Effect of reducing agents on the in vitro protein and starch digestibilities of cooked sorghum

Abstract

A low-tannin sorghum cultivar M-35-1 was used in this study. Investigation showed that the in vitro protein digestibility (IVPD) decreased considerably when sorghum flour was cooked in water, while it increased when cysteine, sodium metabisulphite, or ascorbic acid were added to the cooking medium. The increase in the IVPD was significantly higher with increasing concentrations of cysteine up to 0.25 M and it continued to increase to 0.5 M for sodium metabisulphite; with ascorbic acid it increased up to 0.1 M then decreased. The in vitro starch digestibility (IVSD) of the treated gruel initially increased in the presence of either cysteine, sodium metabisulphite or ascorbic acid. The increase was parallel to that shown by IVPD; however, at high levels of cysteine or sodium metabisulphite the IVSD was low. Removal of cysteine from the gruel by alcohol gave higher IVSD. Altered viscosity patterns for all the treatments led to increase in the gelatinization temperature, peak viscosity and breakdown. However, the setback decreased in all treatments. Cysteine and ascorbic acid gave a negative setback but when the pH was adjusted to 4.5 or 7.0 a normal setback was obtained. ©