Comparison of Two Inoculum Preparation Protocols for Improving the Nutritional Quality of Soybean Meal Fermented with Aspergillus niger and Candida tropicalis

This study compared two inoculum preparation protocols for the mixed fermentation of unsterilized soybean meal (SBM) with Aspergillus niger and Candida tropicalis. The mono-culture-derived (MO-C) inoculation protocol involved separately cultivating A. niger and C. tropicalis and subsequently inoculating both cultures into SBM, whereas the co-culture-derived (CO-C) inoculation protocol involved cultivating both microorganisms together in the same medium before inoculating into SBM. Compared with the MO-C treatment, the CO-C treatment resulted in a slightly higher apparent crude protein content and significantly increased the acid-soluble protein and free amino acid contents in fermented soybean meal (FSBM), with increases of 9.22% and 141%, respectively. The CO-C treatment was also associated with greater reductions in β-conglycinin and raffinose-family oligosaccharides (RFOs), as well as higher DPPH and ABTS radical-scavenging activities and ferric-reducing power of FSBM. These findings suggest that the co-culture inoculum protocol provides a simplified approach for preparing mixed inocula and may improve selected nutritional and functional properties of FSBM under the tested conditions.