Biological valorization of non-food feedstocks: A critical review

Valorizing non-food feedstocks via biological conversion offers a promising route to advance a circular economy without compromising food security. However, non-food feedstocks present distinctive and multifaceted challenges in bioconversion owing to their intrinsic recalcitrance, compositional heterogeneity and inhibitory components. Relative to starch-based, food-derived substrates, these characteristics often translate into lower conversion yields and more complex process integration, thereby constraining cost-competitive deployment at commercial scale. In addition, the recovery and upgrading of waste streams generated during non-food bioconversion are essential for improving system-level carbon efficiency, environmental performance and techno-economic viability. Accordingly, there is a clear imperative for a comprehensive review that clarifies the mechanistic linkages among non-food feedstock characteristics, biological conversion routes and waste upgrading strategies, while synthesizing process-optimization strategies and state-of-the-art advances for non-food bioconversion. Here, we synthesize major non-food feedstocks focusing on sources, composition and pretreatment requirements, and assess how these features shape feasible conversion routes and their limitations. The current challenges and recent progress in biological conversion technologies are then meticulously examined, spanning established platforms (anaerobic digestion and composting) and advanced microbial fermentation driven by synthetic biology and multi-omics. We further examine representative waste-derived intermediates and recent progress in their biological upgrading. Finally, we integrate recent TEA and LCA evidence to identify feasibility drivers and deployment-relevant implications for non-food-feedstock biorefineries, providing clear evaluations and strategic guidance for researchers, policymakers and investors.