Metabolic footprint analysis of volatile metabolites by GC-IMS to discriminate different fermentation temperatures during Streptococcus thermophilus 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.