Protein quality in seed meals of Phaseolus vulgaris and heat‐stable factors affecting the utilisation of protein

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Abstract
The polyphenol content of several cultivars of Phaseolus vulgaris was measured. The true digestibility (TD) and biological value (BV) of the cooked meals of two high‐polyphenol cultivars and one low‐polyphenol cultivar were determined by nitrogen‐balance experiments with growing rats. Removal of the testa caused a significant (P<0.05) increase in TD of Canadian Wonder and Masterpiece (high‐polyphenol) of 7.2 and 8.4%, respectively, but no significant increase in that of Processor (low‐polyphenol). Testa removal caused no significant change in BV. Supplementation of the three cultivars with methionine and cystine raised the BV to values similar to those obtained with egg albumin. Supplementation of Masterpiece with tryptophan caused no further increase in BV. The polyphenol content of the bean meals did not appear to affect the utilisation of their sulphur‐containing amino acids. Isolated Masterpiece globulins were found to be highly digestible (TD 95.4%) but with a low BV (28.36%) which was caused by a low sulphur‐containing amino acid content. The effects measured by rat feeding experiments could not be accounted for by trypsin inhibitors of phytohemagglutinin activity as the heat treatment used prior to the N‐balance studies was sufficient to destroy their activity. For the samples Masterpiece, Canadian Wonder and Processor, both with and without testa, and the isolated globulin fraction, the BV was highly correlated (P<0.001) with the total protein sulphur‐containing amino acid content.