Recent Progress in Conventional and Environmentally Sustainable Extraction of Phenolic Compounds from Barley Waste

Barley grits (BSG), a major byproduct of the brewing industry, represents an abundant and underutilized lignocellulosic biomass enriched with valuable bioactive phenolic compounds, including phenolic acids, flavonoids, and anthocyanins. This review evaluates 83 peer-reviewed studies focused on the extraction of phenolic compounds from barley waste. It provides a comprehensive overview of conventional extraction techniques, such as Soxhlet extraction, solid-liquid extraction (SLE), and alkaline hydrolysis (AH), and advanced green extraction technologies, including ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), and other green methods such as supercritical fluid extraction, pulsed electric field extraction, and pressurized liquid extraction, highlighting the performance of each method for the recovery of total phenolic content (TPC) and the individual phenolic compounds obtained. Furthermore, biotechnological approaches including enzymatic hydrolysis, as well as the application of a new deep eutectic solvent (DES) for phenolic extraction is disscussed. In addition, the influence of extraction parameters, including extraction temperature, extraction time, solvent type and solid to solvebt ratio is discussed to clarify their role in extraction efficiency and process optimization.