UTILIZATION OF TETRAZOLIUM BLUE FOR THE COLORIMETRIC ASSAY OF TANNINSIN TEA

A simple, rapid and accurate colorimetric method is proposed for the d etermination of tannins, based on the reduction of tetrazolium blue in alkaline medium by tannins at 90 +/- 2 degrees C for 15 min, leading to the formation of a highly coloured formazan derivative. Absorbance measurements were made at 527 nm and the calibration graph was linear for 0.2-9.0 mu g/ml of tannic acid. For more accurate analysis, the Ri ngbom optimum concentration range was found to be 0.5-8.2 mu g/ml. The relative standard deviation for the determination in a tea sample con taining 7.55% tannins was 1.65%. Most of the ingredients commonly foun d in tea samples do not interfere with the determination. Several tea samples were analysed using the proposed method.