β-Glucan from Highland Barley Spent Grain: Yield, Molecular Weight, Physicochemical Properties, Antioxidant Capacity, and Gel Characteristics

β-Glucan from Tibetan highland barley (THB) is an excellent edible gel polysaccharide due to its unique hypoglycemic and antioxidant activities. However, direct extraction of β-glucan from THB exhibits low yields with higher costs. Given that highland barley spent grain (BSG) is a byproduct of the brewing process and is frequently considered waste, the efficient extraction of its β-glucan could promote high-value repurposing of BSG. In this study, 2.74% β-glucan (BSG-B) was extracted from Rhizopus oryzae (R. oryzae)-fermented BSG, which is lower than those from THB (THB-B: 4.62%) yet enabled value-added utilization of BSG. The molecular weight of BSG-B was 5.24 × 106 Da, which significantly increased by 124.89% compared to that of THB-B. Fourier-transform infrared (FT-IR) spectroscopy showed similar absorption peaks in BSG-B and THB-B, except for structural modifications in the β-glucan pyranose ring induced by the fermentation of R. oryzae. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) indicated that BSG-B possesses a more compact structure and lower aggregation heights compared to THB-B. Moreover, BSG-B demonstrated superior antioxidant capacities to THB-B in NO/DPPH/ABTS/reducing power assays, and lower apparent viscosity and oil adsorption capacity, likely attributed to the fermentation of R. oryzae. This study establishes a foundation for extracting higher-molecular-weight antioxidant β-glucan from BSG.