When we pickle vegetables, nitrate reductase produced by bacteria is the decisive factor for nitrate reduction to nitrite in pickles. There are many bacteria that can produce nitrate reductase in nature, about 100 species, such as Escherichia coli, Corynebacterium diphtheriae, Candida albicans, Staphylococcus aureus, Bacillus, Proteus, Actinomycetes, Yeast, Mold, etc. In particular, Escherichia coli, diphtheria bacillus, Staphylococcus aureus and Serratia marcescens can reduce NO3- to NO2- anaerobically, so that NO2- can be accumulated.

At present, kimchi is mainly fermented by lactic acid bacteria naturally brought in by vegetable plants. These lactic acid bacteria are attached to vegetable plants, and some harmful bacteria inevitably exist. In the early stage of pickling, the acidic environment has not yet formed, some harmful fungi have not been inhibited, and nitrite will be produced in the process of nitric acid reduction. In the middle and later period of curing, some acid-resistant, salt-tolerant and anaerobic bacilli, cocci, yeasts and molds still have certain activity ability.

Therefore, the formation of nitrite is inevitable even in normal pickling. If the conditions are well mastered, the nitrite content will be less; Improper control of conditions, such as non-anaerobic environment, mixed bacteria pollution, etc. , produce a lot of nitrite.