The nutritive value of decorticated mill fractions of wheat. 3. Digestibility experiments with boiled and enzyme treated fractions fed to mink

The nutritive value of boiled and enzyme treated whole grain wheat and six mill fractions prepared by successive decortication of wheat kernels was studied in 15 digestibility trials with mink. The decortication process yielded mill fractions varying considerably in morphological and in chemical composition. The whole kernels (WK) and each of the six mill fractions (F1–F6) were provided to five adult black male mink either raw (only WK), boiled with water (15 min) or after treatment with cell wall degrading and proteolytic enzymes and mixed into complete diets to comprise 450–500 g wheat dry matter (DM) kg−1 DM. Diet composition was (g kg−1): cod meat 750, wheat fractions 190, lard 40, soy oil 10, vitamin mixture 5 and mineral mixture 5. The amount of starch in the diets based on boiled fractions ranged from 85 g kg−1 DM in the diet prepared from the outer fraction enriched in pericarp/testa (F1) to 300 g kg−1 DM in the diet containing the endosperm enriched inner fraction (F6). Total non-starch polysaccharides (NSP) ran conversely, constituting 213 g kg−1 DM in diet F1 vs. 50 g kg−1 DM in diet F6. Enzyme treatment resulted in a significant depolymerisation and solubilisation of cell wall NSP and of starch. The digestibility of energy was significantly higher (∼3 absolute units) in the diets with enzyme treated wheat fractions compared to the corresponding diets with boiled products in which energy digestibilities ranged from 70.7% to 90.6%. These differences were mainly due to higher digestibility of the carbohydrates and to a lesser extent to higher protein digestibility in the enzyme treated fractions. The digestibility of crude carbohydrates (calculated by difference) was consistently higher in enzyme treated fractions compared to boiled fractions, the differences varying from 13.4 absolute units (37.4% vs. 24.0%) in F1 to 5.4 absolute units (80.7% vs. 75.3%) in F6. Starch digestibility was about 90% for both boiled and enzyme treated fractions showing slightly, but significantly lower values in enzyme treated diets. Therefore, the higher digestibility of crude carbohydrates in enzyme treated fractions could not be ascribed to higher starch digestibility but to depolymerisation of NSP and to higher digestibility of the remaining NSP in enzyme treated fractions (28.1–34.9%) compared with boiled fractions (12.4–25.3%). A significant positive digestibility of NSP in both boiled and enzyme treated fractions combined with substantial amounts of short-chain fatty acids in faecal material suggest that microbial degradation of cell wall NSP can occur in mink despite the high rate of passage and the relatively low microbial activity in the gastrointestinal tract.