Tannin content of tea and coffee.

The tannin content of over-the-counter Indian tea, of green coffee beans and of the roasted coffee beans prepared from the same green beans was determined with a radial diffusion-protein precipitation technique and with a spectrophotometric method. The green beans contained 6.6 +/- 0.6 mg g-1 weight tannic acid equivalents as found by protein precipitation (n = 5, +/- SD) or 6.8 +/- 2.3 mg g-1 by spectrophotometry. The same figures for roasted beans were 18 +/- 1.7 and 17 +/- 2.7 mg g-1, respectively. Tea contained 37 +/- 2.6 mg g-1 weight tannic acid equivalents as analysed by spectrophotometry and 24 +/- 2.8 mg g-1 by the protein precipitation technique. The latter finding may show that the biological reactivity of tannins is variable, although no major changes in the tannin-precipitated albumin occurred as shown by electrophoretic analysis. Both methods provide an easy analysis of the reportedly carcinogenic plant tannins.