A critical evaluation of the vanillin reaction as an assay for tannin in sorghum grain

Several parameters of the vanillin assay, for studying sorghum tannin, were examined to determine which must be most closely controlled to ensure accuracy and reproducibility. A 20-min extraction in methanol was found to be adequate. When corrected for background colour, the modified vanillin assay was found to give nearly identical values with those obtained with the regular vanillin assay, except with group II sorghum. The reactions of tannin and catechin with vanillin were found to differ markedly in reaction kinetics. It was shown that the use of catechin equivalents overestimates tannin content. The assay was found to be very temp. dependent. Revised procedures for the vanillin assay are given which give good reproducibility