Screening And Study On Fermenting Conditions Of Thermotolerant And High Ammonia Assimilation Rate Yeast Strains

In China, forage protein resource was much deficient always. For the past fewyears,with the development of livestock farming,forage protein resource wasseriously lack. It was necessary to develop new protein resource.Lots of reportsshowed that lots of protein, vitamin, minerals and kinds of enzymes were got fromyeasts and yeasts were well palatable.So yeasts were mainly used as fermentationstrains.Some reports showed that inorganic nitrogen could be inverted to organicnitrogen in some kinds of yeasts. The main purpose of this study was to get one orseveral yeast strains which had a strong ability of ammonia assimilation.A total of90yeasts strains including Candida(34strains in all， concludingCandida tropicalis24strains, Candida utilis8strains, Candida guilliermondii1strain andCandida palmioleophila1strain),Saccharomyces cerevisiae(3strains),Kluyveromycesmarxianus(17strains),Issatchenkia orientalis(10strains)and so on were investigatedin fermentation experiments.The result showed that the ammonia assimilation rate of I.orientalis was at lower level,under10%; the ammonia assimilation rate of K.marxianus was at middle level,between30%and55%; the ammonia assimilation rateof Candida was at higher level,between40%and80%.The consequence of thepreliminary screening experiment showed that the difference of the ammoniaassimilation rate of I. orientalis respectively among K. marxianus and Candida wasmore than20%and30%,which signified that there was a large difference among theammonia assimilation rate of different genus yeasts.The difference of the ammoniaassimilation rate of C. palmioleophilaBK-9respectively among C. tropicalis HG3-1and C. utilis SG607was21.9%and32.2%, which signified that there was a largedifference among the ammonia assimilation rate of yeasts which belonged to the samegenus but different species.The difference of the ammonia assimilation rate between S.cerevisiae WHH-3strain and S. cerevisiae WHH-5strain was68.6%, the difference ofthe ammonia assimilation rate between K. marxianus2.1549strain and K. marxianusWZ-1strain was43.4%, and the difference of the ammonia assimilation rate betweenC. tropicalis BK-8strain and C. tropicalis AA-3strain was31.3%, all of whichsignified that there was a large difference among the ammonia assimilation rate ofyeasts belonged to the same genus and specie but different strains.Finally, C.utilis SG607was abtained from the preliminary screening test. Thegrowth character of C. utilis SG607was studied. The viable count of C. utilis SG607in YPD liquid culture in shake flask for12h at28℃、30℃、33℃、35℃、37℃and40℃respectively was1.12×cfu·mL-1、1.38×cfu·mL-1、1.29×cfu·mL-1、1.15 ×cfu·mL-1、0.8×cfu·mL-1and0.74×cfu·mL-1. The consequence indicated that theoptimum growth temperature of C. utilis SG607was30℃although the strain couldgrow at40℃.The result of the second screening experiment showed that the ammoniaassimilation rate of C. utilis SG607was81.1%which was higher than C. utilis1807by23%. The result of preliminary amplification fermentation test showed theammonia assimilation rate of C. utilis SG607was upper to96.7%, the crude proteincontent of the strain was50.2g·100g-1, the biomass was3.43g·L-1and the net proteinyield was1.72g·L-1.SG607-U17strain was got in the UV mutagenesis experiments with C. utilisSG607as the starting strain. The ammonia assimilation rate of SG607-U17strain was92.7%which was higher than C. utilis SG607by11.6%. Then the fermentationmedium was altered.Under this condition,the result of preliminary amplificationfermentation experiment showed that the ammonia assimilation rate of SG607-U17strain was higher than C. utilis SG607by2.4%, the ammonia assimilation amountwas higher by12.2mg·L-1, the biomass was higher by0.16g·L-1, the crude proteincontent was higher by0.7g·100g-1, the net protein yield was higher by1.72g·L-1.