Solubility-digestibility relationship of legume proteins

Protein solubility of raw and cooked faba bean, lentil, chickpea, and dry bean was tested in water and in NaCl in the pH range 1.0-13.0. The solubility of all legume proteins in water typically increased on both sides of pH 4.0. In NaCl, only solubility of raw dry bean proteins was improved. A marked reduction in protein solubility was observed after cooking of all legumes up to pH 10.0, where solubilization occurred, suggesting that it was dependent on deprotonation of lysine and arginine. Amino acid analysis showed that the protein fraction that retained solubility in water (pH 6.5) after cooking had a high amount of arginine and glutamic acid, low levels of hydrophobic amino acids, and, therefore, a much higher charge density than proteins in the whole flour. The SE-HPLC profiles indicated that water-soluble raw faba bean and lentil had main protein peaks of a higher molecular weight than those of dry bean or chickpea, thus suggesting a higher trend toward association. In vitro protein digestibility of faba bean and lentil, unlike that of chickpea and dry bean, was not improved upon cooking. The results indicate that, in addition to hydrophobic forces, basic residues are involved in the stabilization of heat-induced aggregates of legume proteins, possibly contributing to their low digestibility.