Relationship between theoretical and experimentally detected tannin content of common beans (Phaseolus vulgaris L.)

The present study was undertaken to determine the degree of any relationship between the theoretical and the experimentally determined tannin content in common beans. With this objective, theoretical tannin content was calculated as a function of seed coat tannin content and seed coat percentage weight in 19 varieties of common beans, grown in two localities of the Mexican highlands. The percentage of undetected tannins was calculated by the difference for all cultivars. The theoretical tannin content was correlated with the tannins detected experimentally in whole bean flour. The undetectable tannins ranged from 9.9 to 66.4% for varieties grown in Aguascalientes, and from 4.1 to 69.7% for cultivars harvested in Durango. A significant positive correlation was observed between the values of the theoretical tannin content and tannins detected experimentally in cultivars from Aguascalientes (r = 0.81, P<0.01) and Durango (r = 0.72, P<0.01). Nevertheless, there were many examples that showed the real content of tannins to have been underestimated. In accordance with this study, it is propose that, in order to obtain the real content of tannins of beans, these compounds should be determined in the seed coat flour.