Valorization of Residual Fractions from Defatted Rice Bran Protein Extraction: A Carbohydrate-Rich Source for Bioprocess Applications

Defatted rice bran (DRB) is the by-product of rice bran oil extraction and presents approximately 66% carbohydrates and 15% proteins, a composition with the potential to integrate biorefinery systems. This study aimed to investigate the feasibility of residual fractions from ultrasound-assisted protein extraction from DRB as sources of carbohydrates in bioprocesses. First, DRB was exposed to protein extraction in an alkaline medium assisted by ultrasound. The residual fractions, including the precipitate from the extraction process (P1) and the supernatant from protein precipitation (S2), were combined and autoclaved to gelatinize the starch. Enzyme activity tests showed that a temperature of 70 °C was optimal for the simultaneous application of α-amylase and amyloglucosidase (AMG). To study enzymatic hydrolysis, a Full Factorial Design (FFD) 22 was employed, with α-amylase and AMG concentrations ranging from 0.12 to 0.18 mL∙L−1 and a substrate concentration (P1/S2 ratio) between 30 and 70 g∙L−1, resulting in a maximum of 18 g∙L−1 of reducing sugars (RS). Fermentation assays with Saccharomyces cerevisiae demonstrated that the hydrolysate of the residual fractions was effective for ethanol production (8.84 g∙L−1 of ethanol; YP/S: 0.614 gethanol∙gRS−1; η: 120.24%), achieving results comparable to control media (with sucrose as the substrate), indicating its potential for application in bioprocesses. These outcomes highlight a promising technological approach for utilizing DRB in integrated biorefineries.