Non-targeted Metabolomics Analysis of Metabolic Differences among Liuyang Douchi Fermented by Different Strains

In order to investigate the metabolic differences among Liuyang Douchi fermented by different starter cultures, the contents of total acid and amino nitrogen in Douchi fermented naturally and by using Aspergillus flavus 7214 (AF 7214), A. flavus 7622 (AF 7622), their mixture (AF 7214 + AF 7622) or A. orzyae were determined, and the difference in metabolites among these fermentation strategies was explored by liquid chromatography-mass spectrometry (LC-MS). The results showed that among the five Douchi samples, Douchi fermented by AF 7214 had the highest contents of total acid (3.52%) and amino nitrogen (1.47 g/100 g). The results of partial least squares discriminant analysis (PLS-DA) indicated that significant differences in metabolites were observed among the Douchi samples, and the composition of metabolites in Douchi fermented by AF 7622 showed the smallest difference from that in naturally fermented Douchi. Further analysis revealed that the differential metabolites between Douchi fermented naturally and by using starter cultures were mainly involved in amino acid metabolic pathways, especially arginine biosynthesis. A total of 62 key differential metabolites among the five samples were identified using variable importance in the projection (VIP) greater than 1.5 and P < 0.05 as criteria, including 26 amino acids and their derivatives such as L-lysine, L-serine and 2-methylserine, which indicated that enhanced fermentation showed the most notable influence on the metabolism of amino acids. This study has provided a new understanding of the formation of metabolites during the enhanced fermentation of Liuyang Douchi.