Development of a Liquid Microbial Enzyme Synergistic Fermentation Process for Strong-Aroma and Soy-Sauce-Aroma Fresh Distiller’s Grains and the Evaluation of Their Nutritional Value in Pigs

China annually produces around 20 million tons of distiller’s grains, whose utilization is important for resource efficiency. These grains are rich in crude protein (CP), crude fiber (CF), and other nutrients, though their composition varies by grain type. This study applied a microbial–enzyme synergistic liquid fermentation process to two types—strong-aroma and soy-sauce-aroma grains—using a composite inoculant (Lactobacillus, Saccharomyces cerevisiae, Bacillus, each >1 × 109 CFU/g) along with xylanase, cellulase, and protease. Optimal fermentation conditions were identified: for strong-aroma grains—water-to-grain material ratio 1.8, inoculum 0.25%, 25 °C, 16 h; for soy-sauce-aroma grains—ratio 1.8, inoculum 0.3%, 25.5 °C, 13 h. After fermentation, CP increased by 13.62% and CF decreased by 30.37% in strong-aroma grains, while in soy-sauce-aroma grains an 8.83% CP increase and 31.31% CF reduction were observed. Structural analysis of both grains showed looser fibrous formations and changes in protein secondary structure.