Construction And Application Of Thermotolerant Yeast Strain

Soy sauce and soybean paste are delicious and nutritious fermented condiments.They are an indispensable part of the daily diet of Asian people.The low salt solid-state fermentation process has been widely used in some countries because of its high fermentation temperature,short fermentation time and high economic benefits.However,the high fermentation temperature in low salt solid-state fermentation limits the growth and metabolism of medium temperature yeast Torulopsis versatilis(C yeast).The quality of fermentation products is reduced due to the lack of flavoring yeast under high temperature.Firstly,the yeast cultured in the period of 12-21 h was chosen to prepare sensitive state of yeast by determining the growth curve of strain.According to the dilution experiment,Saccharomyces cerevisiae(H yeast)was selected as the foreign gene donor strain and T.versatilis was selected as the initial strain of this experiment,and screened at 42 °C.After that,the genome of S.cerevisiae was transferred into T.versatilis twice under 600 V for gene recombination,and 8 new strains M-1-M-8 were selected finally.Then,M-4 and M-7,two excellent strains with the highest biomass accumulation and the best heat resistance at42 °C,were screened out.The RAPD results showed that the new strains M-4 and M-7 had different RAPD profiles from parental strains,which confirmed the success of gene recombination.The study on the characteristics of the new strain showed that M-4 and M-7could grow normally at 37 °C and 42 °C,but T.versatilis could not grow;Especially at50 °C,the survival rates of M-4 and M-7 were 40.43% and 25.43% respectively,while the initial T.versatilis died,and the higher intracellular trehalose accumulation of the new strain(M-4: 131.55%;M-7: 145.04%;C: 102.57%)may explain the reason for this difference.These results showed that the heat resistance of the new strains was improved.In this paper,the initial T.versatilis,new strains M-4 and M-7 were applied to the fermentation of soy sauce and soybean paste at 30 ° C,37 ° C and 42 ° C.Based on the contents of amino acid nitrogen,total nitrogen,total acid and reducing sugar in soy sauce and soybean paste,M-7 was selected as the final excellent strain.At high temperature,compared with the initial T.versatilis,in the soy sauce samples fermented by M-7,the amino acid nitrogen increased by 27.25 %,the total nitrogen increased by 11.39 % and the total acid increased by 13.67 %;In soybean paste fermented by strain M-7,amino acid nitrogen,total nitrogen and total acid increased by 12.20 %,12.03 % and 51.67 %respectively.At the same time,the soy sauce and soybean paste fermented by M-7 have richer flavor compounds and better senses.The biogenic amine content produced by M-7 in soy sauce and soybean paste samples will not affect human health and is safe.This study proved that the excellent yeast strain M-7 could significantly improve the comprehensive quality of soy sauce and soybean paste under the condition of high-temperature fermentation.