Biotransformation of okara (soybean residue) through solid-state fermentation using probiotic Bacillus subtilis and Bacillus coagulans

Okara or soybean residue is a side-stream product of soybean processing generated in large quantities. However, okara is often underutilised as it is prone to spoilage and has a short shelf life. In this study, we explored the feasibility of biotransformation of okara through solid-state fermentation using probiotic Bacillus spp. to modify its nutritional value and flavour property. Two probiotic Bacillus species, namely Bacillus subtilis CU1 (CNCM I-2745) and R0179, and Bacillus coagulans IS-2 and 123 were firstly screened. Further analyses were performed on the B. subtilis R0179 and B. coagulans 123 fermented okara due to their better growth after 24 h (10.28 and 9.24 log colony forming units (CFU)/g respectively). Our results showed that B. subtilis R0179 converted a significant portion of okara insoluble dietary fibre into soluble dietary fibre after fermentation, but not B. coagulans 123. In addition, the samples fermented with B. subtilis R0179 showed higher amounts of maltose, hemicellulosic sugars as well as aromatic, branched-chain and hydrophobic amino acids. Furthermore, the B. subtilis R0179 transformed okara had greater amounts of acetoin, carboxylic acids, pyrazines and branched-chain fatty acids, which may contribute to characteristic natto-like and cheesy flavour. Overall, our study shed light on the possibility of using probiotic bacilli to transform okara with enhanced nutritional value and flavour property, which may increase okara usage in foods.