Current Single-cell Proteins as Ingredients: Functions, Challenges, and Regulatory Frameworks.

Single-cell proteins (SCPs) broadly comprise microbial biomass or biomass-derived proteinaceous products produced from bacteria, yeasts, filamentous fungi, or microalgae. In the broader field of cell-derived alternative proteins, animal- and plant-cell-derived platforms are also being explored because they can reproduce the structural, compositional, and sensory equality of conventional food materials. In contrast, microbial SCPs are distinguished by diverse cellular structures, metabolites, cell-wall components, and bioactive fractions that may confer functionality beyond nutritional supplementation. This review summarizes recent progress in SCP-related platforms by first contrasting animal- and plant-cell-derived proteins with microbial SCPs, and then focusing on bacterial, yeast, filamentous fungal, and algal SCPs. Recent animal, human, and cell-based studies indicate that microbial SCPs and related microbial biomasses may modulate the gut microbiota, intestinal barrier function, immune and oxidative-stress responses, lipid metabolism, muscle protein synthesis, disease resistance, and product-quality traits. Nevertheless, microbial SCP development remains constrained by microbial-group-specific safety and processing issues, including nucleic acid reduction, toxin and contaminant control, and cell-wall-associated digestibility. Finally, this review summarizes regulatory frameworks for SCPs in the United States and the European Union, where commercialization remains product-specific rather than governed by a harmonized SCP category. Overall, SCPs represent promising but heterogeneous food and feed platforms that require integrated evaluation of nutritional value, functionality, safety, processing, and regulatory compliance.