Chemical Components of Grapes and Wine
Phenolic Compounds in Grapes and Wine
Polyphenolic compounds also referred to as phenolics have a fundamental role in wine quality. Following sugars and acids, they are the most abundant constituents present in grapes. Grape phenolics are largely found in the skin and seed of the berry, while the juice and pulp have much lower concentrations. Red grapes have higher total phenolic concentrations than white grapes, due to the presence of the red anthocyanins in the skin. They play a vital role in determining the wine’s flavor and color, especially in red wines. Phenolic compounds also have antimicrobial and antioxidant properties and they provide much of the health benefits associated with drinking wine. They are involved in browning reactions in grapes and wines and play a key role in the maturation and aging of wines. Phenolic compounds are principally associated with bitterness and astringency. Bitterness is a taste while astringency is a drying and puckering tactile sensation. Bitterness generally diminishes with the increase in the degree of polymerization whereas astringency increases with the degree of polymerization (Cagnaso, 2015).
The flavonoids constitute more than 85 percent of the phenolic content in red wines, due to the nature of processing, i.e., in contact with the skins and seeds where the majority of these compounds are found (Jackson, 2008). In white wines, flavonoids typically constitute less than 20 percent of the total phenolic content. The degree to which flavonoids are extracted during wine production depends on many factors. Extraction is ultimately limited by the amount of phenolic compounds present in the fruit.
Flavanols are found in monomeric form (catechin and epicatechin) and in their polymeric form (proanthocyanidins, also called condensed or non-hydrolysable tannins). Flavanols are responsible for the stabilization of both the color and sensory characteristics (mainly astringency and bitterness) of wines. Flavonols are found in higher concentration in red wines because alcoholic fermentation occurs on grape skins, unlike in white wines.
Flavonols are a class of flavonoids that tend not to receive as much attention as the other flavonoids such as tannins and anthocyanins; however, flavonols can increase the color intensity by several fold through copigmentation, particularly in younger red wine. Also, the amount of flavonols in a wine is highly correlated with its market value.
Anthocyanins are a type of polyphenol from the flavonoid group that is the red pigment found in grape skins and sometimes in the flesh. At véraison, the red berries start to turn pink and then red due to the biosynthesis of anthocyanins in the skin. Similar to tannins, many environmental factors such as sun exposure, temperature, ripening stage, vintage, and the grape variety have an impact on the amount and composition of anthocyanins in grapes.
In wine, tannins are derived from grapes (e.g., the pips (seeds), skins, and the stems) as well as from oak (e.g., oak barrels). On a fresh fruit basis 70 to 80 percent are seed tannins and 20 to 30 percent are skin tannins. Tannins provide texture and mouthfeel to wine as well as a sense of weight and structure. Tannins derived from skins, seeds, and stems differ by size and chemical composition giving rise to different mouthfeel characteristics.
In grapes, most of the non-flavonoids are found in the berry pulp, with some of them also present in the skins. All-grape derived non-flavonoids can display a bitter taste and elicit a sensation of astringency; however, because their concentration is close to their sensory threshold their influence is negligible. So, they do not play a direct role in the taste of wine.
Red wines have a much higher concentration of phenolic compounds because the fermenting must contain skins and seeds during and after fermentation, which are extracted into the juice/wine. Polyphenols are key actors in red wine, since they are involved in its sensory properties (e.g., color, flavor, astringency, and bitterness), in its aging potential, and in beneficial health effects attributed to moderate wine consumption.
The Role of Oxygen
Phenolic compounds are the main substrates for oxidation in must and wine. The addition of oxygen to wine leads to polymerization of certain phenolic compounds. Polymerization reactions can provide better color stability and intensity while decreasing the astringency, thus affecting flavor.
The phenolic concentration of wine is an indication of its capacity for oxygen, with higher phenol-content wines being able to accommodate higher concentrations of oxygen. Winemaking practices that lead to higher phenolic concentrations, such as skin contact, hard pressing, and barrel ageing of wine, should lead to a higher capacity of wine for oxygen.
Extraction of Phenolic Compounds
Most red grape varieties do not contain pigments within their pulp but rather in their skins, so an essential step in the red winemaking process is the extraction of anthocyanins and tannins from the solid components of the grape. The management of skin fermentation affects the sensory profile of the resulting wines because other compounds—aromatic substances and precursors, nitrogen compounds, polysaccharides, and minerals are released during the maceration process. This means that it is necessary to keep skins and seeds in contact with the must (i.e., cap management) in the tank during fermentation to extract and diffuse these phenolic compounds in the must and to obtain the end color of the wine provided by the skin’s anthocyanins, as well as the antioxidant capacity and structure provided by tannins.
Phenolic Management Strategies
Generally speaking, grape processing, whether it be viticultural management practices or winemaking techniques, can impact the phenolic-makeup of wines, i.e., flavonoid content.
Managing Phenolics in the Vineyard
Like almost anything to do with wine, managing phenolics starts in the vineyard. The level of anthocyanin and tannin will depend on the grape variety as well as factors such as temperature, vine vigor, bunch exposure, and harvesting.
Managing Phenolics in the Winery
Aside from viticultural factors, the actual tannins and anthocyanins that end up in the final wine are highly dependent on the winemaking process and techniques employed. While stem tannins can be eliminated from the winemaking process prior to fermentation, skin tannins tend to be more easily extracted than seed tannins during early maceration stages. Skin tannins are generally extracted quickly during the initial part of fermentation once the yeast is active and also extracted during the latter part of fermentation (Scrimgeour et al., 2014). Seed tannins tend to be more readily extracted with longer maceration times and continue to be extracted during extended maceration. Extraction of seed tannins can also be facilitated when seeds are cracked or damaged.
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