There are several different types of tannins naturally occurring in grape stems, skins, and seeds. Tannins attribute to a wine's astringency, mouthfeel, aroma, and color stability due to their reaction with proteins (including those in wine, in our saliva, and in our food). Tannins (also referred to as complex phenols or polyphenols) are polymers of phenols, and can be classed as hydrolyzable or condensed.
Hydrolyzable Tannins - These tannins 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, and 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. (These tannins will be discussed at more length during a following post).
Condensed Tannins - 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) sourced from grape seeds (> 50%) and, to a lesser extent, grape stems and skins. Catechins from seeds and stems are perceived as very bitter and astringent due to their low degree of polymerization, while catechins from skins are seen as more beneficial to wine quality due to a higher degree of polymerization and are the easiest to extract.
Condensed tannin levels decrease during maturation as they react with proteins (including anthocyanins) 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 tend to be perceived as less bitter but more astringent, considered more favorable for wine quality.
Anthocyanins - Anthocyanins attribute the color of red grapes and are present in grape juice as glyosides. Composed largely of luecoanthocyadinins and luecoanthocyanins flavonoid phenols, this group has very little effect on a wine's bitterness or astringency. There are typically five forms found in wine grapes/juice (vitis vinifera), the most common being malividin-3-D-glucoside.
Anthocyanin stability and resulting color is dependent on several factors including pH, sulfur, and polymerization. Here's some examples:
- Potential red color is only 50% visible in wine with pH values greater than 3.0 (almost all wine).
- Sulfur dioxide binds to carbon 4, the same site as other phenolic compounds (i.e. condensed tannins), inhibiting polymerization which may lead to temporary or permanent decolorization (depending on stage of processing, which will be discussed in a following post).
- 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.
Continue to read White Varietal Processing Considerations.