Effect of cooking and of amino acid supplementation on the nutritive value of black beans (Phaseolus vulgarisL.)

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Abstract
The effect of pressure cooking for various periods on the nutritive value of black-bean (Phaseolus vulgaris L.) protein was studied by chemical analysis and microbiological assessment of changes in methionine, lysine and valine contents, as well as by determination of free [epsilon]-amino groups of lysine and biological tests with rats. The results indicated that cooking for from 10 to 30 minutes at 16 lb pressure (121[degree]) was optimal. Longer cooking times decreased the nutritive value of bean protein. Open-kettle cooking for 4 hours was as good as pressure cooking within the 10-30 minute period. Rats fed on raw-bean diets with or without a methionine supplement died in less than 14 days. Cooking destroyed the toxic factor in beans. The concentrations of lysine, methionine and valine, microbiologically determined, were not changed by cooking. On the other hand, the content of free [epsilon]-amino groups of lysine decreased as cooking time increased. The first limiting amino acid in bean protein was found to be methionine, and the addition of 0.20% of this amino acid to the bean diet significantly improved the weight gain of the rats, the protein efficiency ratio (PER) and the biological value. The true digestibility of the protein was not improved by addition of methionine or other amino acids. It was also found that the addition of lysine and tryptophan to the methionine-supplemented bean diet improved the PER but not the weight gain of the rats. This improvement in PER seems important since beans have always been considered good sources of both amino acids. It is concluded that these amino acids are limiting in bean protein because of incomplete availability to the animal, as corroborated by the demonstrated low true digestibility of bean protein.