The nutritive value of decorticated mill fractions of wheat. 2. Evaluation with raw and enzyme treated fractions using adult cockerels

Abstract

The nutritive value of raw and enzyme treated whole grain wheat and six mill fractions produced by successive decortication was studied. The mill fractions, which varied considerably in chemical composition, consisted of one fraction enriched in pericarp/testa, one fraction enriched in aleurone and four fractions with graded levels of endosperm tissues. The whole wheat kernels and the six mill fractions constituted 895 g kg −1 of dry matter (DM) of the experimental diets, with fish meal as the only other main component added. The highest content of protein and gross energy (GE) was found in the fraction enriched in aleurone cells. However, a higher amount of non-starch polysaccharides (NSP) was also found in the pericarp/testa (385 g kg −1 DM) and aleurone (182 g kg −1 DM) enriched diets versus the endosperm enriched diets (less than 100 g kg −1 DM). Significantly negative correlations were found between apparent protein digestibility (APD) and insoluble non-starch polysaccharides (I-NSP) ( P < 0.05) and the apparent metabolisable energy (AME n) and I-NSP ( P < 0.001). In contrast, soluble NSP (S-NSP) did not significantly contribute to any negative effect on neither digestibility nor AME n. The APD and AME n values in the aleurone enriched diet were 84.7% and 12.8 MJ kg −1 DM, and in the pericarp/testa enriched diet 75.2% and 8.0 MJ kg −1 DM, respectively. The APD and AME n values in the endosperm rich diets were on average 88.6% and 15.6 MJ kg −1 DM, respectively. Results from this study also revealed that the cockerels were able to digest from 10.7% total NSP in pericarp/testa to an average of 42.8% in the endosperm based diets. The lower digestibility and AME n in pericarp/testa and aleurone enriched diets were caused by the thick cell walls in these layers made up of especially insoluble cellulose and arabinoxylans, which impose a reduction in the availability of nutrients. Enzyme treatment with cell wall degrading and proteolytic enzymes resulted in significant depolymerization and solubilisation of cell wall NSP and of starch. The AME n increased significantly in the diets based on the whole wheat kernel and pericarp/testa ( P < 0.05). The enzyme treatment improved APD significantly in diets with whole wheat kernel and aleurone ( P < 0.05). It is assumed that disruption of cell walls owing to enzyme action and release of cell wall content was primarily responsible for the observed improvement in digestibility and AME n. It is further believed that released cell wall monosaccharides probably have, to some extent, contributed energy to the cockerels.