Screening For Streptococcus Thermophilus Having Desirable Fermentation Properties And The Study Of Their Flavor Characteristics

Streptococcus(S.)thermophilusis widely used in the industrial production of dairy products as yogurt starter.In this study,solid phase microextraction and gas chromatography coupled with mass spectrometry(SPME-GC-MS)were applied to analyze the volatile flavor compounds in fermented milk produced with 30 S.thermophiles strains from different regions.We also analyzed the key aroma compounds at the fermentation endpoint by means of odor activity value(OAV)and principal component analysis(PCA).Wesuccessfully identified six S.thermophiles strains that produceddesirable flavor.We profiled the volatile flavor compounds generated over thefermentation process and postfermentation storage.Additionally,wecompared the profiles of flavor compounds among the six strains.The main results are as follows:1.The fermentation time,volatile flavor substances,and key flavor substances of fermented milk produced by 30 S.thermophilus strains were analyzed.Six strains of S.thermophiles,including MGA21-4,MGA45-4,MGB79-3,MGB90-4,G72-5,and G81-1,were screened out.The fermentation time of 18 strains was less than 8 hour;127 volatile flavor compounds,including 13 key flavors were detected.Eleven of these flavor compounds had important effect on the overall flavor of the fermented milk.2.The fermentation characteristics of 6S.thermophilusstrains in fermenting full fat milk were studied.The number of viable bacterial counts of S.thermophilus fermented milk increased rapidly during the fermentation,and it reached 8.71log10 CFU/m L at the end of fermentation.During fermentation and storage,the p H value of the fermented milk decreased,and the titratable acidity increased significantly.At the end of the fermentation,the p H value fluctuated between 4.26~4.5,and the average acid production was75.79 °T(G81-1),81.84 °T(MGA21-4),84.57 °T(MGB90-4),88.12 °T(MGA45-4),90.37 °T(MGB79-3),91.58 °T(G72-5),respectively,for each strain.During fermentation and storage,significant difference in viscosity and dehydrating shrinkage were shown.The viscosity of the MGA21-4 fermented milkwas the highest,reaching 2149 m Pa · s after storage for 12 h.The lowest water holding capacity was observed in MGA45-4 fermented milk,which decreased to 3.44% after fermentation for 4 h.3.During fermentation and storage,a total of 116 kinds of volatile flavorcompoundswas detected from fermented milks produced by the 6 selected strains,including Acids(7),aldehydes(31),ketones(31),alcohols(34),and esters(13).Among them,there were 23 key flavor compounds,including aldehydes(11),ketones(5),alcohols(5),and esters(2).The key aroma compounds,namely hexanal,heptanal,octanal,nonanal,2,3-Butanedione,2-octanone,1-octen-3-ol,3-methyl-1-butanol,hexanoic acidethyl ester were the key aroma compounds in the period of fermentation;hexanal,heptanal,octanal,2,3-butanedione,acetoin,3-methyl-1-butanol,and formic acidethenyl ester,were detectedduring the storage period.4.By comparing the fermentation characteristics and the profile of volatile flavor compounds of fermented milks produced by the 6 S.thermophilus strains,it was found that the MGA45-4,MGB79-3 and G72-5 fermented milk had the most desirable properties.The variety and content of volatile flavor compounds produced in the fermented milk are superior to those of other strains,and can be used as the fermentors for dairy products.