Biochemical basis of the in vitro digestibility of proteins in raw and cooked indica and japonica rices (Oryza sativa L.) with low amylose content
2000
Duldulao, J.B.A. | Juliano, B.O. | Mendoza, E.M.T. (Philippine Rice Research Inst., Maligaya, Munoz, Nueva Ecija (Philippines))
Endosperm proteins of two low amylose indica rice, both cooked and uncooked, were found to be less digestible than that of five japonicas in vitro. Cooking reduced protein digestibility from 84.4% to 79.3%. Mean true digestibility of cooked rice protein in growing rats was significantly lower in indica (94.0%) than in japonica (98.7%) rice. Nevertheless, the biological value and net protein utilization of indicas were comparable with japonicas. Lysine (3.2%) was limiting in cooked rice proteins. Cystine content showed significant correlation to in vivo (r = -0.83*] and in vitro (r = -0.88*] protein digestibility. Prolamin content of protein did not show a significant correlation to protein digestibility. In vivo protein quality of the japonicas (58%) was relatively higher than the indicas (49%). Prolamin: albumin-globulin: glutelin ratios of raw and cooked rice were 16:5:79 and 17:4:79. Characteristic eletrophoretic bands were noted in the prolamin (16- and 13-k Da), albumin-globulin (26-kDa) and glutelin (57-, 40- and 20- kDa) fractions. Cooking reduced prolamin extractability and albumin-globulin content more in indica varieties. Electrophoretic analysis of residual proteins showed the resistance of the waxy gene protein to digestion and the predominance of low molecular weight peptides of 10-18 kDa, presumably prolamins, particularly in indica varieties
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