tannins

[Jerry D Mead]

They are the astringent part of wine...coming from the skins rather than the juice or pulp, which is why tannin is almost exclusively a red wine issue...as red wine is fermented in contact with the skins to extract color and flavor.

It is that rough puckery feeling in the mouth...like alum if you know what that is...or as in tea when brewed too strongly, too concentrated, which will give you the same feeling of astringency.

Harder to explain is the differing chemical compositions of tannins...some of which are very offensive in the mouth, others which seem to have round edges and be actually complimentary to a red wine's mouthfeel.

Red wines meant to be softened by aging and time tend to have more tannin that do simple, fruity red wines meant to be quaffed young.

Some of the technical guys can probably add to this.