Tannins are present in naturally occurring substances and organic matter, including leaves and wood. It is used in the tanning of leather, hence the name. Tannins in tea, give the beverage its astringency.
Teas with high levels of tannins have a bitter taste accompanied by strong astringency, seen especially in green and black tea. The tannins found in tea are thearubigins, most prominently theaflavins. When the anti-oxidising agents such as catechin in the tea become oxidised, theaflavins are produced.
Tannins in tea are responsible for the antioxidant activities of black and other dark teas. They have both positive and negative effects on the body. The positive health benefits of tannin come from its anti- carcinogenic and anti-mutagenic properties, mostly due to its anti-oxidising nature.
Tannins also remove harmful microbes from the body, and fight against harmful bacteria, viruses and fungi. By speeding up blood clotting, tannins also have a healing effect on cuts and wounds. Other beneficial properties of tannins include stabilizing blood pressure.