NMR- and GC/MS-based metabolomic characterization of sunki, an unsalted fermented pickle of turnip leaves

This study revealed the compositional characteristics of sunki, a traditional, unsalted, lactic acid-fermented pickle produced using turnip leaf in Kiso district, Japan. Comprehensive compositional analysis by two metabolomic approaches based on NMR and solid-phase microextraction-GC/MS methods was used to determine its chemical composition by annotating 54 water-soluble and 62 volatile compounds. Principal component analysis showed that samples had different compositions, depending on the agricultural processing factory and production year. This variation potentially resulted from the differences in the lactic acid bacterial community produced during the spontaneous fermentation of sunki and in the initial nutritional composition of the turnip leaf. Partial least squares regression revealed that the acetic acid level showed a strong positive correlation with pH (R = 0.810), in contrast to the negative correlations of lactic acid and ethanol levels (R = -0.533 and -0.547). This indicated the crucial impact of acetic acid-related metabolism on acidification during sunki fermentation.