Linking tannin reduction with structural and functional changes in sorghum flour during Rhizopus oligosporus fermentation

Solid-state fermentation using Rhizopus oligosporus offers a promising strategy to improve the nutritional and functional quality of sorghum flour. This study investigated the effects of varying inoculum concentrations (1-4%, v/w) on the physicochemical, structural, and functional properties of sorghum flour during 48 h of fermentation. The results demonstrated a significant reduction in extractable condensed tannin content, with a maximum decrease of approximately 79.07%. Fermentation was also associated with an increase in protein content from 7.01% to 8.12%, enhanced DPPH radical scavenging capacity, and increases in total phenolic acid- and flavonoid-related parameters. Functional properties were also modified, characterized by reduced water solubility index (WSI) and viscosity, while water absorption index (WAI) and swelling power (SP) remained relatively stable. Structural analyses using FTIR and SEM indicated fermentation-associated modifications in the sorghum flour matrix, accompanied by progressive microstructural changes that may influence molecular accessibility. In addition, fermentation induced measurable color changes, characterized by increased lightness (L*) and a shift toward yellowish hues, which may be associated with changes in phenolic-related compounds during fermentation. Overall, the results demonstrate that R. oligosporus fermentation is associated with compositional, structural, and functional changes in sorghum flour. These findings highlight the potential of controlled fungal fermentation as an approach to develop sorghum-based ingredients with modified nutritional and functional characteristics.