Towards Sustainable Recovery of Phenolics, Proteins, and Arabinoxylans from Brewer's Spent Grain Through Deep Eutectic Solvents: Extraction Strategies, Structure-Function Relationships, and Food Biorefinery Perspective.

Brewer's spent grain (BSG) is the primary by-product of the brewing industry and a low-cost lignocellulosic resource rich in phenolics, proteins, and arabinoxylans (AXs). Conventional recovery methods using acids, alkalis, and organic solvents often involve energy-intensive processes, generate hazardous waste, and limit food-grade applications. This review critically examines the evolution of extraction methodologies for BSG bioactives, highlighting the potential of deep eutectic solvents (DES) as sustainable alternatives. Key factors such as solvent chemistry (polarity, pH, and water content) influence bioactives' recovery and selectivity. Process intensification techniques such as ultrasound, microwave, and pressurised liquid extraction enhance efficiency by reducing extraction time and temperature. The review assesses how various processes modify the structure-function properties of BSG bioactives, including antioxidant activity, protein functionality, and rheological behaviour. A significant finding is that DES research has primarily focused on single compounds, while integrated DES biorefineries for comprehensive valorisation remain underexplored. Future research should therefore prioritise integrated process design that balances recovery, structural preservation, functionality, and sustainability to support scalable, near-zero-waste BSG valorisation for food and nutraceutical applications.