Fermentation dynamics of Lactobacillus helveticus H9 revealed by ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometry

Lactobacillus (L.) helveticus H9 is a probiotic strain that can produce antihypertensive peptides during milk fermentation. This study analysed the dynamics of skim milk fermentation by L.helveticus H9 by ultra-performance liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (UPLC/Q-TOF MS). A total of 1992 metabolites were detected from all of the fermented samples in the LC-MS analysis by multivariate statistical analysis. Metabolites with variable importance in projection (VIP) values >= 2 were considered differentially abundant among samples and were responsible for the unique taste and nutritional and functional qualities of fermented milk. Valine, threonine, L-methionine, tyrosine, asparagine and leucine were the predominant amino acids produced during fermentation, and their quantities changed remarkably during the fermentation process. Citric acid and uric acid were the major, and only detectable, organic acids. Some intermediate metabolites, such as N-acryloylglycine and nicotinamide-N-oxide, were also detected. Moreover, certain oligopeptides such as Val-Leu, Lys-Gly, Ala-Glu, Asp-Ser, Leu-Pro and Val-Phe-Ala were not detected until the middle and late fermentation periods. This study demonstrated dynamic metabolic changes, providing a strong foundation for deciphering the physiological and biochemical mechanisms of the fermentation process.