| The development and application of microbial rennet is more and more attention by researchers.In the present study,in order to breed high activity milk-clotting enzyme from strain,Mucor miehei was used as the origin strain and mutagenized by Ultraviolet and Ultraviolet combining lithium chloride. Meanwhile,the culture medium was optimized to improve the activity of milk-clotting enzyme from the selected strain. Subsequently,used milk-clotting enzyme from Mucor miehei to produce rennet casein and made researches on production technology and charactersof it,which provide a theoretical basis to the development and application of microbial rennet and the low-cost industrial production for rennet casein.The results are as follows:1 Through the experiment determination ultraviolet ray mutagenesis’ s time was 90 seconds.After screening repeatedly,a mutant strain UV-13 was screened.The best condition of compound mutation that the time of Ultraviolet was 90 seconds, the concentration of lithium chloride was 1.5%. On the basis of multiple mutation, a mutant strain UV-Li C l-6 was screened. The milk-clotting activity of UV-Li C l-6 reached 2703.58SU/m L and was 57.70% higher than that of the parent strain.The ratio of milk-clotting activity to proteolytic activity reached 355.06. Results of the pass-generation test indicated that two strains had good genetic stability.2 By using single-factor test, we obtain that the best carbon source, the optimal nitrogen source, and the best enzyme inducer were glucose, ammonium nitrate, and whey powder, respectively. On this basis, through the orthogonal experimental design, the optimal extraneous components we obtained were as follows: 2.5% glucose, 1.0% ammonium nitrate, and 2.0% whey powder. In the optimized medium, milk-clotting activity of Mucor miehei reached 3649.33SU/m L and increased by 32.13%.3 By using single- factor test and Box-Behnken design experiment, the technological parameters of casein production with milk-clotting enzyme from Mucor miehei were further optimized.The results showed the optimum technological parameters were chymosin addition amount of 0.49%, Ca C l2 addition amount of 0.37 mg/m L, p H 6.08 and curd temperature of 35 ℃. Under the optimal conditions,the maximum yield of rennet casein was 3.39%.4 By comparing the yield of casein with rennet from different sources can draw a conclusion that the yield of casein with calf rennet close to casein with milk-clotting enzyme from Mucor miehei.The yield of both rennet casein were 3.41%,3.39%,respectively,and significantly higher than the yield of casein with papain(p<0.05), 2.90%.5 The parameters of the colored was determined by colorimeter.The results showed that the a* and b* value of the casein with papain were significantly higher than casein with calf rennet and the casein with milk-clotting enzyme from Mucor miehei. Also,the L* value was significantly lower the other caseins.Among them,the a* and b* value of the casein with milk-clotting enzyme from Mucor miehei were the smallest. Casein with milk-clotting enzyme from Mucor miehei and casein with calf rennet was not significant in the L* value(p>0.05).6 The analysis of nutritional ingredient of rennet casein in revealed that rennet casein with milk-clotting enzyme from different sources were not significant in water, protein, fat and ash(p>0.05).7 The result of functional characteristics for rennet casein with milk-clotting enzyme from different sources showed that the solubility,emulsification and viscosity of rennet casein with milk-clotting enzyme from different sources changed remarkably under different p H,but the samples under the three treatment systems were not significant(p>0.05). The thermal stability of casein with milk-clotting enzyme from Mucor miehei were significantly higher than casein with calf rennet and the casein with papain(p<0.05). |
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