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.