Brewer's Spent Grain as a Source of Proteins and Valuable Polysaccharides

Brewer's spent grain (BSG) is an abundant by-product rich in proteins and polysaccharides. This study evaluated ultrasound (US) to enhance alkaline protein extraction in 110 mM NaOH and to obtain a polysaccharide-enriched residue, with mechanical agitation (AG) as the control. First, 40 min extraction curves were evaluated at 25 and 50 °C and fitted to the Weibull model. At 25 °C, US increased the 40 min protein yield (40.8 ± 0.1 g/100 g initial protein) by 2.5-fold compared with AG; heating increased AG yield, whereas US showed negligible temperature sensitivity between 25 and 50 °C. Subsequently, extractions were performed at 25 °C in one or two 20 min cycles using fresh solvent. Extracts were recovered by pH-shift precipitation and freeze-dried. US maximised protein recovery (47.6 ± 0.1 g/100 g initial protein after two cycles) but co-extracted other compounds, reducing purity. AG extracts showed higher emulsifying activity (up to 9.6 ± 0.1 m²·g^-1), while US increased emulsion stability (up to 46 min). US residues showed up to ~35% lower glucose content than BSG (42 ± 2 g/100 g dry matter for BSG) and higher arabinoxylans (up to 23.5 ± 0.6 g/100 g dry matter; ~2.4-fold BSG), supporting a dual valorisation route based on the production of a protein-rich extract and an arabinoxylans-rich concentrate.