Lactic acid bacteria(LAB)are industrially important bacteria that are widely used in the fermented food industry,especially in the manufacture of yogurt.Characteristic flavors are produced by LAB during fermentation and storage that affect the quality and acceptability of fermented milk products.In this study,the volatile compounds in milk fermented by Streptococcus thermophilus IMAU80842 alone,Lactobacillus delbrueckii subsp.bulgaricus IMAU20401 alone,or both species together were identified using Solid Phase Microextraction methods coupled with gas chromatography/mass spectrometry(SPME-GC/MS).There were 53,43,and 32 volatile compounds identified in milk fermented by S.thermophilus alone,L.delbrueckii subsp.bulgaricus alone,or both species together,respectively.The presence of some important flavor compounds was confirmed: acetic acid,acetaldehyde,acetoin,2,3-butanedione,ethanol,and 1-heptanol.Our results demonstrate that the composition of the volatile compounds in fermented milk depends on the species of LAB used and whether they are used alone or in combination.This is important for the selection of appropriate starter cultures for the production of different types of fermented milk product with particular flavors.