Metagenomic insights into Chinese northeast suancai: Predominance and diversity of genes associated with nitrogen metabolism in traditional household suancai fermentation

Abstract

Chinese northeast suancai represents a typical and valuable food product that has been handed down by traditional household procedures over centuries. Nitrite is formed and accumulated during the suancai fermentation process and commonly causes food safety problems. The biogeochemical cycle of nitrite may provide a reference and guidance for the enzymatic degradation of nitrite in fermented food. The potential nitrogen metabolic pathways in the microbially driven suancai fermentation were reasonably inferred through monitoring nitrogen conversions and detecting the genes of different functional enzymes. Complex microbial metabolism is responsible for the unique nitrogen conversions during suancai fermentation. The metagenomic results showed that Pseudomonas with nitrate reductase genes (narG, narH, narI) and nitrite reductase genes (nirB, nirD) contributed the most to both nitrite reduction and nitrate reduction. The majority of the sequences of nitrate reductase and nitrite reductase were derived from the families of Pseudomonadaceae, Erwiniaceae and Yersiniaceae. According to the physicochemical analysis, the nitrite concentration of the fermentation broth reached the peak value (0.48 mM) and gradually decreased to the minimum (0.02 mM). The downward trend of the pH and nitrite concentration were closely associated with the nitrite enzymatic degradation period before the acid degradation period. Our results indicated that nitrite removal in suancai fermentation involved the reduction of nitrite to ammonia and denitrification, which were mainly contributed by the reduction of nitrite to ammonia mediated by the nirB/nirD enzyme (Indentified ECs: 1.7.1.15). This research offers new insights into the metagenome-based bioinformatic roles of the previously unstudied microorganisms in spontaneous suancai fermentation for the enzymatic degradation of nitrite. It provides helpful basis for the detection and even elimination of nitrite in suancai and for improving the safety level of suancai.