Radial Diffusion Assay for Tannins

Abstract

Tannins are polyphenols familiar to most of us as astringent compounds in red wine (originating in skins, seeds, and stems of grapes), tea leaves, and unripe fruit. In trees, they occur in bark, buds, and fruits, and are found in cell vacuoles or surface waxes. Tannins are assumed to defend plants against microorganisms, insects, and vertebrate herbivores. These brownish or yellowish compounds are used for tanning and dyeing. Tannins precipitate with proteins, the basis for tanning leather. In this assay, the interaction of tannins with protein in an agar gel is quantified. The insoluble precipitates form rings around an origin. The diameter of the rings is proportional to the tannin amounts present. It needs to be added that not all tannins bind to proteins, and not all precipitates are insoluble. The method of this exercise follows the Tannin Assay as described by Hagerman (1987). The material analyzed in this exercise can be known amounts of tannins such as “tannic acid” (a mixture of tannins), or plant extracts with unknown amounts of tannin. This exercise is written for tests with plant materials whose tannin level we wish to determine. These can be bud scales of trees such as aspen, or different parts of acorns (tip vs. base).