Composite Microbial Solid-State Fermentation Enhances the Fermentation Quality, Nutritional Value, and Safety of Cottonseed Hulls: Insights Based on Physicochemical Detection and Untargeted Metabolomics

Cottonseed hulls (CSH) are by-products of cotton processing, but their use in livestock feed is constrained by lignocellulose and free gossypol. In this study, solid-state fermentation of cottonseed hulls was optimized using Aspergillus niger, Aspergillus oryzae, and Lactobacillus plantarum through an orthogonal experimental design. Fermentation quality, nutrient composition, safety indicators, microstructure, and metabolic profiles were subsequently evaluated under the optimized conditions. The results showed that composite microbial fermentation significantly reduced pH and decreased NH3-N/TN by 42.35%, while increasing lactic acid content by 20.01 g/kg. Meanwhile, butyric acid was not detected. Compared with the non-inoculated control, neutral detergent fiber and acid detergent fiber were further degraded by 5.78% and 7.37%, respectively. In addition, free gossypol was reduced by 79.79% compared with untreated cottonseed hulls, and aflatoxin B1 content decreased by 60.11% compared with the non-inoculated control. Untargeted metabolomics revealed increased abundances of amino acids and bioactive small peptides, including L-arginine, Ile-Lys, Glu-Met, and L-isoleucyl-L-arginine. Taken together, these findings indicate that composite microbial fermentation may serve as an effective strategy for the detoxification and nutritional improvement of cottonseed hulls, providing a theoretical basis for the application of fermented cottonseed hulls as a feed resource.