IN-VITRO PROTEIN DIGESTIBILITY OF DEVELOPING AND MATURE SORGHUM GRAININ RELATION TO ALPHA-KAFIRIN, BETA-KAFIRIN, AND GAMMA-KAFIRIN DISULFIDE CROSS-LINKING

Sorghum (P721N, 1992) was harvested at selected days after half-bloom (DAHB) and at maturity, and analyzed for protein and moisture contents , protein digestibility, alpha-, beta-, and gamma-kafirin contents, an d unextractable disulfide-bound complexes. alpha-Kafirin synthesis beg an before 10 DAHB, and beta- and gamma-kafirin at 20 DAHB. AU the kafi rins were as abundant at 40 DAHB as at maturity. Protein digestibiliti es of uncooked flour were about 90%, and dropped to 73% at maturity. D igestibilities of cooked flour dropped markedly at 35 DAHB, 40 DAHB an d maturity. A comparison of 1992 and 1987 data indicates that digestib ility decreases with moisture content, not days of development. The am ount of disulfide ''crosslinked'' beta- and gamma-kafirins was insigni ficant at the earlier stages, though increased as the grain matured. S ince kafirin synthesis is complete by 40 DAHB, we suggest that the dec rease in digestibility in maturing grain is due to the drying effect a nd formation of disulfide-bound complexes involving beta- and gamma-ka firins. Cooking the flour may further promote protein interactions, pa rticularly after kafirin synthesis has ceased. (C) 1995 Academic Press Limited