Combined Rhizopus oryzae Fermentation and Protein Extraction of Brewer’s Spent Grain Improves Protein Functionality

Abstract Brewer’s spent grain (BSG) is a protein- and fiber-rich biomass with minimal food functionality that limits its usability in foods. Solid-state fermentation (SSF) of BSG using the filamentous fungus Rhizopus oryzae increased the protein content from 16.8 to 23.3% of which 95% is mycoprotein. Alkaline extraction combined with ultrasonication was used to produce protein extracts from BSG and fermented BSG (F-BSG) having 44.9% and 46.2% protein content, respectively. In addition, the protein extracts had a lower total fiber content while composition of fibers changed to have a higher proportion of soluble fibers than insoluble fibers. LC–MS/MS analysis provided a detailed biophysical characterization of the proteins to shed light on the differences inferred by fermentation and protein extraction. This revealed an enrichment in the F-BSG protein extract of the mycoprotein “hydrophobic surface binding protein” that contributes hydrophobic properties to the sample. BSG and F-BSG exhibited poor protein solubility (below 15%) in the pH range 3–8, while the F-BSG protein extract displayed very high protein solubility (85%). The F-BSG extract stood out from the non-fermented and non-extracted samples in terms of smaller particle size, higher surface hydrophobicity, and improved oil holding capacity. Furthermore, the F-BSG protein extract displayed improved functionality by producing 534% foam overrun with desired bubble morphology and by stabilizing a coarse emulsion. In summary, by combining detailed characterization and functional analyses, we have shown the potential of SSF using a food-grade fungi combined with protein extraction as a bioprocessing technology to upcycle side streams into climate-neutral protein-rich ingredients.