Dietary fibre contents and compositions of sorghum and sorghum-based foods

Abstract

The compositions of dietary fibre (DF) and acid detergent fibre (ADF) fractions of sorghum-based diets common in Sudan were investigated in relation to the nutritional characteristics of the diets as determined in rat feeding experiments. Energy availability decreased on cooking and 60% of the loss could be explained by the formation of a form of starch that resists enzyme digestion during DF determination. Polyphenols caused significant increases in the levels of lignin in the DF fraction from the various products, both before and after cooking. Fermentation to pH 3·9 before cooking counteracted the formation of enzyme-resistant starch, whereas immediate acidification to pH 3·9 did not. Low pH (3·9) inhibited the formation of lignin during cooking. Large amounts of protein were associated with the DF and ADF fractions of sorghum, which differs in this respect from cereals, such as wheat, maize, rye and barley. The levels of ADF-bound protein were correlated more closely with true protein digestibility values in the sorghum diet material than were those of DF-bound protein. The amino acid composition of the (low lysine) ADF proteins in all diets resembles that of kafirin. The amounts of protein that bind to the DF and ADF fractions increased on cooking the foods or if sorghum with high polyphenol content was used. Fermentation or acidification to pH 3·9 inhibited this protein binding effect in materials containing either high or low levels of polyphenols, reflecting the reduced binding of the kafirins under these conditions. The results are consistent with the observed beneficial nutritional effect of the traditional method of fermentation of sorghum diets in Sudan.