What are tannins and how do they relate to flavan-3-ols?

Tannins are plant polyphenolics that possess the property of precipitating proteins from aqueous media. Tannins can be separated the tannins into two classes: hydrolyzable tannins and nonhydrolyzable or condensed tannins. According to Haslam, a world-renowned phenolic chemist, the term proanthocyanidins means the condensed or nonhyrolyzable tannins. Chemically speaking, the terms condensed tannin and proanthocyanidin can be used interchangeably and both are comprised of two or more flavan-3-ol units, but proanthocyanidin seems to be preferred in the nutraceutical industry. Species in the Vitis genus (i.e. grapes) contain non-hydrolyzable tannin; or complex polyphenols which can not be degraded under hydrolytic conditions – either acid, alkaline or hydrolytic enzymes – into simpler fragments, mainly sugars and some phenolic acids.

Billot, J., Fleuriet, A., Macheix, J-J. 1990. Fruit Phenolics. CRC Press, Inc. Boca Raton, FL.
Haslam, E. 1979. Vegetable tannins. In “Recent Advances in Phytochemistry”, Vol. 12, Swain, T., Harborne, J.B., and Van Sumere, C.F. Eds. Plenum Press, New York, 475.

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