Efficient conversion of insoluble dietary fiber to soluble dietary fiber by Bacillus subtilis BSNK-5 fermentation of okara and improvement of their structural and functional properties

Bacillus subtilis exhibits strong adaptability and biotransformation potential in the fermentation of okara, but the effects of fermentation on their dietary fiber remain unclear. This study explored the impact of Bacillus subtilis BSNK-5 fermentation on converting insoluble dietary fiber (IDF) to soluble dietary fiber (SDF) in okara, focusing on structural and functional changes. After 72 h of fermentation, SDF increased 7.51-fold. The surface folds of fermented IDF were reduced. Meanwhile, SDF displayed a more porous structure with significant changes in its crystalline structure. FTIR analysis showed that surface disruption exposed both hydrophilic and hydrophobic groups. Thermal analysis showed that the peak of maximum degradation moved to a lower temperature. Both fermented SDF and IDF exhibited antioxidant activity, effective lipid- and glucose-lowering effects. These findings suggest that BSNK-5 effectively transforms IDF into SDF, with fermented dietary fiber showing great potential as a functional ingredient in the food industry.