Tannins are plant defense substances that exhibit a strong astringent taste and precipitate proteins, leading to the inhibition of protein functions; however, owing to their relatively low toxicity, tannins must be accumulated in high concentrations in cell vacuoles. Therefore, the solubility of tannins is crucial for their functions. In this review, the structure and reactions of tannins related to solubility; insolubilization of persimmon proanthocyanidins on fruit ripening; pigment formation from cinnamon procyanidins by reaction with cinnamaldehyde in case of wounding; and insolubilization of ellagitannins in chestnut wood is discussed. In addition, the development of functional polyphenols including oil-soluble tea catechins is introduced.
Keywords: antioxidant; catechins; cinnamaldehyde; essential oil; polyphenols; proanthocyanidins; tannins.