Rice bran undergoes a heat stabilization step to preserve oil quality, and the defatted bran is termed, Heat-stabilized Defatted Rice Bran (HDRB). During defatting the proteins are denatured and aggregate with other cellulosic components in the rice bran making the proteins difficult to be extracted. Our objective was to explore solid-state fermentation (SSF) using Bacillus subtilis natto to extract proteins and protein hydrolysates from HDRB and evaluate for antioxidant activity.
The Response Surface Methodology (RSM) was used to optimize the effect of Bacillus subtilis inoculum log, initial water content, and time as independent variables on the extraction of the maximum water-soluble proteins (WSP) and protein hydrolysates (WSPH). The solubilized proteins and hydrolysates were subjected to TCA precipitation, and SDS-PAGE separation to determine their molecular size. The antioxidant activities of WSP and WSPH were determined.
The optimum conditions for extracting proteins and WSPH were: water content of 41 % v/w, fermentation time of 48 h, and inoculum size of 106 CFU/g of HDRB. The WSP and WSPH produced were 81%. The SDS-PAGE showed 12 bands with molecular sizes ranging from 5 KDa to 100 KDa. The antioxidant activity with DPPH assay showed higher scavenging activity of 72 % ± 2.3 compared with protein extracts from non-fermented HDRB (12% ± 1.7) at the same concentration.
The SSF method is an efficient method to extract proteins and hydrolysates from HDRB and can find application as an ingredient in suitable products. Furthermore, the higher antioxidant activity is indicative of its potential function in emulsified food.