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 oat s-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 t he beta-glucan in the diet, regardless of origin. MW was reduced 7-35- fold after passage through the stomach and the proximal small intestin e of pigs, oat bran being least affected. Depolymerization toward a sp ecific MW range of about 100 000 was seen in some digesta samples, ind icating cleavage at randomly distributed susceptible bonds. The oat fr actions themselves had some capacity to degrade beta-glucan, but this was insufficient to account for the MW loss during digestion. Degradat ion also took place when isolated beta-glucan was incubated with jejun al digesta from pigs fed an oat-free diet. Fractionation of the digest a by ultracentrifugation showed that the degrading activity was mostly associated with the supernatant. Since autoclaved digesta were withou t effect, the activity was presumably enzymatic, originating from micr obes present in the gut. Neither isolated beta-glucan nor crude extrac ts from oat bran were sensitive to trypsin, indicating that cleavage o f peptide bonds was not responsible for the reduction in molecular wei ght and loss of viscosity in digests from pigs.