Brewer’s spent grain (BSG) is the most abundant (85%) by-product generated from the beer-brewing process. Currently, BSG is predominantly used as cattle feed or buried in landfills, leading to substantial resource losses. The high fiber content (~70%) and protein content (14 ~ 30%) make BSG attractive as feedstock for the production of biofuels and protein-related products. In this study, an integrated approach was investigated to produce both protein concentrate and 2,3-butanediol (2,3-BDO) from BSG.

BSG was first subjected to a wet fractionation process to produce protein concentrate and fiber-rich product (FP) using three commercial proteases (Alcalase, Neutrase, and Pepsin) at different loadings. The FP obtained from the optimized process was pretreated with sulfuric acid (0.5 - 3%, v/v) for different times (15 - 60 min) and then hydrolyzed with cellulase (10 - 40 FPU/g biomass) for 72 hr to obtain reducing sugars. Finally, the reducing sugars obtained were fermented to 2,3-BDO by a newly isolated thermophilic and alkaliphilic B. licheniformis YNP5-TSU.

The results showed that the optimal wet fractionation condition for protein concentrate production was 20 µL Alcalase/g dry BSG enzyme loading, 4 h incubation time, 50 ^oC incubation temperature, pH 8.0. Under the optimal condition, up to 83% protein in BSG was separated and concentrated into protein product. The protein concentration in protein concentrate was 41%, which about doubled that in BSG (22%). The removal of protein from BSG significantly improved reducing sugar production. FP pretreated with 0.5% H₂SO₄ for 60 min at 121 ^oC can release 65% of xylose. The peaked glucose concentration (51 g/L) and glucose yield (80%) were achieved when the cellulase loading was 10 FPU/g biomass. B. licheniformis YNP5-TSU was able to ferment the hydrolyzed sugars into 2,3-BDO. The highest 2,3-BDO yield was 0.5g/g total reducing sugars and the productivity was 0.3g/L/h. Overall, this study demonstrated that BSG can be upcycled into multiple value-added products via an integrated biorefinery process.

Joshua OHair

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