Isolation And Identification Of Yeasts In Fermented Cucumber And Its Effects On The Flavor Of Fermented Cucumber

In this paper,the dominant yeasts in five regions of pickled cucumber were isolated and identified.The enzyme activities,tolerance and probiotic properties of different yeasts were analyzed.The effects of yeast on the flavor of fermented cucumber were investigated.The results were summarized as fol-lows:(1)54 strains of yeast were isolated from pickled cucumbers in five regions including Guangxi,Hunan,Guangdong,Yunnan and Jilin Yatai.The yeast strains were identified by using 26S rDNA se-quence.The results showed that the yeast species from different sources were different,and Pichia yeast was the most widely distributed.According to the isolation regions and species,8 strains of yeast were selected for further experiments:Debaryomyces hansenii-X1(Dha_X1);Kodamaea ohmeri-X10(Koh_X10);Candida humilis-H3(Chu_H3);Rhodotorula mucilaqinosa-T4(Rmu_T4);Pichia occiden-tails-D9(Poc_D9);Pichia kudriavzevii-Y4(Pku_Y4);Pichia fermentans-Y5(Pfe_Y5);Hanseniaspora meyeri-Y10(Hme_Y10).(2)The salt and acid tolerance,enzymatic activities and probiotic properties of the 8 yeasts were analyzed.At 60 g/L salt concentration,the inhibition rate of each strain was between 18.300%and72.528%,and Dha_X1 and Koh_X10 were more tolerant to salt than others.Except Pku_Y4 and Poc_D9,other strains could not grow at 0.9%acetic acid concentration.Each strain was able to grow normally under 1.2%lactic acid conditions.The intracellularβ-glucosidase activity of yeasts was34.770-93.901 nmol/h/ml.Some strains demonstrated strong extracellularβ-glucosidase activity,for example the activity of Pfe_Y5 was up to 62.179 nmol/h/ml.The esterase was mainly present in the intracellular,and the intracellular esterase activity of each strain was 39.413-55.713nmol/h/ml.Yeasts had lower lipase activity.The probiotic results showed that the survival rate of each strain in the simu-lated gastrointestinal fluid digestion was different,and the survival rate of Pku_Y4 was up to 89.257%.Each strain had DPPH free radical scavenging ability and automatic cell aggregation ability,but the dif-ference between the strains was not significant.(3)The effects of 8 strains of yeast on the growth of Lactobacillus plantarum,Lactobacillus pen-tosus and Leuconostoc mesogenes were studied in a Chemical-defined Medium,which can not maintain the growth of above bacteria.The results showed that 8 strains of yeast all promoted the growth and reproduction of these three kinds of lactic acid bacteria.Yeast had a strong promoting effect on Leuco-nostoc mesogenes.The growth stimulating effect of Pku_Y4 on Lp_GR3,Lactobacillus pentosus and Leuconostoc mesogenes was significantly stronger than that of other strains,and the growth multiples were 93.182,95.122 and 203.937,respectively.(4)Based on the number of lactic acid bacteria,pH,total acid,nitrite,organic acid and volatile components,the effects of Dha_X1,Pku_Y4,Lp_GR3 fermentation and natural fermentation on cu-cumber fermentation were compared.At the end of the fermentation,the number of lactic acid bacteria fermented by yeast was significantly higher than that of natural fermentation,reaching 10~7-10~8 CFU/mL.The initial pH value of the fermentation broth was lower at the initial stage of fermentation.As the fer-mentation progressed,the pH values of the four kinds of pickled cucumbers gradually increased and then stabilized.And the cucumbers inoculated with yeast were not significantly different from others.The total acid content gradually decreased.During the fermentation,the nitrite content was low,and the“nitrous peak”appeared on the second day,and was not detected after the fourth day.The"nitrous peak"naturally occurring on the second day was higher than the inoculation fermentation.The content of cit-ric acid and succinic acid was significantly higher in the organic acids measured in the late fermentation stage of cucumber inoculated with yeast.The volatile matter in fermented cucumber was analyzed by headspace solid phase microextraction combined with GC-MS.A total of 86 volatile substances were detected.Among them,the content of alcohols is the highest,accounting for 88%-93%of the total vola-tile components.The amount of volatile substances containing special aroma in cucumbers inoculated with Pku_Y4 was higher than that of the other three fermented cucumbers,such as alcohols include decyl alcohol,octanol,and citronellol;aldehydes are octanal,furfural,citral and ketones,nerylacetone,etc.