The production characteristics of volatile organic compounds and their relation to growth status of Staphylococcus aureus in milk environment

In our previous research, 3-methyl-butanal and 3-methyl-butanoic acid were identified as representative and specific volatile organic compounds released by Staphylococcus aureus in broth. In this study, we explored the production of the 2 volatiles and their correlation to Staph. aureus growth in milk under different conditions. We found significant correlations between the production of 3-methyl-butanoic acid and cell counts of 5 Staph. aureus strains in sterile milk, and there were no obvious differences for its production among 5 tested strains. The intensities of the 2 volatiles were similar and positively correlated with bacterial counts in cultures at 25°C and 37°C despite delayed production of volatiles at 25°C; however, neither compound could be detected at 4°C. The production of 3-methyl-butanoic acid was strongly correlated with growth of Staph. aureus mixed with Streptococcus agalactiae, Escherichia coli O157:H7, and Shigella flexneri, whereas correlations for 3-methyl-butanal were not statistically significant. Compared with the monoculture of Staph. aureus, in mixed cultures, production of 3-methyl-butanal was decreased and that of 3-methyl-butanoic acid was comparatively higher. In pasteurized and raw milks, production of 3-methyl-butanoic acid was correlated with growth of Staph. aureus, and 3-methyl-butanoic acid could be detected when Staph. aureus populations reached 106 to 107 cfu/mL in pasteurized milk and 105 to 106 cfu/mL in raw milk; the correlations for 3-methyl-butanal were not statistically significant. Our results suggest that 3-methyl-butanoic acid is a more suitable marker for high counts of Staph. aureus in milk, whereas 3-methyl-butanal is a transient metabolite and easily depressed by environmental factors.