As the grape berry ripens, tannins are present in practically every part of it. However, as ripening progresses, tannins tend to disappear from the pulp, until only very small amounts remain. Meanwhile, in almost all varieties, the tannins in the skins and seeds begin to polymerize, which makes them more pleasant to the palate. This polymerization is considerably greater in the skin, so for the same harvest date, the tannins obtained from the skins are more velvety and elegant than those obtained from the seeds.
To achieve proper ripeness of the seed tannins, it is often necessary to let the grapes overripen to give these tannins time to polymerize. In warm climates, this overripening can become a real headache for winemakers, since it creates a very large time gap between the optimal potential alcohol and the moment of tannin ripeness, or phenolic ripeness.
Tannins are probably among the most important polyphenolic compounds in wine, since the wine’s color and sensory characteristics depend to a large extent on them. They are found in many parts of the cluster, but mainly in:
- The rachis (stem)
- The seeds
- The skin
The tannins in the rachis, or stem, usually give the wine green, bitter and astringent notes that consumers find unpleasant, which is why very few wineries include the stems during alcoholic fermentation. As a result, the tannins that actually shape the sensory characteristics of a wine made by traditional fermentation are those from the seeds and those from the skins, since there are practically no polyphenolic compounds in the pulp (around 2%, including all types).
Seed tannins are procyanidin-type tannins, which are found either free or esterified with gallic acid, forming galloylated tannins that are proportionally more astringent and bitter than skin tannins. Skin tannins, on the other hand, are usually a combination of procyanidins and prodelphinidins, which tend to bind with cell-wall components such as proteins and polysaccharides, making them less reactive than in their free state.
Tannins form through the polymerization of basic molecules with a phenolic function, such as phenolic acids and flavonoids. This great variability in their synthesis leads to the formation of a range of substances that differ from one another but share certain properties.
The most important of these properties are:
- Their ability to inhibit enzyme activity by binding to the protein fraction of the enzyme.
- Their role in the evolution and stabilization of color in wines through the formation of condensed tannin-anthocyanin complexes.
- Their role in wine oxidation, both as pro-oxidants that enable the autoxidation of ethanol to ethanal, which is essential for forming tannin-anthocyanin, anthocyanin-tannin and tannin-tannin complexes, and as antioxidants.
Tannins bind with salivary proteins, producing the sensation of astringency. This sensation is directly related to the molecular weight of the tannin molecule. Molecules or polymers that are too small cannot form enough bonds with the proteins to cause astringency. On the other hand, when the chains are highly polymerized, they are unable to react with the proteins’ active sites, simply for stoichiometric reasons.
In short, within the range of molecular weights at which tannins react with salivary proteins, the lower the molecular weight, the greater the astringency of the wine; conversely, molecular weights close to the upper end of the range usually result in much softer astringency that is more pleasant on the palate.
All of this is easy to verify by tasting a young red wine right after alcoholic fermentation, and then tasting the same wine during barrel aging and later in bottle, when the condensed tannins polymerize and the initial tannic harshness of the wine is significantly reduced.
Based on their structure, tannins can be classified into two types:
- Condensed tannins
- Hydrolyzable tannins
Condensed tannins (or catechol tannins)
These are the tannins of wine, or rather of the grape, formed by polymers of flavans (catechins, their isomers and gallic esters). When characterizing the tannins present in wine and in the berry, condensed tannins are defined as a family of phenolic substances in which catechin polymers (or catechin tannins) are the most abundant. These tannins are called proanthocyanidins. Within the proanthocyanidin family, two groups can be distinguished:
- Procyanidins: polymers of catechin and epicatechin
- Prodelphinidins: polymers of gallocatechin and epigallocatechin
This group of condensed tannins plays a very important role in red wine, since it is directly involved in forming the colored complexes that arise when free anthocyanins combine with condensed tannins, specifically proanthocyanidin tannins. These tannin-anthocyanin complexes form over time when free anthocyanins and proanthocyanidin-type condensed tannins are present along with small amounts of oxygen in the wine. This combination stabilizes the color over time, protecting the free anthocyanins from oxidizing or precipitating. These tannin-anthocyanin complexes give the wine a redder hue, characteristic of wines made for aging.
Although catechins and leucoanthocyanins on their own do not have the tannic properties described above, the polymers formed from these units do begin to show tannic properties, which are directly related to the molecular weight of the polymer (within a certain range).