Symbolic Metabolite Analysis of Pickled Wax Gourd in Eastern China by 1H-NMR Spectroscopy and Multivariate Data

Pickled wax gourd is a traditional fermented vegetable product popular in Zhejiang province, eastern China; however, information on their quality is quite limited. In the present study, we investigated the quality of pickled wax gourd at different processing phases using nuclear magnetic resonance, principal component analysis, and orthogonal partial least squares-discriminant analysis. According to the spectra of ¹H NMR, 36 kinds of metabolites were found at the 20th day of fermentation. Principal component analysis score plot showed that diverse marker metabolites occurred in all designated sampling points (every other 5 days), apart from the samples between the 10th day and 15th day of fermentation. Symbolic metabolites at the first 5 days were L-rhamnose, lactate, fatty acid, arginine, N-acetylglutamate, succinate, phosphorylcholine, glutamate, ornithine, lysine, and β-glucose. Lactate, succinate, glycine, proline, and choline were detected at the following period from the 5th to the 10th day of fermentation. Lactate, succinate, glycine, mannitol, choline, and α-ketoglutaric acid were the main metabolites on the 15th and 20th day. The diversity of metabolites fits with the microbial species structure in different pickling periods. This method was proved to be effective for monitoring the pickling process in real time.