Extraction of Heat Stabilised Defatted Rice Bran Protein by Solid-State Fermentation Using Heterofermentative Microbes from Asian Traditional Starters

Research background. Heat stabilised defatted rice bran (HSDRB) is a primary by-product of rice bran oil extraction industries and a nutritious protein source. However, despite the unique nutritional profile of RB protein, the protein-rich by-product, HSDRB is underutilized as low-value animal feed. Research on protein extraction from HSDRB via enzymatic hydrolysis has gained the attention of numerous scholars. However, a cost-effective extraction method is required to mitigate the high expenses associated with enzymes. Therefore, we presented an alternative economical and natural approach for HSDRB protein extraction using solid-state fermentation (SSF) with heterofermentative microbes. Experimental approach. SSF of HSDRB with two kinds of Asian traditional fermentative starters, namely Loog-pang and Koji, were investigated for enzyme production and their efficacy in extracting protein from HSDRB. For this purpose, HSDRB fermentation for 0, 12, 24, 48, 72 and 96 h followed by 24 h hydrolysis was carried out to evaluate the extracted RB protein. Moreover, the microbiome diversity in the fermentative starters was also determined by metagenomic sequencing of 16S rRNA and internal transcribed spacer for bacterial and fungal identification, respectively. Results and conclusions. The microbial community in the fermentative starters revealed the dominance of lactic acid bacteria (LAB) such as Bacillus subtilis in Loog-pang and Streptococcus lutetiensis, Bacillus pumilus, Lactococcus cremoris, Lactococcus garvieae, and Pediococcus pentosaceus in Koji whereas yeast species, Saccharomycopsis fibuligera, and Saccharomyces cerevisiae were dominating the fungal diversity in Loog-pang and Koji starters, respectively. Results suggest that Loog-pang and Koji could produce cellulase, neutral, and acid proteases during fermentation. Despite their microbial diversity discrepancy and the enzyme activity during SSF, both starters were effective in enhancing protein extraction from HSDRB. A positive relationship between the SSF duration and extracted protein was noted. SSF by Loog-pang and Koji after 72 h followed by 24 h hydrolysis was found to extract 65.66 and 66.67 % protein from HSDRB, respectively. The amino acids profile of the protein hydrolysate prepared by non-fermented and fermented method showed no difference and displayed an abundance of glutamic acid, aspartic acids, leucine, arginine, alanine, and glycine amino acids, accounted for approximately 58 % of the total amino acids. Novelty and scientific contribution. Loog-pang and Koji (Thai and Japanese traditional fermentative starters, respectively), were found effective in extracting protein from HSDRB by SSF despite being inexpensive microbial enzyme sources, and future research aimed at scaling up HSDRB protein extraction for usage in industrial applications can rely on our findings.