Perspectives into factors limiting in vivo digestion of legume proteins: antinutritional compounds or storage proteins?

The in vivo protein digestibility of raw and cooked common bean (Phaseolus vulgaris L.) and faba bean (Vicia faba L.) and of protein fractions extracted from them was determined with growing rats. Overnight-fasted rats were intubated with a protein suspension or fed the same amount of protein added to a basal diet. The rats were killed 1 h later, the contents of stomach and small intestine were washed out, and their protein contents were measured. The in vivo digestibility of proteins of raw common bean flour was 72.4% and not significantly improved after cooking. In contrast, the digestibility of faba bean proteins was decreased from 86.5 to 60.6% by the thermal treatment. Globulins from either species had similar digestibilities (approximately 70%). Proteins in the soluble fraction of cooked beans were more digestible than those in the insoluble fraction, which contained the bulk of the proteins. Hemagglutination assay and trypsin inhibitor determination indicated that after the thermal treatment only very low, nonharmful, levels of both lectin and inhibitor remained. Faba bean contained more polyphenols than common bean samples, with most of the polyphenols being bound to globulins. However, protein-bound polyphenols were markedly decreased after cooking. SDS-PAGE characterization of the gastrointestinal digesta of globulins and amino acid analysis of undigested proteins of whole cooked common bean and faba bean suggested that it is mainly the structural properties of the storage proteins and not their binding of polyphenols, which determines the extent of protein aggregation on autoclaving and may therefore be responsible for their low digestibility.