Study On The Screening And Fermentation Characteristics Of Yogurt Starters With Different Compound Proportions

Yoghurt starter is of great significance to the development and utilization of fermented dairy products.In this study,the commercial starter provided by Kohansen Company was used as the control group.On the basis of previous studies,Lactobacillus delbrueckii subsp.bulgaricus IMAU20312 and Streptococcus thermophilus IMAU80809 which had good fermentation characteristics was isolated from traditional fermented dairy products.Viable bacteria,p H,titrating acidity,viscosity and water holding capacity of yoghurt were detected during milk fermentation(0,2,4,6 h,fermentation end point)with different proportions of L.delbrueckii subsp.bulgaricus and S.thermophilus.At the same time,the volatile flavor compounds produced by different proportions of starters at the fermentation end point were detected and analyzed by SPME-GC-MS technology and the best compound ratio was determined.Then the characteristics of the compound ratio during milk fermentation and storage(0,1,3,7,14,21 days)were further analyzed and the results were as follows:At the end of fermentation,the titration acidity of A1(1:1)compound group was the highest,which was 86.15 °T,but lower than that of the control group(97.5 °T).The viscosity of fermented milk of A3(1:100)group reached 621 m Pa·s,which was better than that of the control group(600 m Pa·s),A2(1:10)and A3 which the water-holding capacity of fermented milk reached 54% and 56% respectively was better than that of the control group(50%).At the end of fermentation,72 volatile flavor compounds were detected in the fermented milk of the compound group,including 19 kinds of acids,7 kinds of aldehydes,11 kinds of alcohols,10 kinds of ketones,5 kinds of esters,16 kinds of alkanes and 4 kinds of nitrogen compounds.Some important flavor compounds,such as acetic acid,caproic acid,octanoic acid,acetaldehyde,benzaldehyde,n-nonanal,heptanal have important effects on the formation of fermented milk flavor.The results of heat map cluster analysis and principal component analysis of fermented milk with different compound ratios showed that the type and content of volatile flavor compounds in fermented milk of A3 compound group were close to the control group,and the results of sensory evaluation also confirmed this conclusion.A total of 117 volatile flavor compounds were produced in A3 compound group during milk fermentation and storage,including 24 acids,16 alcohols,13 aldehydes,18 ketones,11 esters,23 alkanes and 12 nitrogen compounds.The results of partial least squares discriminant analysis showed that fermentation period(4,6 h)and the early storage period(1 d)were similar.In addition,the middle storage period(3,7 d)and the late storage period(14 d)were similar.However,the types and contents of volatile flavor compounds in fermented milk at the late storage period(21 d)were significantly different from those at the other time.Meanwhile,the results of electronic nose system detection and analysis also confirmed this conclusion.To sum up,L.delbrueckii subsp.bulgaricus IMAU20312 and S.thermophilus IMAU80809 has a great influence on the number of living bacteria,p H value,titration acidity,viscosity,water-holding capacity and the type and content of volatile flavor compounds in yoghurt.When the compound ratio is 1:100,the taste and flavor of fermented milk are closer to those of the control group,indicating that the compound ratio has good application potential in the development and utilization of dairy starter.