Tannins are a family of astringent compounds called polyphenols. Polyphenols constitute over 48.5% of the total solids in a cup of tea. They’re found in several kinds of plants, including dark sorghum, some apples and yerba mate, a popular South American drink made from the leaves and branches of an evergreen tree.
Tea is often included in the list of tannins-containing foods. The tea leaves contain varying amounts of tannins: Green teas, which have unfermented leaves, contain the most tannins; black teas including pekoe, which are fermented, contain the fewest.
By definition, tannins are biologically active, that is tannins bond protein, a process that is pH dependent. Most of the phenolic groups in tannins are free. Tannins have a molecular weight between 500 and 3000, and generally are soluble in water.
Tannins are usually divided into two classes: those can be hydrolyzed (react with water) and those that cannot. The nonhydrolysable tannins found in tea are condensation polymers of catechin.
The occurrence of tannins is reported in unripe fruits such as mango, dates and persimmons. The tannins diminish in amounts as the fruits ripen.
Other food sources of tannins are grapes (condensed tannins, on average 5000 mg/kg of grapes). Grape juice, wines (red wine: 1.2 – 4l4 g/L of wine) and some sorghum grain varieties.
Tannins in tea
