Study On The Degradation Of Nitrite By Lactobacillus Plantarum

Nitrite in food and water is very harmful to human health. Lactobacillus is a probiotic bacterium that can produce nitrite reductase. The bacteria can be applied to control the content of nitrite in food and water. In the thesis, we used Lactobacillus plantarum, which was screened and identified in our laboratory, firstly we studied the strain on degradation mechanism of nitrite, and then by breeding and optimizing fermentation conditions, the enzyme production was improved,so as to improve the ability of degrading nitrite, finally choosing a suitable embedding material and method, immobilized good strain, got immobilized cell which reused and less affected by environment of high efficient degradation of nitrite.In the second part of the thesis, by studying Lactobacillus plantarum on degradation mechanism of nitrite, it was concluded that at low p H, especially when p H<4.5, nitrite is not stable, in a certain degree of nature degradation. Based on the fermentation process monitoring, the degradation of Lactobacillus plantarum to nitrite was divided into two parts, enzyme degradation and acid degradation. In the early stage of fermentation, fermentation liquor p H >4.5, mainly enzyme degradation, late fermentation, p H<4.0, the degradation of nitrite was mainly acid degradation. In order to reduce the acid degradation, increase the enzyme degradation part, by adjusting the fermentation liquid p H=6.0, and then to determine the ability of nitrite degradation. By drawing nitrite degradation curve, under the induced environment, after 8h, nitrite degradation rate reached 75%, the fermentation liquid p H=4.66, was greater than 4.5, 12 h degradation rate reached 93%, p H=4.36, at the time of 24 h degradation rate reached 95%, p H=4.34, less than 4.5, concluded that measuring strain degradation ability of nitrite by producing enzyme, the suitable time was 1.5-8 h.In the third part of the thesis, the original strain by UV and DES mutagenesis, then the new stain UV6-DS2 was screeded. Under the 200 mg/L nitrite environment condition,after 8 h, nitrite degradation rate increased from 82.8% to 90%, which increased by 7.2% as compared with the original strain. The new good strain, by the optimization of fermentation conditions, it was concluded that the optimal culture condition: The initial p H 6.5, inoculation quanity 8%, temperature 35 ℃, cultivation way for rest, the optimal induction amount of nitrite for 400 mg/L, the optimum carbon source for glucose, which added 2%, the optimum nitrogen source was compound nitrogen source-soya peptone and yeast, according to the mass ratio of 1:1, which added 2.5%, metal ions Fe2+, Cu2+ can significantly improve the enzyme activity, improve the degradation rate of nitrite, Mg2+, Mn2+, Ca2+ had few affect on enzyme activity.In the fourth part of the thesis, comparing three methods of immobilization, SA embedding method, PVA embedding method, ACA embedding method,by the mechanical strength of gel ball, the complexity of the preparation and the ability of nitrite degradation, it was concluded that the best embedding method for SA embedding method. Then by orthogonal optimization of SA embedding method, established the most suitable embedding condition, SA concentration 2.25%, Ca Cl2 concentration 1.25%, immobilization time 1.5 h. Finally adding different molecular weight of PEG, changed the aperture of gel ball, improved permeability of the ball, increased the combination of substrate and enzyme, it was concluded that the best was PEG 6000, at the same time the degradation rate of nitrite reached 76.9%, increased by 27.3%.