Dynamics of the glucosinolate–myrosinase system in tuber mustard (Brassica juncea var. tumida) during pickling and its relationship with bacterial communities and fermentation characteristics

Pickled tuber mustard is a traditional fermented pickle widely consumed in China, and it is characterized by the presence of glucosinolates (GSLs). To understand the biotransformation of GSLs in tuber mustard during pickling, the dynamics of the glucosinolate-myrosinase (GSL-MYR) system and its potential associations with bacterial communities and fermentation characteristics (i.e., salinity, titratable acidity [TAA], and pH) were investigated. In total, 18 GSLs were identified in fresh tuber mustard; 12 were aliphatic, 4 were indolic, and 2 were aromatic, with aliphatic sinigrin and aromatic gluconasturtiin being the dominating components. The pickling process resulted in complete degradation of GSLs, with isothiocyanates (ITCs) and nitriles being the main breakdown products. Total ITCs reached maximum concentrations on day 21-28, while total nitriles peaked at the end of pickling. Based on Spearman's correlation analysis, our study showed that lactic acid bacteria (LAB) species might contribute to GSL transformation in pickled tuber mustard. Specifically, Weissella paramesenteroides, Pediococcus pentosaceus, and unclassified Lactococcus exhibited positive correlations with GSL contents (p < 0.01), suggesting that they might contribute to the increasing amounts of GSLs in the initial pickling, while the Lactobacillus-related populations that dominated in the later stages (i.e., Companilactobacillus alimentarius and Lactiplantibacillus Plantarum) were positively correlated with nitrile product concentrations. Moreover, redundancy analysis showed that pH and TAA had strong effect on myrosinase activity during tuber mustard pickling, which was dictated via the organic acids produced by microorganisms. This study provided a perspective for understanding the effect of fermentation on the transformation of tuber mustard GSLs.