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

Abstract

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 (d_H) 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 (d_H ∼ 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.