Circular Valorization of Agricultural Residues Into Food‐Grade Single‐Cell Protein via Carbon‐Fixation Fermentation

The conversion of agricultural residues into food‐grade single‐cell protein (SCP) has emerged as a promising strategy to simultaneously address increasing protein demand, food‐processing waste, and carbon emissions. However, current technological developments remain largely compartmentalized, limiting the realization of fully integrated circular biorefinery systems. This review critically evaluates the potential of coupling fruit‐processing residues with CO 2 /H 2 ‐based carbon‐fixation fermentation as an integrated platform for sustainable protein production and carbon utilization. Current evidence indicates that fruit‐processing residues provide abundant fermentable carbohydrates, although substantial variability in substrate composition, nutrient imbalance, and fermentation inhibitors frequently compromise microbial growth, protein yield, and product consistency. Experimental studies have demonstrated SCP concentrations of 18.83 g/L following optimized pretreatment, suggesting that selective release of fermentable substrates may be more influential than increasing pretreatment severity. Hydrogenotrophic gas fermentation has also produced microbial biomass containing 52%–74% protein, highlighting the potential of autotrophic cultivation to generate nutritionally valuable biomass while reducing dependence on food‐grade carbon sources. Recent metabolic engineering has further enhanced autotrophic performance, achieving biomass concentrations of 23.0 ± 0.8 g/L, indicating that targeted strain optimization can substantially improve carbon conversion efficiency. Collectively, the available evidence identifies gas–liquid mass transfer limitations, renewable hydrogen supply, feedstock heterogeneity, downstream processing, food safety assurance, and regulatory compliance as the principal barriers to industrial implementation. Importantly, current evidence indicates that experimental studies remain scarce on fully integrated continuous platforms that combine agricultural‐residue valorization, renewable hydrogen generation, CO 2 fixation, and food‐grade SCP production within a single process. Future advances will likely depend on coordinated innovations in reactor engineering, microbial robustness, nutrient recycling, food safety validation, and techno‐economic optimization to enable scalable circular protein biorefineries.