POSSIBLE EFFECTS OF SEED COAT POLYPHENOLICS ON THE NUTRITIONAL QUALITY OF BEAN PROTEIN

Studies were carried out to determine the possible relationship between the color of the seed coat of beans and the nutritive value of its protein. Beans with white, red and black seed coats and a black coated bean and its white mutant were chosen for the study. Hemagglutinin activity was located in the cotyledons of all samples with low activity in the seed coat. No activity was found in the cooked beans or in the cooking broth. Trypsin activity was influenced by a heat labile factor (true trypsin inhibitor) and by a heat resistant factor (tannins). The heat labile factor or true trypsin inhibitor was higher in the cotyledons (16-18 TUI/mg sample) than in the seed coat, while the heat resistant factor was found in highest concentration in the seed coat. Red and black seed coats had a higher concentration (23-31 TUI/mg sample) of the heat resistant factor than the white seed coat of the normal white bean and of the mutant (7-9 TUI/mg sample). Cooked beans and their broth showed trypsin inhibitor activity of the heat resistant type. Cooked cotyledons had 5-9 TUI/mg sample. Tannin concentrations was high in colored seed coats (38-43 mg/g) and low in white coated beans (1.3 mg/g) while values ranged from 3.8-5.9 mg/g in the cotyledons. A highly significant correlation (r = 0.88) was found between tannin concentration in the seed coat and trypsin inhibitor activity. No correlation between these 2 components was found in the cotyledons. Samples of cooked beans supplemented with methionine without the cooking broth had higher protein quality values (2.9-3.3) than samples fed with the broth (1.7-2.1). Protein digestibility was lower for red (70.4%) and black beans (75.0%) fed with the broth than beans fed without the broth (78.7% and 77.9%, respectively), but the broth had no effect on the protein digestibility of white coated beans (81.3 and 81.4%). Color of the seed is apparently related to the protein quality of beans.