Impact of Tetragenococcus halophilus CICC 10286 Inoculation on the Fermentation Dynamics of Soybean Paste

Fermented soybean paste, a traditional high-salt condiment, faces challenges in standardization and quality control due to its reliance on natural fermentation. This study systematically evaluated the effects of a defined starter culture, Tetragenococcus halophilus CICC 10286, on soybean paste fermentation by comparing natural fermentation (NF) and fortified fermentation (FF). Compared with NF, FF maintained a higher moisture in the later stage (NF-LS: 50.30%; FF-LS: 60.08%) and lower total acid levels in the middle and later stages (NF-MS: 1.58 g/100 g; FF-MS: 0.96 g/100 g; NF-LS: 2.23 g/100 g; FF-LS: 1.11 g/100 g). Although protein degradation was more pronounced in the FF group at the midpoint (p < 0.0001), the lower accumulation of amino acid nitrogen suggests a potential shift in nitrogen metabolism, possibly toward enhanced transamination or deamination processes. Free amino acid profiling indicated that FF facilitated earlier accumulation of umami and sweet amino acids, but the total free amino acid content in the later stage was lower. Specifically, Glu and Asp reached 724.47 nmol/L and 305.52 nmol/L, respectively, in NF-LS, whereas the corresponding values in FF-LS were 397.16 nmol/L and 275.46 nmol/L. Meanwhile, Pro reached 337.81 nmol/L in FF-MS, indicating earlier accumulation of some amino acids under FF. Notably, the proportion of bitter amino acids in the FF group was reduced in the later stage. Microbial community analysis showed that FF promoted the enrichment of Tetragenococcus and halotolerant bacteria, such as Halomonas, at the midpoint, and increased the relative abundance of the aroma-producing yeast Zygosaccharomyces (NF-MS: 37.73%; FF-MS: 65.11%). Functional prediction based on PICRUSt2 suggested a higher predicted abundance of genes involved in pyruvate metabolism and branched-chain amino acid degradation in the FF group. These findings demonstrate that T. halophilus CICC 10286, as a starter culture, can effectively modulate the fermentation of soybean paste, providing a scientific basis for developing standardized and quality-controlled fermentation processes.