Abstract: Solid phase microextraction coupled with gas chromatography-mass spectrometry (SPME-GC-MS) and relative odor activity value were used to study the key volatile compounds of fermented cow milk produced by pure and mixed cultures of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. The results showed that 100 volatile compounds, including acids, ketones, aldehydes, alcohols, estersds, hydrocarbons and aromatic compounds were detected. Principal component analysis (PCA) indicated that 2,3-butanedione, nonanal and toluene were the characteristic aroma compounds of fermented cow milk produced by a pure culture of S. thermophiles while hexanal, butanoic acid-2- methylpropyl ester and 1-heptanol were the characteristic aroma compounds of fermented cow milk produced by a pure culture of L. bulgaricus. Acetaldehyde, 3-methyl-butanal, acetoin, 2-nonanone, 2-heptanone, acetic acid ethenyl ester, carbonic acid-heptyl phenyl ester, formic acid-ethenyl ester, and 2-nonanol were the characteristic aroma compounds of fermented cow milk produced by mixed cultures. Compare to fermented cow milk produced by pure cultures, the compositions of volatile compounds and their relative contents and the key volatile compounds were all changed in fermented cow milk produced by mixed cultures.

Key words: fermented cow milk, key volatile compounds, solid phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS), principal component analysis