Radial diffusion method for determining tannin in plant extracts

Tannin in plant extracts can be determined by reacting the tannin with a protein and quantitating the precipitated complex. In the new assay described here, a tannin-containing solution is placed in a well in a protein-containing agar slab. As the tannin diffuses into the gel and complexes with protein, a visible ring of precipitation develops. The area of the ring is proportional to the amount of tannin in the extract. The detection limit of the method is 0.025 mg tannic acid or condensed tannin and the precision is 6% (relative standard deviation). Tests with extracts of a variety of plants show that the new method gives results comparable to other precipitation methods and that the new method is superior for samples of unusual composition, such as aspen buds. The method has several advantages over other methods for determining tannin: The new method is very simple and requires neither complex reagents nor instruments. Components of the plant extract such as non-tannin phenolics or water-insoluble compounds do not interfere with the method. The assay is not subject to interference from the organic and aqueous solutions which are commonly used to extract tannin from plants.