Effect of cooking and treatment with sodium bisulphite or ascorbic acid on the in vitro protein digestibility of two sorghum cultivars

Two sorghum cultivars, Karamaka (high in tannin, 3.1%) and Gadam Elhamam (low in tannin, 0.35%), were used in this study. The effect of sodium bisulphite and ascorbic acid on in vitro protein digestibility of the two sorghum cultivars was investigated. Varying concentrations of reducing agents, 0.05, 0.1, 0.25 and 0.5 M, and pepsin digestion times, 30, 60, 90, 120, 150 and 180 minutes, were considered. Reducing agents improved digestibility for cooked and uncooked samples. 0.1 M concentration provided the highest level of digestibility for both reducing agents. Cooking reduced in vitro protein digestibility for both cultivars. Formation of disulphide bonds in cooked samples is probably responsible for the reduced protein digestibility. Reducing agents prevent formation of disulphide bonds and improve the in vitro protein digestibility. For all treatments, Karamaka holds a lower level of in vitro protein digestibility than Gadam Elhamam cultivar. This could be attributed to the higher tannin content in Karamaka cultivar.