Influence of fermentation conditions by Rhizopus oryzae on the release of phenolic compounds, composition, and properties of brewer's spent grain

Abstract

This study aimed to (i) assess the influence of moisture and particle size on the release of phenolic compounds from brewer's spent grain by Rhizopus oryzae fermentation, (ii) determine the release kinetics of these compounds, and (iii) characterize brewer's spent grain before (BSG) and after (BSGF) fermentation. BSG and BSGF were evaluated for chemical composition, phenolic profile, and antioxidant and technological properties. The release of methanol-soluble phenolic compounds (MSPC) was influenced by both moisture and particle size, with the highest value (300.25 mg 100 g−1) obtained at 71% moisture and a 50:50 mixture of particles measuring 0.50 and 0.30 mm. This maximum MSPC content was obtained after 48 h of fermentation, being 761% higher than that contained in BSG. Catechin was the major component of the MSPC fraction (37.08 mg 100 g−1), whereas ferulic acid (36.38 mg 100 g−1) was the predominant component of the bound phenolic fraction. BSGF showed higher ash (8%) and protein (17%) content, but lower soluble dietary fiber (10%) and lipid content (67%) than BSG. Fermentation altered the protein fractions of the residue, increasing the proportion of albumin and globulin and decreasing that of prolamin and glutelin. These changes resulted in improved technological properties (oil absorption, water solubility, and swelling volume) and antioxidant activity. It is concluded that the proposed fermentation method can contribute to expanding the utilization of BSG, thereby diversifying its applications, and adding value to the production chain.