Phenolic compounds are present at rather low levels in wine, but their concentrations are a primary factor in the differences between wine types and styles. Phenolic compounds effect several sensory components of wine, including wine color, astringency, bitterness, and olfactory profile. They are also important oxygen reservoirs and substrates for browning reactions, and play an important role in chemical processes such as reactions with wine proteins. Their concentrations in wine are largely due to processing considerations (read more: White Wine Processing Considerations and Red Wine Processing Considerations).
The basic phenol structure is carbolic acid, also known as hydroxybenzene: C6H5OH
Several hundred different phenolic compounds are naturally occurring in grapes, divided into two basic groups referred to as nonflavonoid and flavonoid phenols:
The majority of nonflavonoid phenols are sourced from grape pulp. These are hydoxycinnamate derivatives present as free acids, ethyl esters, and tartrate-glucose esters. Phenols arising from oak maturation are primarily hydrolizable nonflavonoids such as vanillin.
Nonflavonoid levels are largely effected by fermentation. Up to 20% of total nonflavonoids are absorbed by yeast, while many are hydrolized to free acid and ester forms such free cinnamic acids and ethyl phenols.
Most nonflavonoids are present in wine below their sensory threshold. Collectively, they can have an impact on bitterness and astringency. Some nonflavonoids are also indicators of spoilage. For example, 4-ethyl phenol can be used as an indicator of Brettanomyces.
For white grape varieties, nonflavonoids represent the overwhelming majority of finished wines' phenol content.
Flavonoids have much more impact on a wine's structure and color compared to nonflavonoids. They are found in skins, seeds, and stems of both white and red grapes. They represent 25% of total phenol content in white wines made without skin contact and 80-90% of total phenol content in red wines produced in a traditional manner.
Flavonoids can exist in monomeric forms, or polymerized to other flavonoids, nonflavonoids, sugars, or a combination of these. Polymeric flavonoids make up the majority of total phenolics found in all stages of red winemaking. Further polymerization yields flavenoid polyphenolic compounds, or tannins and condensed tannins.
- Catechins and epicatechins are flavon-3-ols. Precursors for browning and bitterness in both red and white wines, they polyermize to create procyanidins, also known as condensed tannins. Catechins and epicatechins account for the majority of white wine flavonoid content and up to 14% of total red wine phenol content. Catechins are primarily sourced from grape seeds and stems, and these are perceived as very bitter and astringent due to their low degree of polymerization. Catechins sourced from grape skins are seen as more beneficial to wine quality due to a higher degree of polymerization.
- Luecoanthocyanidins and luecoanthocyanins serve as precursors to larger polymeric forms of anthocyanins. These compounds are very closely related to catechins; luecoanthocyanidins have an additional hydroxyl group, and luecoanythocyanins have an attached sugar molecule. These compounds have minimal effect on a wine's bitterness.
- Flavonols are primarily found in grape skins, thus their concentrations in wines produced without skin contact are minimal or nonexistent. Quercitin commonly represents the majority of a wine's flavonol content, though kaempforol and myricetin are also found in significant concentrations. These compounds effect a wine's bitterness.
There are several different types of tannins naturally occurring in grape stems, skins, and seeds. Tannins are also referred to as complex phenols or polyphenols because they are polymers of phenols.
Tannins attribute to a wine's astringency, mouthfeel, aroma, and color stability. This is due to their reactions with proteins, not only in the wine itself but also proteins present in our saliva and food.
These are based on nonflavonoid phenols and exist primarily as esters. They are oxidative and relatively easily decomposed via hydrolysis. Hydrolyzable tannins are derivatives of gallic acid, which esterifies to the most simple form known as gallotannins. The most common form of gallotannin is pentagalloyl glucose (PGG), though there are many variations. When coupling of galloyl groups occurs, ellagitannins are formed.
Hydrolyzable tannins represent a small portion of tannins present in wine. They are primarily derived from commercial tannin additives and oak during maturation. They are generally considered as soft tannins (i.e. are not harsh or very astringent) that lead to a pronounced but rounder mouthfeel.
Also referred to as proanthocyanidins/proanthocyanins because of their ability to polymerize with anthcyanins, condensed tannins are based on flavonoid phenols. These tannins cannot be easily decomposed by hydrolysis, and are made of catechins and epicatechins.
Condensed tannin levels decrease during maturation as they react with proteins and polymerize to form long-chain polymers. They are typically present in younger wines as dimers or trimers, but can polymerize during maturation to molecules with up to 14 flavonoid units. Longer-chain tannins are considered more favorable for wine quality as they tend to be perceived as less bitter but more astringent.
Anthocyanins are composed largely of luecoanthocyadinins and luecoanthocyanins flavonoid phenols. While they attribute the color of red grapes and wine, anthocyanins have very little effect on a wine's bitterness or astringency.
There are typically five forms found in wine grapes/juice, the most common being malividin-3-D-glucoside.
Stability of anthocyanins and the color of red wine is dependent on several factors including wine pH, presence of sulfur, and polymerization.
- Potential red color is only 50% visible in wine with pH values greater than 3.0. This would include almost all wines.
- Sulfur dioxide binds to carbon 4, the same site as other phenolic compounds. The inhibition of polymerization may lead to temporary or permanent decolorization, depending on stage of processing.
- Polymerization rate rapidly increases in the presence of acetaldehyde.
- A fine balance between enough and too much polymerization has been noted; reactions between anthocyanins and highly polymerized anthocyanidins (condensed tannins) can form instable compounds that will precipitate and decrease color.