quality is also driven by the balance of color and tannin. Though winemaking goals vary, most quality-oriented producers seek a balance between drinkability and long-term aging potential.

During fermentation, winemakers capture anthocyanin and tannin from the fruit through extraction. Warmer temperatures and more frequent cap management, including punchdowns and pumpovers, result in a higher concentration of color and tannin. By adjusting the intensity of extraction based on the progress of fermentation, winemakers can select for different phenolic compounds. For example, heavier extraction early in fermentation favors skin tannin and color, while extraction later results in more seed tannin and catechin. Extraction from the skins comes easily in some vintages, while more work is required to obtain the desired level of extraction in others. Winemakers monitor the first few fermentations closely to understand what is needed in a given year, and enzymes may be used to increase the overall rate of extraction.

Other extraction practices, such as cold soak and extended maceration, have more mixed results. A cold soak is a low-temperature, pre-fermentation maceration on the skins. In practice, cold soaking can be done at different temperatures and for different durations. Some will intentionally hold the grape must at 40 to 60 degrees Fahrenheit for a period of roughly 3 to 14 days, while others will achieve a similar effect by allowing cold fruit to slowly warm up naturally. Some producers look to cold soaking as a way to achieve deeper color intensity while limiting seed tannin extraction. Among Pinot Noir producers, cold soaking is commonly used because the variety is prone to rapid fermentation and has particularly tannic seeds. For many wines, including some Pinot Noir, the practice will not enhance color intensity since anthocyanin is fully extracted within a few days. Yet a longer pre-fermentation maceration gives naturally occurring grape enzymes time to break down the skins prior to fermentation, which may allow for gentler handling during fermentation. Furthermore, cold soak has other benefits beyond color.

In extended maceration, wine remains on the skins after alcoholic fermentation is complete. This results in wines with a higher concentration of polymeric pigments, tannins, and catechins. While extended maceration may be used for any variety, it is often practiced with Nebbiolo in Barolo to produce ageworthy wines. In some cases, extended maceration results in plush, silky wines, while in others, bitter, seed-derived tannins dominate the flavor profile.

After fermentation, several factors influence the formation of polymeric pigments. Slow oxygen exposure increases the rate of polymerization, so barrel aging, microoxygenation, and racking can be used to stabilize color and soften tannins. Warmer temperatures also increase the rate of polymerization, and wines may be intentionally warmed to 80 to 90 degrees Fahrenheit after alcoholic fermentation to promote the formation of polymeric pigments. Wines aged under warm or oxidative conditions may have a more mature color and tannin profile.

Finally, it’s worth noting that both color and tannins may be added to wine. Tannin additives (also called exogenous or enological tannin) can be produced from hydrolysable or condensed tannin. Recommended doses do not actually appear to contribute much tannin to the wine. While the addition of acid, sugar, or water closely mimics nature, phenolic chemistry is complicated, and these additions cannot reflect the nuances imparted by genetics or terroir.

Back to the Land

The amount of polyphenols in a wine is limited by fruit composition, so many winemakers also look to the vineyard to improve the phenolic composition of their wines. The idea that wine is made in the vineyard is validated by the complexity of phenolics and their importance as quality parameters: it is largely these compounds that differentiate a great vineyard from a mediocre one. While farming is clearly important, it is more difficult to influence phenolic content in the vineyard than in the winery.

The production of phenolic compounds is driven by genetics. Grape varieties differ in their overall amounts of tannin, anthocyanin, and other compounds, as well of the ratio of particular species within each of these categories. The genome of Tannat, a cultivar that has exceptionally high levels of phenolics, includes a larger number of genes that prompt flavonoid production than most other varieties. This sort of genetic variability drives diversity among grape varieties. Some winemakers look to blending to compensate for deficiencies in their fruit—for example, blending a tannin-rich Left Bank Cabernet with a color-rich Petit Verdot.

