Characterization of Chinese liquor starter, "Daqu", by flavor type with 1H NMR-based nontargeted analysis

"Daqu" is a fermentation starter and substrate complex that is used to initiate fermentations for the production of Chinese liquor (alcoholic spirit). Several different types of Daqu are customary used, having different flavours, i.e. light, strong, or sauce flavor. With the aim to develop objective methods to characterize and distinguish such different types of Daqu, nontargeted analyses of extracts from three typical flavor types of Daqu were carried out using (1)H nuclear magnetic resonance (NMR) spectroscopy. A significant separation of spectra of Daqu of light-flavor, strong-flavor and sauce-flavor types was achieved using principal components analysis. The separation could be attributed to higher levels of glycerol, malate, acetate and N-acetylglutamine in light-flavor Daqu; higher levels of mannitol, betaine, trimethylamine and pyroglutamate in strong-flavor Daqu; and higher levels of lactate, isoleucine, leucine, isovalerate and valine in sauce-flavor Daqu. These metabolites were regarded as the representative metabolites or biomarkers characteristic for each type of Daqu and could be associated with some of the microorganisms that have been reported in Daqu. This study highlights the application of nontargeted analysis techniques based on NMR in process research and quality control in Daqu production and liquor fermentation.