Characterisation of an oat protein-beta-glucan co-extract

We characterise a novel, dilute protein concentrate produced via alkaline extraction of defatted oat flour (DOF) without additional protein purification and identify a self-assembly of oat protein and dietary fibre (primarily oat β-glucan). The oat protein-β-glucan co-extract (OPBG) was characterised using sodium dodecyl-sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and showed multiple protein bands, with major bands assigned to oat 12S globulin monomers (65-67 kDa) and its subunits (45 and 22 kDa). Circular dichroism (CD) spectroscopy revealed high quantities of β-sheet and random coil structures with evidence of partial protein unfolding at pH 2.0. The co-extract showed various degrees of aggregation when subjected to different pH (pH 2.0-10.0) and ionic strength (0-1.0 M added NaCl) conditions. At pH 2.0, β-glucan-β-glucan aggregates appeared dispersed in a continuous protein phase. At pH 7.0, OPBG displayed a compact aggregated microstructure with a hydrodynamic diameter (dH) of ∼100-150 nm and a polydispersity index of 0.28, where binary protein-β-glucan clusters co-existed with singular entities, i.e. protein-protein and β-glucan-β-glucan aggregates. Complementary ζ-potential measurements confirmed a negative surface charge at pH 7.0 and isoelectric point (pI) of 4.0-4.5. As the pH increased to 10.0, confocal laser scanning microscopy (CLSM) images revealed strong colocalisation of the protein and β-glucan, with consequent evolution of a smaller peak (dH ∼ 20 nm) resembling oat 12S globulin hexamers. Overall, hydrogen bonding appeared to dominate the protein-polysaccharide interactions in OPBG, whilst its colloidal properties, including responsiveness to pH and added salt, seemed to be largely governed by the proteinaceous fraction.