Excessive yields can result in lower levels of color and tannin; however, aggressively reducing yields will not increase the concentration of these compounds. Because polyphenol levels are driven by genetics, the plant compensates by adjusting phenol production within a reasonable range of yields. The growing environment also significantly impacts the production—and the degradation—of phenols. In general, seed phenols seem much less impacted by environmental conditions than those in the skins.

While sunlight and heat both impact grape polyphenol concentration, the details are not fully understood. Flavonol concentration is greatly increased by pre-veraison light exposure. But the effects of sunlight and heat on the other polyphenols are confounding, since light seems to stimulate polyphenol production, while heat can reduce concentrations through degradation. Moderate sun exposure is generally considered beneficial for red wine quality, and red grapes are often farmed with this in mind. Trellis systems and practices such as removing leaves to allow dappled light to reach the fruit zone may help increase the levels of phenolics in the grapes.

Maintaining a slight deficit in the vine’s water status, especially before veraison, seems to induce the plant to produce more skin tannins. Similarly, excess nitrogen prompts the plant to stop producing polyphenols. Many producers who choose to irrigate adopt a deficit irrigation strategy to increase the overall amount of polyphenols in the fruit, and nutrient levels are carefully managed.

Wines made from grapes grown at high elevation tend to be rich in polyphenols compared with their lower-elevation counterparts and provide an interesting example of the growing environment’s impact. High-elevation grapes receive more sunshine, both in terms of intensity and duration. The light is more concentrated at high elevation; for every 1,000-foot increase in elevation, there is a 2% increase in ultraviolet light exposure. Cooler average temperatures prolong the ripening period, so tannins and color are riper at lower sugar levels. Hillside vineyards also tend to have less water availability, which contributes to their higher polyphenol production. These observations can be extended more broadly: dry, sunny climates and longer hangtimes typically result in wines with elevated levels of color and tannin.

Continued Exploration

The color and phenolic structure of a wine tell a story, albeit a cryptic one. They give us clues about the origin of the wine—grape variety, environment, and winemaking each leave their mark. They can also tell us where the wine is going, since the phenolic profile of ageworthy wines is distinct from that of wines intended for early drinking. While much remains unknown, scientists are slowly untangling the web, deconstructing processes into their components, and trying to put together the pieces of the puzzle. While future research will offer more in-depth understanding, the behavior of phenolics is one of wine’s great mysteries—it’s likely some questions will always remain.


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  • Brava, Jennifer! Great article. Lots of useful information. 

  • Let me clarify  'asexual' propagation.  Asexual propagation is taking a part of the host plant and grafting it onto another host (rootstock).  This ensures that the exact genetic material is transferred to the new plant.  This is how all grape vine nurseries operate.  'Sexual' propagation would be taking the seed and using that to create a new vine.  The issue, though, is that the seed has the DNA from both parents hence it is not the same (genetically) as either parent.  The vast majority of wine cultivars are 'hermaphroditic' meaning the vines have both male and female (both parents) and therefore 'self pollinate.'  With pollination comes the seed and with the seed(s) come the grape.

  • Good question. But in the absence of a pollen vector (grape flowers self-pollinate) or 'Asexual Propagation'  the need for insects is basically '0' as compared to other fruit species like almonds, citrus, apple, etc.  So most insect species would be 'feeding' on the grape itself or leaf tissue and NOT feeding on it, and by the physical act of introducing pollen to the flower.  To the specific light wavelengths for insects, realize that at absorbance of the non-colored phenolic compounds (280mn) in the UV region is very different from where 'we' see color (520nm) in the 'visible region.'  To the Somm audience I pose a question, " is there a relationship between 'self-pollenating' and 'vegetative propagation?"

  • Scott, is there a spectrum of light, UV or IR that is predominant in the colors?   I believe many insects who would eat the grapes see in those spectrums and perhaps there is a genetic correlation as either an attractant or visual repellent?