Free Gossypol Removal and Nutritional Value Enhancement of Cottonseed Meal via Solid-State Fermentation with <i>Rhodotorula mucilaginosa</i> TG529

The presence of free gossypol (FG) in cottonseed meal (CSM) greatly limits the use of CSM as a high-quality protein feed. Microbial fermentation is an effective method to simultaneously reduce FG and improve the nutritional value of CSM. In this study, using potato dextrose agar containing acetic gossypol as a selective medium and humus soil from cotton fields as the source, we isolated six strains of fungi capable of tolerating FG. With an inoculation ratio of 8% (8 mL × 10⁶ CFU/mL cells or spores in 100 g fermented CSM), 50% moisture content, and a temperature of 30 °C, CSM was fermented for 5 days. The results showed that strain F had the highest FG removal rate at 56.43%, which was identified as <i>Rhodotorula mucilaginosa</i> (<i>R. mucilaginosa</i>) and named <i>R. mucilaginosa</i> TG529. Further optimization revealed that when the fermentation time was extended to 11 days, TG529 achieved a maximum FG removal rate of 73.29%. Compared to the original sample, treatment with TG529 significantly increased the contents of crude protein, acid-soluble protein, and 18 amino acids, while significantly reducing the contents of crude fiber, neutral detergent fiber (NDF), and acid detergent fiber in fermented cottonseed meal (FCSM). Using atmospheric and room temperature plasma for mutagenesis of TG529, it was found that the mutated TG529 significantly increased the contents of acid-soluble protein and phenylalanine in FCSM, significantly reduced the NDF content, and enhanced the FG removal rate to 76.50%. In summary, this study screened and mutagenized a strain of FG detoxifying fungus, <i>R. mucilagnosa</i> TG529, which can effectively reduce the FG content and improve the nutritional value of CSM by solid-state fermentation.