| Natamycin as a kind of natural anti-fungal substance with broad-spectrum and highly effective, was widely used in biological control of plant disease, food preservation, health care, etc.At the current, the application of natamycin is limited due to low and long fermentation production. Streptomyces lydicus G117 is a new strains of high-yield natamycin. It has been breeded by gene shuffling with A01 and A02. Its fermentation period is signigicantly shorter and its yield of natamycin is higher than the original strain. The fermatation process of natamycin produced by S.lydicus G117 was systematically studed in this paper. Through optimizing the medium composition and fermentation conditions, the yield of natamycin was increased substantially, and the fed-batch fermentation kinetics model was established which provides a theoretical basis for further research and industrial production. The main research results are as follows.(1) Fermentation medium for strain G117 was optimized with methods of fractional factorial design, path of steepest ascent and central composite design. The average yield of natamycin for the optimized fermentation medium was 2718.57mg/L, which there was no significant difference with the theoretical prediction yield, and increased by 33.2% in comparison with basic fermentation medium. Fermentation medium optimized composition: corn starch 46.15 g / L, soybean powder 14.80 g / L, Na Cl 5.88 g / L, glucose 10 g / L, peptone 6g / L,(NH4)2SO4 7.5g / L, Mg SO4 0.5 g / L, KH2PO4 0.3g / L, Ca CO3 10 g / L, temperature amylase 0.05 g / L.(2) It was determined that the optimum seed culture time is 24 h and the optimum amuout of inoculation is 9%. The temperature have great influence to the yield of natamycin in the process of fermaentation. Higher fermentation temperature is advantageous to the growth of bacteria, and the synthetic of natamycin needs lower fermentation temperature.So a two-stage temperature control strategy, in which temperature was controlled at 30℃for the first 36 h and then switched to 28℃ till the end of the fermentation, was developed. By using this temperature-shift strategy, the yield of natamycin reached 3516.65mg/L, which increased by 19.1% and 20.34%, compared to the constant temperature operation at 28℃and 30℃.(3) The optimal initial p H of fermentation is 7.3.The yield of natamycin is 5208.88mg/L when fermentation broth p H was 6.3 by feeding glucose, which increased by 47.79%. The addition of 0.1% sodium propionate could increase the yield of natamycin(3982.53mg/L) at cultivation time of 24 h, increased by 10.26%.(4)With the optimized medium and culture conditions, fed-batch fermentation experiments of S.lydicus G117 for natamycin production was carried out in the study. The yield of natamycin is 11.81g/L, increased by 69.57%. Mathematical kinetics model was established based on the unstructured model of biochemical process: cell growth kinetics model: ( )0.1908t0.1908t1.374X t =0.897+0.103ee, R2=0.9978. Product formation kinetics model: ( ) ( )0.1908 tP t =0.686182 ln 0.897 +0.103 e, R2=0.9600. Substrate consumption kinetics:0.1908t0.1908t0.1908t1.5952S(t) 50.7052 0.8998 ln(0.897 0.103)0.897 0.103eee= - - ++, R2=0.9819. Fitting of mathematical kinetics model and measured values is very well. Mathematical kinetics model can describe the time-course of biomass, products and substrates. |
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