Biochemical composition of mature winged beans, Psophocarpus tetragonolobus (L.) DC
The proximate composition has been determined and information on the carbohydrates and fatty acid composition has been obtained for the mature seeds of five varieties of winged bean, Psophocarpus tetragonolobus (L.) DC. The proteins of one of these varieties have been fractionated and partially characterized.
The results confirm that winged beans have a high protein and fat content, which is similar to soybeans. The mean protein content of the five varieties analyzed was 40.12%; the mean fat content was 16.94%. The endosperm in TPT-2 bean was found to constitute about 84% of the dry weight of the seeds, the hulls about 16%. Whole beans contain about 17% “dietary fiber", mostly (72%) in the hulls.
Similar to soybeans, starch could not be detected in the mature beans and the soluble sugars (9.7 to 13.8%) consisted almost entirely of verbascose (0.2 to 0.9%), stachyose (2.2 to 3.6%), raffinose (1.1 to 2.0%) and sucrose (5.6 to 8.2%).
The fatty acid composition of winged beans was also similar to soybeans, with a high degree of unsaturation (mean, 62.0%). The major unsaturated fatty acids were oleic and linoleic. Behenic and stearic acids were the major saturated fatty acids. Parinaric acid was not present in the five varieties of winged beans analyzed, although the presence of this toxic acid in winged beans had been reported earlier. The solubility of protein nitrogen was found to be a function of pH and salt concentration. The nitrogen solubility in water was lowest (17.6%) at pH 4.0 (the apparent isoelectric point). The maximum solubility in water at neutrality (pH 6.70) was about 60.4%. Solutions of Na₃PO₄ (0.5%, pH 11.0) extracted the most protein (86.0%), compared to 70% in 0.75-1.0 M NaCl (pH 6.95), 69% in 0.25 M Na₂SO₄ (pH 7.20) and 66% in 0.20-0.30 M Na₂HPO₄ (pH 9.20).
This study also describes the preparation of protein concentrates (extracts), isolates (acid precipitates) and “wheys” from defatted bean flour. The amino acid composition of the concentrates, isolates, and “wheys” obtained from water, NaCl and Na₃PO₄ extraction were similar to the defatted bean flours, except that the precipitate from NaCl extraction contained a high amount of methionine. Lysine was higher than that found in soybeans with cystine and methionine as the limiting amino acids.
Fractionation of the extracts on Biogel A-1.5M gave three to four fractions, two high molecular weight fraction components and one or two low molecular weight fractions. The acid precipitated protein from the water extract yielded two fractions of higher molecular weight proteins and a fraction of lower molecular weight. The “whey” from the water extract yielded two fractions, both of relatively low molecular weight proteins.
The fractions from gel filtration, the original extracts, the acid precipitate and “whey” were all analyzed via polyacrylamide gel electrophoresis. The Na₃PO₄ extract was apparently highly aggregated as indicated by non-migration of the proteins. The NaCl and water extracts appeared to be similar with regard to complexity and molecular size. These extracts also showed evidence of aggregation. The acid precipitates were also highly aggregated especially that of the NaCl extract which did not migrate at all. The wheys were all well resolved.
Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the winged bean proteins indicated the presence of well-resolved proteins and/or subunits. SDS-PAGE patterns did not show any aggregation. The high molecular weight fractions appeared to consist of proteins with a molecular weight of about 74,000 while the low molecular weight fractions consisted of proteins with molecular weights of about 20,000.