The sensation of astringency in a wine is due to tannins, molecules that belong to the polyphenol family. They produce a tactile sensation – also referred to as somesthetic – as tannins stick to the salivary proteins and dry the mouth out by reducing the amount of water molecules. The stalk (or skeleton of the grape bunch) contains a lot of tannins.
Grapes are rarely fermented nowadays with the stalks as they give the must a very vegetal character. Grape seeds also contain tannins and are a very good ripeness indicator: they are green and acidic and affect your taste buds for several hours if ripeness is lacking. Conversely, if the seeds are ripe, they have a slight hazelnut and chocolate flavour and are very crunchy. The best tannins, however, are located in the grape skins. They are often fatter and fleshier though can also be a little herbal depending on how ripe the grape is.
Wines aged in oak casks – both red and white – also absorb tannins from the wood. Tannins form the ‘backbone’ of red wines, imparting structure and balance and guaranteeing development and ageing potential. A sensation of astringency is rarely if ever found in white wines which are made from the juice only.
One noteworthy fact is that polyphenols are antioxidant compounds which impart real biological protection to red wines. This is why white wines, which contain few or no tannins, need greater amounts of sulphites.