Preparation of nitrite-reducing Lactiplantibacillus plantarum SYS-4 starter culture and its impact on the quality and flavor of fermented mustard

Mustard is traditionally prepared through natural fermentation. However, this method is associated with food quality issues, including long fermentation periods, inconsistent quality, and high nitrite (NIT) levels. In this study, Lactiplantibacillus plantarum SYS-4 was prepared as a direct solid fermentation agent using vacuum freeze-drying. The response surface method was employed to optimize the concentration of the protective agent and evaluate its storage stability. Fresh mustard was inoculated with the fermentation agent, and the quality parameters of the mustard during fermentation were comprehensively analyzed. The findings showed that the optimal concentration (g/100 mL) of the lyophilized protective agents, determined by response surface analysis, was 12.50 trehalose, 5.50 mannitol, and 12.00 skimmed milk. Under these conditions, the survival rate of Lactiplantibacillus plantarum SYS-4 freeze-drying agent reached 80.15 ± 1.04 %. The viable bacteria count was significantly negatively correlated with storage time, storage temperature, and water activity (Aw) (P < 0.05), with maximum storage stability observed at Aw levels between 0.10 and 0.12. Compared to natural fermentation, inoculation with Lactiplantibacillus plantarum SYS-4 significantly decreased the time required for acid accumulation, accelerated the fermentation endpoint (pH 3.45, total acid content of 0.74 %), prevented the formation of the “nitrite peak,' and decreased NIT levels. Headspace Solid Phase Micro-extraction Gas Chromatography-mass Spectrometry (HS-SPME-GC-MS) analysis identified esters, alcohols, aldehydes, ketones, alkanes, and nitriles. Esters were the dominant flavor compounds in both methods. However, the relative ester content increased by 7.01 % with Lactiplantibacillus plantarum SYS-4 fermentation, while the content of characteristic flavor compounds, such as allyl isothiocyanate, decreased. Sensory evaluation showed that Lactiplantibacillus plantarum SYS-4 fermentation reduced the spiciness of mustard, yielding a milder flavor profile with a more pronounced sour flavor. Furthermore, the inoculated fermentation preserved the desirable texture of natural fermentation while enhancing overall acceptability.