Valorization of soy sauce lees through sequential aerobic-anaerobic solid-state fermentation: Process optimization, nutritional transformation and bacterial community restructuring

Soy sauce lees (SSL) are a protein-rich agro-industrial by-product, but their feed utilization is limited by high salt and fiber contents. This study developed a water-washing and two-stage solid-state fermentation strategy to improve the nutritional characteristics of SSL and characterize associated bacterial community changes. Water-washing conditions, microbial combinations, and fermentation parameters were optimized using single-factor experiments and a Box-Behnken design. The optimal pretreatment conditions were a solid-to-liquid ratio of 1:5 and soaking for 18 h. Fermentation consisted of an aerobic stage with Bacillus subtilis and Aspergillus niger, followed by an anaerobic stage with Lactiplantibacillus plantarum. The optimized conditions were 75 h of aerobic fermentation, 72 h of anaerobic fermentation, and 55% initial moisture, with only 0.89% deviation between the measured and predicted crude fiber (CF) degradation rates. Water washing significantly decreased ash and NaCl contents. Subsequent fermentation significantly increased crude protein (CP) content and decreased CF, neutral detergent fiber (NDF), acid detergent fiber (ADF), and ash contents. Fermentation did not alter bacterial α-diversity but reshaped community structure, with enrichment of Lactiplantibacillus and Bacillus and reductions in Escherichia-Shigella and Clostridium. Distance-based redundancy analysis linked bacterial community shifts to nutritional changes, with CP associated with fermented profiles and CF, NDF, and ADF with pre-fermentation profiles. Functional prediction indicated shifts in carbohydrate metabolism, glycolysis/gluconeogenesis, pyruvate metabolism, and amino acid metabolism pathways. Overall, pretreatment and fermentation alleviated SSL compositional constraints, increased CP concentration, and reshaped bacterial communities and predicted metabolic potential, supporting its potential application as a processed feed ingredient.