Tannins can be considered as being grape’s natural antioxidants, able to protect the color and aroma compounds from the action of oxidasic enzymes, such as laccase, as well as from the free radicals that are formed from the oxidation of polyphenolic molecules.
The stage between grape crushing and the start of alcoholic fermentation is a crucial one, since considerable quantities of dispersed oxygen are present and tannins extraction from grape skin and pips is hindered because of the scarcity of ethanol content.
This wealth is trapped in the berry, preventing it from protecting the anthocyanins and capturing the oxygen. The anthocyanins, on the other hand, are rapidly extracted from the skins during this stage making them prone to oxidation just as quickly.
The wine maker can avoid this obstacle by adding exogenous tannins which preserve the coloring matter by creating stable bonds, protecting it from oxidations during the breakdown of sugars into alcohol, up to the time when the grape tannins are extracted.