Tannin is a kind of natural polyphenol in plants, which can combine with protein and other polymers, such as cellulose, hemicellulose and pectin, to form a stable compound. Tannin is only a small part of a large class of plant phenols. Tannin, also known as tannic acid or plant polyphenols, belongs to polyphenol compounds and widely exists in leaves, seed shells, fruits and seed coats of plants.