Solid-State Fermentation of Black Gram: Improvement of Protein Digestibility and Functional Properties for Composite Flour Applications

This study evaluated the effect of solid-state fermentation (SSF) using Aspergillus awamori on the nutritional, functional, antioxidant, structural, and protein-digestibility properties of black gram (Vigna mungo) flour and its application in wheat-based composite flour. Black gram was fermented for 0–12 days, and the 6-day fermented sample was selected for flour fortification based on improved nutritional and functional responses. SSF reduced tannin and phytic acid contents while increasing total phenolic content from 2.69 to 3.37 mg GAE/g, total flavonoid content from 0.83 to 1.65 mg QE/g, and DPPH radical-scavenging activity from 2.13 to 4.09 mg AAE/g by day 6. Protein content increased from 24.14% to 27.11%, and in vitro protein digestibility improved from 70.86% to 82.89%. Total amino acid content also increased from 88.71 to 98.24 g/100 g protein by day 6, followed by a decline during prolonged fermentation. Mineral contents, including iron, zinc, and calcium, were also improved after fermentation. Incorporation of 6-day fermented black gram flour into wheat flour enhanced the protein, crude fiber, water-holding capacity, oil-holding capacity, and in vitro protein digestibility of the composite flour. Overall, SSF with A. awamori improved the nutritional and functional quality of black gram and demonstrated its potential as a protein-rich ingredient for fortified cereal–legume composite flour development.