Screening Of Yogurt Starter Strains And Study On Its Fermentation Characteristics

Lactobacillus delbrueckii subsp.bulgaricus and Streptococcus thermophilus were crucial starter widely used in the production of dairy products.This study was sought to evaluate the potential of fascinating flavor and fermentation characteristics production by starter strains in fermented milk.It is of great significance to the production of dairy products.Lactobacillus delbrueckii subsp.bulgaricus and Streptococcus thermophilus were used in combination as experimental strains for fermentation.Physicochemical indexes were measured in fermented milk,including pH,titration acidity,viable count,and viscosity during milk fermentation and post-ripening(12 h).Solid phase microextraction and gas chromatography-mass spectrometry(SPME-GC-MS)combined with electronic nose technology was used to analyze the aroma profile in fermented milk at the end of fermentation.The results are as follows:A comparative analysis of fermentation characteristics of 18 groups compound starters and commercial starter JD showed that A2(IMAU40078&IMAU20246),A4(IMAU40078&IMAU80844),and A6(IMAU40078&IMAU40133)compound groups have good fermentation characteristics.Compared with commercial starter JD,A2,A4,and A6 have the shortest fermentation time to reach the end of fermentation,the fermentation time for these compound starters to reach the end of fermentation is less than 10 hours.Compared with commercial starter JD,the titration acidity of A2,A4,A6 fermented milk did not change significantly(P>0.05)at the end of fermentation.At the end of fermentation,A2,A4,and A6 have higher viable counts in fermented milk.A6 compound fermented milk has the highest number of viable bacteria in fermented milk.A6 fermented milk viable count is close to commercial starter JD compound fermented milk number of live bacteria in milk.At the end of fermentation,compared with other compound starters,the viscosity of A6 compound fermented milk was the highest,reaching 1020 mPa.s.Therefore,the results showed that the A6 compound group has good fermentation characteristics.The results of SPME-GC-MS with odor activity values(OAV)indicated that a total of 128 volatile flavor compounds were identified in the fermented milk.The identified compounds were constituted of 18 acids,13 aldehyde compounds,18 ketones,22 alcohols,12 esters,30 alkanes,and 15 nitrogen compounds.10 kinds of key flavor compounds(OAV?1)were contained in fermented milk,including Hexanal,3-methyl-butanal,benzaldehyde,nonanal,decanal,2,3-butanedione,2,3-Pentanedione,2-nonanone,acetoin,formic acid ethenyl ester.Moreover,0.1?OAV<1 compounds incorporate 3-methylbutanoic acid,butanoic acid,octanoic acid,acetaldehyde,3-hydroxybutanal,heptanal,2-heptanone,1-nonanol,sec-butyl nitrite,they play modificatory role in the overall flavor of fermented milk.Compared with commercial starter JD,it was found that flavor compounds such as 2,3-butanedione,acetoin,2-heptanone,and 2-nonanone,produced consortium A6 fermented milk,had significantly higher concentration.These compounds confer fascinating flavor to the fermented milk.The analysis results of GC-MS fingerprint similarity evaluation showed that the similarity between A6 compound starter and commercial starter JD was the highest,reaching 0.680.Principal component analysis(PCA)and linear discriminant analysis(LDA)evidence that the flavors of A1(IMAU40078&IMAU20153),A4,A6,A12(IMAU62081&IMAU40133)and commercial starter JD fermented milk aroma were similar.Therefore,it is shown that among the 18 groups of compound starter,A6 compound group has good aroma producing characteristics during milk fermentation.Comprehensive analysis of the fermentation characteristics and flavor compounds in fermented milk of 18 groups compound starters and commercial starter JD demonstrate that produced consortium(IMAU40078&IMAU40133)fermented milk have good fermentation characteristics and excellent flavor,which were significantly better than other compound starters.Thus,it can be used as yogurt starer for dairy production.