Phenolic compounds in wine

The phenolic compounds in wine include a large group of several hundred chemical compounds, known as polyphenolics, that affect the taste, color and mouthfeel of wine. This large group can be broadly separated into two categories-flavonoids and non-flavonoids. Flavonoids include anthocyanins and tannins which contribute to the color and mouthfeel of the wine. Non-flavonoids include stilbenes such as resveratrol and compounds derived from acids in wine like benzoic, caffeic and cinnamic acid. In wine grapes, phenolics are found widely in the skin, stems and seeds. During the growth cycle of the grapevine, sunlight with 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 secret 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. J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 517-518 Oxford University Press 2006 ISBN 0198609906 ]

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 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. J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 273-274 Oxford University Press 2006 ISBN 0198609906 ]

Anthocyanins

Anthocyanins are phenolic compounds found throughout the plant kingdom, being 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 in the case for many white sparkling wines which are often made from the red wine grapes of Pinot noir and Pinot noir. The exception to this is the small glass of grapes known as teinturiers, such as Alicante Bouschet, which has a small amount of anthocyanins in the pulp which produces pigmented juice. J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 24 Oxford University Press 2006 ISBN 0198609906 ]

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.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.

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 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. J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 680 Oxford University Press 2006 ISBN 0198609906 ]

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 of with the addition of tannin powder. The natural tannins found in grapes are known as proanthocyanins due to their ability to release red anthocyanin pigments when they are heated in an acidic solution. 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. The tannins derived from oak influence are known as "hydrolysable tannins" being created from the ellagic and gallic acid found in the wood.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.

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 Merlot and Pinot noir having high concentrations while Syrah has very low levels. As an antioxidant, there are some studies into the health benefits of moderate consumption of wines high in catechins. J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 144 Oxford University Press 2006 ISBN 0198609906 ]

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. J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 727 Oxford University Press 2006 ISBN 0198609906 ]

Non-Flavonoids

Resveratrol is a phenolic compound found in highest concentration in the skins of wine grapes. 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. 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, interesting 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. J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 569 Oxford University Press 2006 ISBN 0198609906 ]

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