Optimization of Solid-State Fermentation Process for Prodigiosin Production from Soybean Residue Using Serratia marcescens BD2025 and Evaluation of Pigment Properties

Soybean residue, a major by-product of soy milk and tofu processing, is a large-yield and nutrient-rich substrate, but it is currently experiencing low utilization rates. In this study, a prodigiosin-producing strain, Serratia marcescens BD2025, was isolated from naturally fermented soybean residue. Subsequently, prodigiosin was produced by solid-state fermentation of soybean residue, and the fermentation conditions were further optimized by a Box–Behnken (BBD) model involving the soybean residue-to-water ratio, inoculum size, and temperature. Under the optimized conditions of a soybean residue-to-water ratio of 1:4.28, an inoculum size of 5.2%, and cultivation at 27.5 °C for 48 h, the prodigiosin yield reached 19.05 mg/g. High-performance liquid chromatography analysis showed that the major pigment peak had a retention time comparable to that of a prodigiosin standard, with an estimated purity of 96% based on peak area normalization. The results show that prodigiosin remains relatively stable at below 40 °C, in acidic conditions, and in darkness. The extracted prodigiosin inhibited both Escherichia coli and Staphylococcus aureus, with stronger activity against S. aureus. The scavenging ability of DPPH radical was dose-dependent, with the scavenging activity of 93.13% at 5 mg/mL. These findings indicate that soybean residue can serve as a solid-state fermentation substrate for prodigiosin production and provide a basis for the value-added utilization of soybean-processing residues.