Screening Of Yeast For High Non-protein Nitrogen Utilization

The shortage of protein feed resources has always been one of the bottlenecks restricting the healthy and stable development of China's feed industry.Single cell protein(SCP)has great potential in solving the problem of shortage of food and protein in the world.After years of research,industrial production of SCP has made great progress in strain selection,production technology,and fermentation raw materials.However,SCP feed prices on the market are still high.The cost of SCP feeds remains high,causing SCP feeds to remain unappreciated in the feed industry.Increasing the utilization efficiency and growth rate of non-protein nitrogen(NPN)production strains for production of single cell proteins is the key to reducing the production cost of single cell proteins.In this study,yeast strains preserved in laboratory were selected to obtain yeast strains capable of rapid growth with cheap carbon sources,strong NPN utilization capacity,and high yield of yeast proteins as mutagenic strains.Through mutation breeding of UV irradiation,mutagenesis with diethyl sulfate and complex mutagenesis,the dominant mutant strains with higher yield of yeast proteins were screened through primary screening of mutant colonies and liquid fermentation screening.The fermentation process parameters were optimized.The results of the study are as follows:Through the study of 14 species of yeasts using different carbon and nitrogen sources,the most suitable carbon and nitrogen sources were determined and ranked according to the growth potential of 14 yeasts,yield of yeast proteins,and NPN utilization capacity.The S.cerevisiae M(Saccharomyces cerevisiae strain YI59)and Saccharomyces cerevisiae N2(Saccharomyces cerevisiae isolate AA2)with better overall performance were screened.The yeast M and N2 were used as the starting yeast for single-factor mutagenesis and compound cross mutagenesis induced by UV irradiation and mutagenesis with diethyl sulfate.Using glucose and ammonium sulfate as sole carbon and nitrogen sources,primary screening and rescreening were performed according to the plate colony size and the yield of liquid fermentation cell protein.The optimal ultraviolet mutagenesis irradiation time of Saccharomyces cerevisiae N2 was found to be 10 s and 11 s,and Saccharomyces cerevisiae M was 12 s and 13 s.The optimum concentrations of diethyl sulfate for M and N2 mutagenesis were 1.1% and 0.4%,respectively.Through the preliminary screening and rescreening of the mutant strains,it was found that there was no direct correlation between the yeast protein content of the mutants and the yeast protein production and the size of the mutant colonies.Therefore,it is not possible to screen high-yield SCP yeasts by colony size alone,and it mustbe screened by liquid fermentation to detect yeast protein production.Finally,a strain MH12U5 with high yield of yeast proteins was obtained.The yeast protein content reached0.27 g/L,which was 22.72%(P<0.05)higher than 0.22 g/L of original bacteria M.The single factor test and orthogonal test optimization were performed on the amount of glucose added to the mutant MH12U5,the amount of ammonium sulfate added,the amount of potassium dihydrogen phosphate added,and the amount of seed liquid inoculated.The results showed that the optimum combination of the mutant strain MH12U5 was 2.5% glucose,1.5%ammonium sulfate,0.5% potassium dihydrogen phosphate,and 5% optimal fermentation inoculum.Therefore,the optimum fermentation conditions for the strains were as follows:culture and culture temperature 30°C,shaking speed 180 r/min,and fermentation time 84 h.After optimization of fermentation conditions,the yield of mutant MH12U5 yeast protein was optimized from 0.27 g/L to 0.50 g/L(P<0.05).In this experiment,a yeast mutant strain MH12U5 capable of fermenting single cell protein with ammonium sulfate as sole nitrogen source was obtained through yeast screening,mutagenesis and breeding,and fermentation conditions were optimized.It can be applied to the industrial production of single cell proteins.The cost of fermentation has been reduced,providing theoretical support for alleviating the shortage of protein feed in China.