Influence of naturally acid-soluble proteins from beans (Phaseolus vulgaris L.) on in vitro digestibility determination

The in vitro digestibility of bean (Phaseolus vulgaris) protein fractions was studied using a pepsin-pancreatin system. Enzymatic hydrolysis was stopped by adding a strong acid and the extent of proteolysis determined by measurement of free amino groups in the soluble fraction. The in vitro digestibility of bean protein fractions was low when in the native state and was differently affected by denaturation. For phaseolin, the main reserve protein, heating caused a significant increase of susceptibility to hydrolysis, whereas heat had no apparent effect on digestibility of glutelins and albumins (II). For the PIL (protease inhibitor-lectin rich) fraction, which was shown to have a composition similar to total albumins, there was a decrease of digestibility, probably associated to disulfide bond formation upon heating. Results of in vitro digestibility were shown to be strongly dependent on the utilization of a sample blank to account for proteins naturally soluble in the acid used to interrupt hydrolysis, which would otherwise be estimated as digested protein. These proteins are characterized by a high carbohydrate content, probably responsible for their high solubility and low digestibility.