A study of the in vitro protein digestibility of indica and japonica cultivars

Protein digestibility was determined in 18 indica and japonica raw milled rices using an in vitro pH-drop method with three- or four-enzyme system. Similar protein digestibility was found between indica and japonica rices, which is in agreement with the in vivo digestibilities in human. Cooking improved protein digestibility in the four-enzyme assay, while reducing agents exhibited apparent inhibition in multienzyme digestibility of indica and japonica rices. A significant correlation was detected between protein content and the estimates of digestibility, whereas no significant correlation was found between amylose content and digestibility estimates. SDS–PAGE analysis showed a significant difference in the degradation extent of prolamin between multienzyme and pepsin digestion, which might contribute to the inconsistence between results of this study and previous findings that in vitro protein digestibility of japonica rice was higher than that of indica rice. In addition, our results supported the previous report that waxy gene product level is not a major determinant of protein digestibility in milled rice.