Metabolomic analysis of meju during fermentation by ultra performance liquid chromatography-quadrupole-time of flight mass spectrometry (UPLC-Q-TOF MS)

Changes in the water-soluble metabolites of meju during fermentation were analysed by ultra performance liquid chromatography-quadrupole-time of flight mass spectrometry (UPLC-Q-TOF MS), and the resultant data were statistically processed by partial least squares-discriminant analysis (PLS-DA). Various metabolites, including amino acids, small peptides, nucleosides, urea cycle intermediates, and organic acids, which are responsible for the unique taste and nutritional and functional quality of fermented soy foods, were clearly altered by increasing the fermentation period. Changes in these metabolites allowed discrimination among meju samples with different fermentation periods (0, 10, 20, 40, and 60 d) on a PLS-DA score plot, and the fermentation was mainly processed between 10 and 40 d of fermentation. Twenty-two metabolites (phenylalanine, glutamic acid, leucine, adenine, citrulline, arginine, glutamine, γ-aminobutyric acid, proline, acetylornithine, valine, pipecolic acid, methionine, citric acid, xanthine, tyrosine, isoleucine, Glu-Tyr, Ser-Pro, tryptophan, Glu-Phe, and Leu-Val-Pro-Pro) with high PLS-DA values of over 1.00 were determined as the major compounds contributing to the discrimination of meju samples. These metabolites, which were positively related to the sensory quality of meju, can be used as fermentation biomarkers for the production of meju and to construct the meju fermentation metabolic pathway. Therefore, our results indicate that monitoring the changes in metabolites during meju fermentation might be important for producing meju-related foods with good nutritional and sensory quality.