MOLECULAR-WEIGHT CHANGES IN THE (1-]3)(1-]4)-BETA-D-GLUCAN OF OATS INCURRED BY THE DIGESTIVE PROCESSES IN THE UPPER GASTROINTESTINAL-TRACT OF PIGS

Pigs were fed three diets containing different mill fractions from oats-oat flour, rolled oats, and oat bran. The molecular weight of the (1-->3)(1-->4)-beta-D-glucan (beta-glucan) in diets and from the jejunal contents of the pigs was determined by size exclusion chromatography. There was no significant difference in the molecular weight (MW) of the beta-glucan in the diet, regardless of origin. MW was reduced 7-35-fold after passage through the stomach and the proximal small intestine of pigs, oat bran being least affected. Depolymerization toward a specific MW range of about 100 000 was seen in some digesta samples, indicating cleavage at randomly distributed susceptible bonds. The oat fractions themselves had some capacity to degrade beta-glucan, but this was insufficient to account for the MW loss during digestion. Degradation also took place when isolated beta-glucan was incubated with jejunal digesta from pigs fed an oat-free diet. Fractionation of the digesta by ultracentrifugation showed that the degrading activity was mostly associated with the supernatant. Since autoclaved digesta were without effect, the activity was presumably enzymatic, originating from microbes present in the gut. Neither isolated beta-glucan nor crude extracts from oat bran were sensitive to trypsin, indicating that cleavage of peptide bonds was not responsible for the reduction in molecular weight and loss of viscosity in digests from pigs.