Nitrite self-degradation process in radish paocai under the synergistic regulation of prokaryotic microorganisms

Abstract

Nitrite, as the main hazard in pickle production, can be completely self-degraded through microbial biological enzymatic degradation (BED) and chemical acid degradation (CAD), providing a valuable strategy for the in-situ control of nitrite in pickle production. However, the microbial mechanism of the self-degradation of nitrite in radish paocai, a popular pickle in Asia, remains unclear. Herein, this study aimed to reveal the microbial metabolic pathways and processes regulating the self-degradation of nitrite during the pickling of radish paocai. In the early stage of the pickling process, Enterobacter with nitrate reductase (NarG) might reduce nitrate to form a nitrite peak of 27.61 mg/kg through “truncated denitrification”, which is in excess of the Chinese national standard (20 mg/kg). As nir genes encoding nitrite reductase increased to peak values of 9.8 × 104 copies/μL, BED dominates the self-degradation of nitrite during this early stage, which is performed by prokaryotic microorganisms such as Chryseobacterium and Halobacterium with nitrite reductase (NirK) through “complete denitrification”. Meanwhile, Weissella without nitrite reductase may dominate the pH decrease from 6.7 to 4.3 through diverse organic acids generated by metabolizing dominant reducing sugars, such as glucose and fructose of radish, thereby initiating CAD. Subsequently, the acid-tolerant Limosilactobacillus continued to reduce pH by metabolizing glucose, causing CAD to replace BED to dominate the self-degradation of nitrite until the end of pickling. Thus, the nitrite self-degradation process in radish paocai is under the synergistic regulation of diverse prokaryotic microorganisms.