When I studied wine making it was taught that tannins started out as small molecules in the wine and as the wine aged they would join together (polymerise), becoming bigger and heavier and thus settle out of the wine, making it smoother and more mellow. Sounded logical and explained the ‘crust’ or sediment found in older wines.
Now researchers don’t think that’s the case. Recent analysis of the same wine from 1954 to 2004 vintages showed tannin concentrations of similar levels. For instance wines from the 1950’s and 1990’s have the same level of tannin, while wines from around 1980 have slightly less.
Overall, however, the levels are only in a small range, showing that the amount of tannin in wine is not related to wine age. So, what is happening to the tannins? One promising theory is that the shape of the tannin changes. It might be that young wine tannins are long and thin with lots of receptors along it and these are what react and give the astringent, drying sensation in the mouth. As the tannins age in the wine they become more compact and rounded, so there are less receptors and thus less astringency. Again, sounds logical, but only more research well tell us if this is really the case, so don’t go quoting me just yet.