Polyphenols in wine

During the growth cycle of the grapevine, sunlight will increase the concentration of phenolics in the grape berries with the development of phenolics being an important component of canopy management. Most phenols are classified as secondary metabolites and are not active in the primary metabolism and function of the grapevine. They are water soluble and will often secrete into the vacuole of grape berries as glycosides. In winemaking, the process of maceration or "skin contact" is used to increase the influence of phenols in wine. Phenolic acids are found in the pulp or juice of the wine and can be commonly found in white wines which usually doesn't go through a maceration period. The process of oak aging can also introduce phenolic compounds to wine, most notably in the form of vanillin which adds vanilla aroma to wines.^[1]

Grape polyphenols

Grape skins contain significant amounts of flavonoids as well as other polyphenols.^[4] Both red and white wine contain flavonoids; however, since red wine is produced by fermentation in the presence of the grape skins, red wine has been observed to contain higher levels of flavonoids, and other polyphenolics such as resveratrol.

Flavonoids

In red wine, up to 90% of the wine's phenolic content fall under the classification of flavonoids. These phenols, mainly derived from the stems, seeds and skins are often leeched out of the grape during the maceration period of winemaking. The amount of phenols leeched is known as extraction. They contribute to the astringency, color and mouthfeel of the wine. In white wines the number of flavonoids is reduced due to less skin contact that they receive in winemaking. Within the flavonoid category is a subcategory known as flavonols, which includes the yellow pigment inducing phenol-quercetin. Like other flavonoids, the concentration of flavonols in the grape berries increases as they are exposed to sunlight. Some viticulturalists will use measurement of flavonols like quercetin as an indication of a vineyard's sun exposure and the effectiveness of canopy management techniques. There is on going study in the health benefits of wine derived from the antioxidant and chemopreventive properties of flavonoids.^[5]

Anthocyanins

Anthocyanins are phenolic compounds found throughout the plant kingdom, being frequently responsible for the blue to red colors found in flowers, fruits and leaves. In wine grapes, they develop during the stage of veraison when the skin of red wine grapes change color from green to shading from red to black. As the sugars in the grape increase during ripening, so does the concentration of anthocyanins. In most grapes anthocyanins are found only in the outer cell layers of the skin, leaving the grape juice inside to be virtually colorless. Therefore to get color pigmentation in the wine, the fermenting must needs to be in contact with the grape skins in order to extract the anthocyanins. For this reason, white wine can be from red wine grapes as is the case for many white sparkling wines which are often made from the red wine grapes of Pinot noir and Pinot meunier. The exception to this is the small class of grapes known as teinturiers, such as Alicante Bouschet, which has a small amount of anthocyanins in the pulp which produces pigmented juice.^[6]

There are several types of anthocyanins found in wine grapes which are responsible for the vast range of coloring found in wine grapes from ruby red to dark black. Ampelographers can use this observation to assist in the identification of different grape varieties. The European vine family Vitis Vinifera is characterized with anthocyanins that are composed of only one molecule of glucose while non-Vinifera vines such as hybrids and the American Vitis labrusca will have anthocyanins with two molecules. In the mid-20th century, French ampelographers used this knowledge to test the various vine varieties throughout France to identify which vineyards still contained non-Vinifera plantings.^[6]

The color variation in the finished red wine is partly derived from the ionization of anthocyanin pigments caused by the acidity of the wine. In this case, the three types of anthocyanin pigments are red, blue and colorless with the concentration of those various pigments dictating the color of the wine. A wine with low pH (and such greater acidity) will have a higher occurrence of ionized anthocyanins which will increase the amount of bright red pigments. Wines with a higher pH will have a higher concentration of blue and colorless pigments. As the wine ages, anthocyanins will react with other acids and compounds in wines such as tannins, pyruvic acid and acetaldehyde which will change the color of the wine, causing it to develop more "brick red" hues. These molecules will link up to create polymers that eventually exceed their solubility and become sediment at the bottom of wine bottles.^[6] Pyranoanthocyanins are chemical compounds formed in red wines by yeast during fermentation processes^[7] or during controlled oxygenation processes^[8] during the aging of wine.^[9]

