Lactic acid bacteria fermentation-driven protein-based gelation: Mechanisms, structure-function relationships, and...

Protein-based gels are central structural elements in fermented foods, but their formation during lactic acid bacteria (LAB) fermentation cannot be adequately explained by acidification alone. Although pH reduction and isoelectric aggregation initiate gelation in many systems, the final network architecture and functionality are also governed by exopolysaccharide (EPS) production, proteolysis, ionic interactions and the initial colloidal state of the protein matrix. This review reinterprets LAB fermentation-driven protein gelation using a strain-metabolite-protein colloidal state-gel functionality framework. Within this framework, strain-specific traits determine acidification kinetics, EPS yield and structure, proteolytic activity and ionic microenvironment; these factors collectively modulate protein charge, conformational stability, hydrophobic exposure, peptide formation, ion bridging and protein-polysaccharide compatibility. Importantly, the same mechanism may produce opposite outcomes depending on the matrix: EPS can reinforce networks by bridging and pore filling but may also promote incompatibility or phase separation; controlled or limited proteolysis can expose reactive sites, generate crosslinkable peptides and enhance network formation, whereas excessive hydrolysis weakens network continuity; divalent ions can strengthen gels through bridging but may induce coarse aggregation when unbalanced. We further compare dairy, plant, meat and microbial protein systems to identify matrix-dependent control targets, including acidification rate, endpoint pH, EPS molecular features, degree of hydrolysis, ionic strength, fermentation temperature and fermentation duration. Finally, we highlight current knowledge gaps, particularly the lack of standardised quantitative reporting and predictive models linking microbial metabolism, colloidal transitions and gel functionality. This review provides a mechanistic basis for rational starter selection and process design in fermented protein gel systems.