Effects of Fermentation Period on the Non-Volatile Metabolites of Chinese Ultra-Long-Term Solid Fermented Kohlrabi Based on Non-Targeted Metabolomic Analysis

This study aimed to investigate the effects of ultra-long-term fermentation on the formation of non-volatile metabolites of Chinese solid-fermented kohlrabies. Liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) based non-targeted metabolomics coupled with multivariate statistical analysis were employed to respectively analyze the kohlrabies solid fermented for 5 years (5Y), 8 years (8Y), and 11 years (11Y). The results showed that 31, 169, and 123 differential metabolites were identified in the three groups of 5Y and 8Y (A1), 5Y and 11Y (A2), and 8Y and 11Y (A3), respectively (VIP > 1, p < 0.05 and |log2FC| > 1). The differential non-volatile metabolites were mainly organic acids and derivatives, organoheterocyclic compounds, benzenoids, lipids and lipid-like molecules, and organicoxygen compounds. Furthermore, 11 common differential metabolites were screened in the three groups, including diaminopimelic acid, ectoine, 9,10,13-TriHOME, and 9 others. The citrate cycle, glycine, serine and threonine metabolism, pantothenate and CoA biosynthesis, and glyoxylate and dicarboxylate metabolism were the four pathways most significantly correlated with the differential non-volatile metabolites based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis (p < 0.05). The present study describes the effects of ultra-long-term fermentation periods on the formation of non-volatile metabolites in solid fermented kohlrabies, providing a theoretical basis for cooking with the three solid fermented kohlrabies to make different Chinese dishes.