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The tannin content of crude plant extracts or of purified preparations was determined by adding the sample to a standard solution of protein, isolating the insoluble tannin- protein complex, dissolving it in alkaline solution, and measuring the absorbance at 510 nm after adding ferric chloride. Plots of absorbance as a function of the amount of tannin are linear for tannic acid and partially purified sorghum tannins for amounts of tannin ranging from 0.20 to 1.0 mg. Nontannin components of crude methanolic extracts of sorghum and cowpeas do not interfere with the assay. The results of the precipitation method are qualitatively similar to results obtained with the vanillin assay. The precipitation assay can be used to study the effects of pH and other parameters on tannin- protein interactions.

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