Biochemical basis for the in vitro digestibility of proteins in raw and cooked indica and japonica rice (Oryza sativa L.) with low amylose content

Endosperm proteins of two low amylose indica rice varieties, both cooked and uncooked, were found to be less digestible than those of five japonica varieties in vitro. Cooking reduced protein digestibility from 84.4 percent to 79.3 percent and showed a more pronounced effect on japonica varieties than on indica varieties, markedly increasing the quantity of indigestible proteins in the former. Mean true digestibility (TD) of cooked rice protein in growing rats was significantly lower in indica varieties (94.0 percent) than in japonica (98.7 percent). Nevertheless, the biological value (BV) and net protein utilization (NPU) of the indica varieties were comparable to some japonica varieties. Amino acid analysis of cooked rice proteins showed lysine (3.2 percent) to be limiting while cystine content was negatively correlated with in vivo (r = -0.83] and in vitro (r = -0.88*] protein digestibility. Based on amino acid score and TD, protein quality of the japonicas (58 percent) was relatively higher than the indicas (49 percent). Prolamin: albumin-globulin:glutelin ratios were 16:5:79 for raw rice and 17:4:79 for cooked rice. Characteristic electrophoretic bands were noted in the prolamin (16- and 13- kDa), albumin-globulin (26-kDa) and glutelin (57-, 40- and 20-kDa) fractions. Reduction of prolamin extractability and albumin-globulin content were more pronounced in cooked indica varieties. Protein digestibility did not correlate with prolamin content but correlated significantly with the amount of poorly soluble prolamin (r = -0.71* [in vitro]; r = -0.82* [in vivo]. Electrophoretic analysis of residual proteins, from both gelatinized and ungelatinized residues, revealed the predominance of low molecular weight proteins in the range of 10-18 kDa, particularly in indica varieties. These are presumably prolamins resistant to digestion as has been demonstrated in earlier reports. Furthermore, the content of waxy gene protein of raw rice after pepsin/pancreatin digestion was comparable to initial levels (0.08 percent) and accounted for about 6 percent of the total undigestible proteins. Overall, the digestibility differences of indica and japonica rice were explained by the resistance of prolamins to digestion but not the prolamin content per se. The lower protein digestibility of indica rice compared to japonica was attributed to the higher cystine and higher content of poorly soluble prolamin of cooked indica rice. Other possibilities suggested by some results, e.g., disulfide cross-linking and protein-starch interaction need further study