Optimization of solid-state fermentation process of diets and its effects on growth performance and intestinal function in growing pigs

Background Biological fermentation can improve animal growth performance and meat quality by optimizing feed nutritional properties. However, the complex fermentation parameters require further systematic optimization. This study aimed to establish and optimize a solid-state fermentation (SSF) process, evaluate changes in the feed microbial community and flavor metabolites, and investigate their effects on muscle development and intestinal barrier function in growing pigs. Here, we developed a synergistic solid-state fermentation (SSF) strategy for pig feed using combinations of 4 probiotics and 11 enzymatic preparations, 16S rDNA-seq and flavoromics-seq were employed to investigate the dynamic changes in microbial communities and flavor compounds post-fermentation. Subsequently, 32 Duroc × Jinfen White pigs were fed diets containing 10% SSF to assess growth performance, intestinal health and muscle development. Results The optimal fermentation ratio of Bacillus subtilis, Lactobacillus plantarum, Saccharomyces cerevisiae and Aspergillus niger is 1:2:3:3, with a temperature of 36°C, an inoculation rate of 93%, a moisture content of 72%, and a time of 4.1 days. SSF significantly enhanced the nutritional value of feed by increasing the ash, organic matter (OM), crude protein (CP), and ether extract (EE) content, while simultaneously reducing the concentration of anti-nutritional factors. Sequencing identified 17 differential microbes and 116 flavor compounds, with the relative abundance of Lactobacillus and the Firmicutes significantly increased, 2-octenal and vanillin imparting sweet, fruity, and grassy notes to the feed. Meanwhile, RNA-seq revealed 320 DEGs in muscle tissue following fermented feed supplementation, which are mainly enriched in pathways related to cytochrome P450 drug metabolism and arginine biosynthesis. Additionally, H&E staining results indicated that fermentation significantly increased the villus height, crypt depth, and villus-to-crypt ratio in the small intestine of growing pigs, and the levels of tight junction proteins Claudin-1 and ZO-1 in the jejunum were significantly higher than those in the ctrl group. Subsequent correlation analysis indicated that Firmicutes may influence pig growth performance and IL-6, TNF-α levels by affecting their metabolites. Conclusion Our findings establish and optimize an SSF process that markedly elevates feed nutritional value, enriches beneficial microbes, and fosters the production of unique flavor metabolites. When fed to growing pigs, it effectively enhances growth performance and antioxidant capacity while improving small-intestinal morphology and barrier function.