Tannins

Tannins refer to the diverse group of chemical compounds in wine that can affect the color, aging ability and texture of the wine. While tannins can not be smelt or tasted, they can be perceived during wine tasting by the tactile drying sensation and sense of bitterness that they can leave in the mouth. This is due to the tendency of tannins to react with proteins, such as the ones found in saliva. In food and wine pairing, foods that are high in proteins (such as red meat) are often paired with tannic wines to minimize the astringency of tannins. However, many wine drinkers find the perception of tannins to be a positive trait-especially as it relates to mouthfeel. The management of tannins in the winemaking process is a key component in the resulting quality of the wine.^[10]

Tannins are found in the skin, stems and seeds of wine grapes but can also be introduced to the wine through the use of oak barrels and chips or with the addition of tannin powder. The natural tannins found in grapes are known as proanthocyanidins due to their ability to release red anthocyanin pigments when they are heated in an acidic solution. Grape extracts are mainly rich in monomers and small oligomers(mean degree of polymerization <8). Grape seed extracts contains three monomers (catechin, epicatechin and epicatechin gallate) and procyanidin oligomers. Grape skin extracts contains four monomers (catechin, epicatechin, gallocatechin and epigallocatechin), as well as procyanidins and prodelphinidins oligomers.^[11] The tannins are formed by enzymes during metabolic processes by the grapevine. The amount of tannins found naturally in grapes varies depending variety with Cabernet Sauvignon, Nebbiolo, Syrah and Tannat being 4 of the most tannic grape varieties. The reaction of tannins and anthocyanins with the phenolic compound catechins creates another class of tannins known as pigmented tannins which influences the color of red wine.^[12] Commercial preparations of tannins, known as enological tannins, made from oak wood, grape seed and skin, plant gall, chestnut, quebracho, gambier^[13] and myrobalan fruits,^[14] can be added at different stages of the wine production to improve color durability. The tannins derived from oak influence are known as "hydrolysable tannins" being created from the ellagic and gallic acid found in the wood.^[10]

In the vineyards, there is also a growing distinction being made between "ripe" and "unripe" tannins present in the grape. This "physiological ripeness", which is roughly determined by tasting the grapes off the vines, is being used along with sugar levels as a determination of when to the harvest. The idea is that "riper" tannins will taste softer but still impart some of the texture components found favorable in wine. In winemaking, the amount of the time that the must spends in contact with the grape skins, stems and seeds will influence the amount of tannins that are present in the wine with wines subjected to longer maceration period having more tannin extract. Following harvest, stems are normally picked out and discarded prior to fermentation but some winemakers may intentionally leave in a few stems for varieties low in tannins (like Pinot noir) in order to increase the tannic extract in the wine. If there is an excess in the amount of tannins in the wine, winemakers can use various fining agents like albumin, casein and gelatin that can bind to tannins molecule and precipitate them out as sediments. As a wine ages, tannins will form long polymerized chains which come across to a taster as "softer" and less tannic. Oxygen can bind with tannin molecules to make them larger and seem also seem softer on the palate. The winemaking technique of micro-oxygenation and decanting wine use oxygen to partial mimic the effect of aging on tannins.^[10]

A study in wine production and consumption has shown that tannins, in the form of proanthocyanidins, have a beneficial effect on vascular health. The study showed that tannins suppressed production of the peptide responsible for hardening arteries. To support their findings, the study also points out that wines from the regions of southwest France and Sardinia are particularly rich in proanthocyanidins, and that these regions also produce populations with longer life spans.^[15]

Reactions of tannins and anthocyanins with the phenolic compound anthocyanidins creates another class of tannins known as pigmented tannins which influences the color of red wine.^[12] Commercial preparations of tannins, known as enological tannins, made from oak wood, grape seed and skin, plant gall, chestnut, quebracho, gambier^[13] and myrobalan fruits,^[14] can be added at different stages of the wine production to improve color durability.

Effects of tannins on the drinkability and aging potential of wine

Tannins are a natural preservative in wine. Un-aged wines with high tannin content can be less palatable than wines with a lower level of tannins. Tannins can be described as leaving a dry and puckered feeling with a "furriness" in the mouth that can be compared to a stewed tea, which is also very tannic. This effect is particularly profound when drinking tannic wines without the benefit of food.

