Fermentation and Nutrient Bioavailability in Indigenous Crops: A Critical Review of Protein Quality, Glycaemic Response, and Micronutrient Enrichment for Snack Food Applications

Indigenous and underutilised crops such as sorghum, the millets, teff, fonio, cowpea, Bambara groundnut and the Andean pseudocereals combine dense micronutrient profiles with substantial phytate, tannin and protease-inhibitor loads that constrain their nutritional performance in unprocessed form. Fermentation is among the oldest and most widely practised interventions used to reconcile this tension, yet the literature addressing its effects remains fragmented across single-crop studies, single-outcome assays and disciplinary silos that rarely intersect with food product development. This review synthesises evidence on three interconnected outcome domains, namely protein digestibility and amino acid availability, glycaemic and starch-digestibility response, and mineral and vitamin bioavailability, and evaluates the extent to which fermentation-derived improvements in these domains can be translated into indigenous-crop snack foods. Evidence was drawn from peer-reviewed journal articles and institutional sources identified through structured searches of publicly accessible bibliographic platforms. Across crops and fermentation modalities, spontaneous and lactic-acid-bacteria-mediated fermentation consistently reduced phytate and condensed tannin concentrations and improved in vitro protein digestibility, with reported gains that varied considerably by substrate, microbial consortium and fermentation duration. Effects on glycaemic response were less uniform, reflecting a balance between organic-acid-mediated suppression of starch hydrolysis and matrix disruption that can accelerate it. Mineral bioaccessibility gains, expressed principally through reductions in phytate-to-mineral molar ratios, were the most consistently reported benefit, although evidence remains concentrated in a small number of crop-microbe combinations and in vitro assay systems. Extrusion and other snack-relevant processing steps interact with fermentation in ways that are only beginning to be characterised, and few studies have evaluated fermented indigenous-crop snacks under conditions resembling commercial manufacture. The review identifies methodological heterogeneity, limited in vivo confirmation, and a scarcity of studies that integrate all three outcome domains within a single product-development framework as the principal constraints on translating this evidence base into viable snack food innovation.