Circular Valorization of Brewer’s Spent Grain and Brewer’s Yeast: Pathways, Sustainability Implications, Techno-Economic Feasibility, and Policy Perspectives

Brewer’s spent grain (BSG) and brewer’s yeast (surplus yeast) account for approximately 85–90% of total solid residues generated in beer production and represent strategic resources within circular bioeconomy frameworks. This review synthesizes 38 peer-reviewed studies published between 2010 and 2024, identified through a Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)-aligned screening process using Google Scholar. Valorization pathways across food, feed, biotechnology, environmental applications, and energy recovery were assessed with regard to sustainability performance, technology readiness levels (TRL), and techno-economic feasibility. Reported data indicate methane yields of approximately 200–400 m3 methane (CH4) t−1 volatile solids (VS) for anaerobic digestion of BSG, while protein contents range from 19 to 30% in BSG and 45–55% in yeast (dry matter basis). Technology readiness is highest for feed applications and anaerobic digestion (TRL 7–9), whereas advanced biochemical and material applications remain at intermediate levels (TRL 4–7). Sustainability outcomes are strongly influenced by stabilization energy demand, logistics, and substitution effects. Overall, BSG is primarily suited to high-volume, mass-based and energy-oriented pathways, whereas brewer’s yeast enables lower-volume, higher-value applications. Based on comparative assessment, three strategic development trajectories—energy-oriented integration, material and feed valorization, and advanced biochemical pathways—are identified as key transition routes toward brewery-centered circular biorefinery systems.