Tannins are a natural substance found in many fruits and bark of trees and are felt via our tactile senses rather than tasted. Most people are familiar with the tannins of over steeped tea or young red wines designed for long ageing.
Tannins in wine are derived from skins, seeds, stems (vegetative material) and oak. Depending on the molecule shape it can be astringent (drying) or bitter.
In red wine tannin combines with pigment molecules to form larger molecules and feels astringent. In white wine tannin molecules are smaller and often appear bitter.
Tannins are extracted from the vegetative material over time by acid of the juice and alcohol of the fermenting wine. For this reason white grapes are rarely fermented in contact with any of the vegetative parts of the vine.
Red wines become ‘softer’ with age as it takes time for the tannin molecules to randomly bump into pigment molecules and form larger less bitter polymers.
Mature tannins are responsible for that wonderful velvety mouth feel of favorite red wines. In red wine we refer to bitter tannins as young, green, hard, coarse, aggressive, grainy (oak) and astringent tannins as soft, silky, smooth, supple, dry, mature.
Winemakers can employ several different techniques to change the way tannin affects the wine: When fresh juice is aerated some of the molecules causing bitterness are oxidized and precipitate to the bottom of the tank this can be a useful method in white varieties.
In young red wines oxygen encourages the polymerization of tannin and pigmented molecules and softens the wine.
So we use various methods of introducing controlled amounts of oxygen into the winemaking process. Fining of wines with additions of skim milk, egg whites, isinglass (ground up fish bladder), gelatin etc. selectively removes some of the tannin because tannins react with protein molecules.
Laboratory trials are conducted to determine the fining agent and rate to be used.
The protein of isinglass reacts with a different size molecule in the wine than say skim milk and if the wrong size molecule is removed or if too many molecules are removed the wine will appear harsher than the starting sample.
When the correct amount of fining agent is used it all settles to the bottom of the tank and no traces remain in the wine.