Many wine lovers see natural tannins (found particularly in varietals such as Cabernet Sauvignon and often accentuated by heavy oak barrel aging) as a sign of potential longevity and ageability. Tannins impart a mouth-puckering astringency when the wine is young but "resolve" (through a chemical process called polymerization) into delicious and complex elements of "bottle bouquet" when the wine is cellared under appropriate temperature conditions, preferably in the range of a constant 55 to 60 F (13 to 16 C).^[16] Such wines mellow and improve with age with the tannic "backbone" helping the wine survive for as long as 40 years or more. In many regions (such as in Bordeaux), tannic grapes such as Cabernet Sauvignon are blended with lower-tannin grapes such as Merlot or Cabernet Franc, diluting the tannic characteristics. White wines and wines that are vinified to be drunk young (for examples, see nouveau wines) typically have lower tannin levels.

Other flavonoids

Catechins are flavonoids that contribute to the construction of various tannins and contribute to the perception of bitterness in wine. They are found in highest concentrations in grape seeds but are also in the skin and stems. Catechins play a role in the microbial defense of the grape berry, being produced in higher concentrations by the grape vines when it is being attacked by grape diseases such as downy mildew. Because of that grape vines in cool, damp climates produce catechins at high levels than vines in dry, hot climates. Together with anthocyanins and tannins they increase the stability of a wines color-meaning that a wine will be able to maintain its coloring for a longer period of time. The amount of catechins present varies amount grape varieties with varietals like Pinot noir having high concentrations while Merlot and especially Syrah have very low levels.^[11] As an antioxidant, there are some studies into the health benefits of moderate consumption of wines high in catechins.^[17]

In red grapes, the main flavonol is on average quercetin, followed by myricetin, kaempferol, laricitrin, isorhamnetin, and syringetin.^[18] In white grapes, the main flavonol is quercetin, followed by kaempferol and isorhamnetin. The delphinidin-like flavonols myricetin, laricitrin, and syringetin are missing in all white varieties, indicating that the enzyme flavonoid 3',5'-hydroxylase is not expressed in white grape varieties.^[18] Myricetin, laricitrin^[19] and syringetin,^[20] flavonols which are present in red grape varieties only, can be found in red wine.^[21]

Non-flavonoids

Hydroxycinnamic acids are the most important group of nonflavonoid phenols in wine. The four most abundant ones are trans-caftaric, cis- and trans-coutaric, and trans-fertaric acids. In wine they are present also in the free form (trans-caffeic, trans-p-coumaric, and trans-ferulic acids).^[22]

Resveratrol is a phenolic compound found in highest concentration in the skins of wine grapes. The accumulation in ripe berries of different concentrations of both bound and free resveratrols depends on the maturity level and is highly variable according to the genotype.^[23] Both red and white wine grape varieties have resveratrol but more frequent use of skin contact and maceration in red winemaking means that red wines will normally have 10 times more resveratrol than white wines.^[24] It generally produced by grape vines as a means of microbial defense, though production can be artificially stimulated by ultraviolet radiation. Grapevines in cool, damp regions with higher risk of grape diseases, such as Bordeaux and Burgundy, tend to produce grapes with higher levels of resveratrol than warmer, drier wine regions like California and Australia. Additionally, different grape varieties are prone to differing levels with Muscadines and the Pinot family having high levels while the Cabernet family being noted for lower levels of resveratrol. In the late 20th century, interest in the possible health benefits of resveratrol in wine was spurred by discussion of the French paradox involving the health of wine drinkers in France.^[25]

Piceatannol is also present in grape ^[26] from where it can be extracted and found in red wine.^[21]

Vanillin is a phenolic aldehyde most commonly associated with the vanilla notes in wines that have been aged in oak. Some trace amounts of vanillin are found naturally in the grapes themselves but they are most prominent in the lignin structure of oak barrels. Newer barrels will impart more vanillin, with the concentration present decreasing with each subsequent usage.^[27]

See Also:

Home Wine Page
History of Wine
Classification of Wines
Science of Taste
The Science of Wine Aroma
About the Acids in Wine
Polyphenols (Tannins) in Wine
Oak in Wines
The Basic Wine Pairing Rules
Science of Food and Wine Pairing
Sugars in Wine
About Wine Tasting
Wine Tasting Terms
Storage of Wine
Aging of Wine
Wine Acessories
Headaches from Wine
About a Wine Sommelier

 

References