Abstract
Nitrites are potential carcinogens. Therefore, limiting nitrites in food is critically important for food safety. The nitrite degradation capacity of Lactobacillus casei subsp. rhamnosus LCR 6013 was investigated in pickle fermentation. After LCR 6013 fermentation for 120 h at 37°C, the nitrite concentration in the fermentation system was significantly lower than that in the control sample without the LCR 6013 strain. The effects of NaCl and Vc on nitrite degradation by LCR 6013 in the De Man, Rogosa and Sharpe (MRS) medium were also investigated. The highest nitrite degradations, 9.29 mg/L and 9.89 mg/L, were observed when NaCl and Vc concentrations were 0.75% and 0.02%, respectively in the MRS medium, which was significantly higher than the control group (p ≤ 0.01). Electron capture/gas chromatography and indophenol blue staining were used to study the nitrite degradation pathway of LCR 6013. The nitrite degradation products contained N2O, but no NH4 +The LCR 6013 strain completely degraded all NaNO2 (50.00 mg/L) after 16 h of fermentation. The enzyme activity of NiR in the periplasmic space was 2.5 times of that in the cytoplasm. Our results demonstrated that L. casei subsp. rhamnosus LCR 6013 can effectively degrade nitrites in both the pickle fermentation system and in MRS medium by NiR. Nitrites are degraded by the LCR 6013 strain, likely via the nitrate respiration pathway (NO2 −>NO−>N2O−>N2), rather than the aammonium formation pathway (dissimilatory nitrate reduction to ammonium, DNRA), because the degradation products contain N2O, but not NH4 +.
Highlights
Nitrites are potentially strong carcinogens and are generated in the process of vegetable fermentation, which poses a potential food safety issue
Their results suggested that nitrate accumulated during the process of bacterial fermentation and the nitrate concentration was reduced with lactic acid fermentation
The abnormal accumulation of nitrites in kimchi is associated with several factors: (1) the number of coliforms is higher than control groups, (2) the concentration of soluble nitrogen compounds is higher than control groups, and (3) the buffering capacity is higher than control groups
Summary
Nitrites are potentially strong carcinogens and are generated in the process of vegetable fermentation, which poses a potential food safety issue. Nitrites react with amines, products of protein decomposition, to generate N-nitroso compounds. Hashimoto et al have found that inoculation of Lactobacillus could inhibit accumulation of high concentrations of nitrites in kimchi fermentation [4]. It has been reported that inoculation of over 10 different strains of Lactobacillus bacteria, including Lactobacillus brevis [5], Lactobacillus fermentum [6], Lactobacillus acidophilus, and Lactobacillus plantrum [6], can efficiently reduce the concentration of nitrites generated by fermentation or artificially added during the fermentation of pickles and meat. Our previous study showed that inoculation of LCR 6013 dramatically reduced the concentration of nitrites in fermented pickles [7].
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