The nutritive value of decorticated mill fractions of wheat. 1. Chemical composition of raw and enzyme treated fractions and balance experiments with rats

Abstract

The chemical composition and nutritive value of raw and enzyme treated whole grain wheat and six mill fractions were studied. The six mill fractions were produced by decortication and consist of one fraction enriched in pericarp/testa, one fraction enriched in aleurone and four fractions with graded levels of endosperm tissues. It was possible, by the advanced milling procedure, to produce mill fractions varying strongly in chemical composition and nutritive value. In the fractions enriched in aleurone cells the concentration of minerals was thus increased two to three times while fat and lysine levels and utilizable protein were 30–40% higher. The digestibility of energy and true protein digestibility were negatively correlated to the dietary fibre level with the lowest values found in the fraction enriched in pericarp/testa (24.0%) and the highest value in the endosperm rich fraction (74.1%). The lowest digestibility of cell wall non-starch polysaccharides was found in the pericarp/testa enriched fraction where the digestibility of arabinoxylans and cellulose was almost the same while the digestibility of arabinoxylans was significantly higher than that of cellulose in the walls of aleurone and endosperm. The strong negative correlation between the digestibility of xylose residue and the arabinose:xylose ratio indicates that the digestibility of arabinoxylans varies in response to the degree of branching. Biological value was higher in the fractions enriched in pericarp/testa, aleurone and germ tissues (≈ 62%) than the whole kernel (56.6%). The highest protein utilization was found in the fraction enriched in aleurone tissues (11.3–11.7%). Enzyme treatment with cell wall degrading and proteolytic enzymes resulted in a significant depolymerization and solubilization of cell wall non-starch polysaccharides and of starch. The digestibility of energy was increased on average by 0.7% (absolute units) by enzyme treatment. The effect of enzyme treatment on true digestibility and biological value was in most cases limited and it was only for the pericarp/testa rich fraction that true digestibility and biological value were significantly improved after enzyme treatment.