Solid-State Fermentation of Legumes: Multi-Scale Structural Modifications to Unlock Techno-Functional Properties

Legumes are increasingly used as functional ingredients in plant-based foods. However, their compact endogenous structure restricts techno-functional properties. Conventional fractionation methods that isolate proteins further impair these properties and generate co-products. Solid-state fermentation (SSF), which involves growth of microorganisms on moist solid legumes for in situ enzymatic modification, is a gentle and sustainable strategy to enhance techno-functionality. However, the mechanistic understanding of structural changes underpinning functional modifications in legume ingredients remains limited. This review critically evaluates SSF as a multi-scale structural modification strategy across cellular, microstructural, and molecular scales to improve the techno-functionality of legume-based ingredients. Factors influencing the SSF process are briefly discussed. Key research gaps are identified, and future directions are proposed to advance understanding of process-structure-function relationships in SSF-derived legume ingredients. SSF-induced process-structure-function relationships in legume ingredients are established within a multi-scale structural framework. Cellular breakdown during SSF liberates starch and protein, overcoming hydration restrictions. Under the microstructural scale, breakdown of protein aggregates improves solubility and interfacial behavior, while surface erosion and crystalline rearrangement of starch granules modulate pasting and thermal properties. At the molecular scale, hydrolysis of multimeric proteins into smaller peptides; protein unfolding that increases surface hydrophobicity, charge, and sulfhydryl content; and alterations in amylose-amylopectin ratio of starch underpin improvements in water and oil binding, interfacial, gelling, and pasting properties. Integrating multi-scale structural insights with microbial growth, enzyme activity and metabolomics will enable reverse engineering of SSF for targeted design of functional legume ingredients.