In the upper part of the figure, a negative relationship is shown between protein in beans and methionine and tryptophan. These results indicate the limitation of these two amino acids in relation to the protein quality of beans and they suggest how these foods should be utilized in food systems. The Role of Tannins Tannins are common components in plants. Legume foods are no exception, and the ones most frequently consumed are characterized by differences in the colour of the seed coat, which appears to be related to tannin content. Although at present their effect on the protein quality of legume foods is not well known, some evidence suggests that they can interfere with protein digestibility and amino-acid availability when fed to animals.
Tannins inhibit enzymatic activity, thus possibly affecting protein digestion.
Furthermore, tannins may react with amino acids, thus decreasing their biological availability to the animal. Representative results suggesting their interference with protein digestibility are shown in Table 2. In this example, cooking beans of three different colours increased digestibility. It is of interest to note that the digestibility of the whole seed, raw or cooked, is negatively correlated with levels of tannins shown in the last line of the table. Beans were supplemented with methionine.