Tannins are astringent, bitter plant polyphenols that either bin and precipitate or shrink proteins. The astringency from the tannins is what causes the dry and puckery feeling in the mouth following the consumption of red wine, strong tea, or an unripened fruit.. The term tannin refers to the use of tannins in tanning animal hides into leather; however, the term is widely applied to any large polyphenolic compound containing sufficient hydroxyls and other suitable groups (such as carboxyls) to form strong complexes with proteins and other macromolecules. Tannins have molecular weights ranging from 500 to over 3,000. Tannins are incompatible with alkalies, gelatin, heavy metals, iron, lime water, metallic salts, strong oxidizing agents and zinc sulfate.