Comparison of acid and enzymatic hydrolyses of oat bran β-glucan at low water content

The effect of acid- or enzyme-catalysed hydrolysis on partial depolymerisation of β-glucan in oat bran was studied. Hydrolyses were performed at relatively low water content (50% dry matter) using high shear mixing in a twin-screw extruder. The hydrolysed oat brans were extracted with hot water and centrifuged to obtain a water-soluble phase and an insoluble residue. The time-dependent gelling of the water-soluble phase was monitored for 14 weeks at 5 °C. Acid-hydrolysis required a short reaction time (3 min) to depolymerise the β-glucan molecules from their original average Mw of 780,000 to 34,000 g/mol. After acid-hydrolysis, β-glucan had low polydispersity (4.0-6.7). Longer incubation time (3-4 h) was needed for enzymatic depolymerisation of the β-glucan molecules down to 71,000-49,000 g/mol. Enzymatic hydrolysis resulted in high polydispersity (19.0-24.2). The concentration and Mw of β-glucan significantly affected the gelling of hot water extracts. At 1.4-2.0% β-glucan concentration, solutions of β-glucan molecules with Mw > 50,000 g/mol agglomerated rapidly, whereas solutions of smaller molecules (34,000-49,000 g/mol) remained as stable dispersions for longer. Gelling was strongly concentration-dependent and at 1.4 to 1.6% beta-glucan concentration gelling occurred after 7 to 12 weeks of storage, whereas at 1.8 to 1.9% concentration gelling occurred already after 2 weeks of storage.