Optimisation of ultrasound-assisted choline chloride : glycerol NaDES extraction of protein- and β-glucan-rich fractions from defatted oat flakes and comparison with barley flakes

Abstract

Sustainable cereal biorefinery processes require extraction methods that can efficiently recover valuable biomolecules while reducing the environmental impacts associated with conventional alkaline extraction. Oat and barley biomasses are rich in nutritionally essential proteins and β-glucan, but the recovery of these components tends to be limited by the complex structure of cereal cell walls and interactions between proteins, polysaccharides, and other matrix components. The potential of ultrasound-assisted natural deep eutectic solvent (NaDES) extraction to recover protein- and polysaccharide-rich fractions from cereal biomasses was examined in this study. The primary matrix for optimisation was the defatted oat flakes, whereas barley flakes were used as a comparison matrix to evaluate the applicability of the optimised extraction conditions to another cereal biomass. Hydrated choline chloride:glycerol (ChCl:Gly) NaDES systems with molar ratios from 1:1 to 1:3, water contents of 20–50% and ultrasound treatment times of 5–25 min were analysed by a three-factor Box–Behnken response surface design. The primary response was the protein concentration, and the secondary response was the β-glucan concentration. Increasing the water content reduced the viscosity of the NaDES systems from approximately 29–34 mPa·s to 4–5 mPa·s and increased the conductivity, indicating better solvent mobility and mass-transfer potential. Protein extraction was greatly affected by solvent composition and water content. The maximum soluble protein concentration (17.51 g BSA equivalents/100 g dry weight) was obtained using a ChCl:Gly ratio of 1:3, 35% water content and 25 min ultrasound treatment. Response surface modelling gave an acceptable model (R2=0.82; lack-of-fit p=0.295) and a set of optimised extraction conditions at a ChCl:Gly ratio of 1:2, 35 % water content and 15 min ultrasound treatment. Validation experiments indicated satisfactory agreement between the predicted and experimental values. The highest β-glucan content (0.0813 g/100 mL) was obtained at a higher water content than the condition favouring maximum protein extraction. This suggests that protein and β-glucan had different extraction requirements. Overall, the results indicate that the combination of hydrated choline chloride:glycerol NaDES systems with ultrasound provides a promising platform for the recovery of cereal proteins and polysaccharides. Water content was shown to be the most important process variable affecting solvent characteristics, extraction performance and extraction selectivity. The findings support the design of more sustainable cereal biorefinery processes and provide a basis for further optimisation of NaDES-assisted fractionation of protein- and polysaccharide-rich components from oat and barley biomasses.