Response surface tests were used to determine the optimal conditions for Bacillus subtilis solid-state fermentation of highland barley bran. The polyphenol composition and antioxidant activity of the fermented bran were also assessed. The results showed that the optimal fermentation conditions for highland barley bran were 10% inoculum, a liquid–feed ratio of 1.80, a fermentation temperature of 30 °C, and a fermentation time of 93.5 h. Under these conditions, the polyphenol content of highland barley bran was 12.43 mg/g. After fermentation, the ferulic acid content of the bran decreased, catechol and gallic acid contents increased significantly, and catechins shifted from the bound state to the free state. In addition, solid-state fermentation improved the antioxidant capacity of highland barley bran, and the DPPH• radical scavenging rate, Fe ion-reducing capacity, and hydroxyl radical scavenging rate of highland barley bran increased after fermentation.

This study utilised Bacillus subtilis and cellulase combined with Bacillus subtilis to ferment quinoa bran. The effects of different fermentation methods on the functional components, antioxidant activity, and structural changes of quinoa bran were evaluated. Fermentation altered the functional components of quinoa bran and enhanced its antioxidant capacity. The phenolic acid and polysaccharide contents increased in BFQ (Bacillus subtilis-fermented quinoa bran) and BEFQ (bacterio-enzyme co-fermented quinoa bran), whereas the protein content decreased. After fermentation, the phenolic acid content in BEFQ increased by 81.68%, while the DPPH and ABTS radical-scavenging rates increased by 43.99% and 31.44%, respectively. The antioxidant capacity in BEFQ was ranked as follows: ferulic acid > p-coumaric acid > vanillic acid > 4-hydroxybenzoic acid. Thus, the antioxidant ability of quinoa bran phenolic acids was primarily dependent on hydroxycinnamic acid derivatives. The polysaccharide content in BEFQ increased by 80.73%. The DPPH and ABTS radical-scavenging rates increased by 52.59% and 50.48%, respectively, whereas the protein content decreased by 21.88%. Furthermore, the DPPH and ABTS radical-scavenging rates increased by 76% and 75.39%, respectively. These results indicate that fermentation using cellulase combined with Bacillus subtilis has the potential to enhance the antioxidant capacity and utilisation of quinoa bran.