Streptococcus thermophilus is widely used in the dairy industry to produce fermented milk. The gas chromatography-ion mobility spectrometry (GC-IMS) based metabolomics was used to discriminate different fermentation temperatures (37 °C and 42 °C) at three time-points (F0: pH = 6.50 ± 0.02; F1: pH = 5.20 ± 0.02; F2: pH = 4.60 ± 0.02) during S. thermophilus milk fermentation, and differences of fermentation physical properties and growth curves were also evaluated. Fermentation was completed (pH 4.60) after 6h at 42°C and after 8h at 37 °C; there were no significant differences in viable cell counts and titratable acidity (TA); water holding capacity (WHC) and viscosity are higher at 37 °C than those at 42 °C. Different fermentation temperatures affected volatile metabolic profiles. After the fermentation was completed, the volatile metabolites that can be used to distinguish the fermentation temperature are hexanal, butyraldehyde, ethyl acetate, ethanol, 3-methylbutanal, 3-methylbutanoic acid, and 2-methylpropionic acid. Specifically, at 37°C of milk fermentation completed, branched-chain amino acids were higher levels, and leucine, isoleucine, and valine were involved in growth and metabolism, which promoted accumulation of some short chain fatty acids (SCFAs) such as 3-methylbutanoic acid and 2-methylpanprooic acid. At 42°C, at three different time-points during fermentation, ethanol from glycolysis all presented higher levels, including acetone and 3-methylbutanal can producing more pleasant flavour to the fermented milk. This work provides a detailed insight into S. thermophilus fermented milk metabolites which differed between incubation temperatures; these data can be used for understanding and eventually predicting metabolic changes during milk fermentation.

Streptococcus thermophilus is widely used in the dairy industry to produce fermented milk. The gas chromatography-ion mobility spectrometry (GC-IMS) based metabolomics was used to discriminate different fermentation temperatures (37 °C and 42 °C) at three time-points (F0: pH = 6.50 ± 0.02; F1: pH = 5.20 ± 0.02; F2: pH = 4.60 ± 0.02) during S. thermophilus milk fermentation, and differences of fermentation physical properties and growth curves were also evaluated. Fermentation was completed (pH 4.60) after 6h at 42°C and after 8h at 37 °C; there were no significant differences in viable cell counts and titratable acidity (TA); water holding capacity (WHC) and viscosity are higher at 37 °C than those at 42 °C. Different fermentation temperatures affected volatile metabolic profiles. After the fermentation was completed, the volatile metabolites that can be used to distinguish the fermentation temperature are hexanal, butyraldehyde, ethyl acetate, ethanol, 3-methylbutanal, 3-methylbutanoic acid, and 2-methylpropionic acid. Specifically, at 37°C of milk fermentation completed, branched-chain amino acids were higher levels, and leucine, isoleucine, and valine were involved in growth and metabolism, which promoted accumulation of some short chain fatty acids (SCFAs) such as 3-methylbutanoic acid and 2-methylpanprooic acid. At 42°C, at three different time-points during fermentation, ethanol from glycolysis all presented higher levels, including acetone and 3-methylbutanal can producing more pleasant flavour to the fermented milk. This work provides a detailed insight into S. thermophilus fermented milk metabolites which differed between incubation temperatures; these data can be used for understanding and eventually predicting metabolic changes during milk fermentation.