Kinetics of spontaneously fermented vegetables
Fermented vegetables are essentially produced by spontaneous fermentation. The establishment of the fermentative microbial community thus highly depends on the processing conditions. The type of cutting varies from finely grated to large pieces of vegetables in domestic productions, but its effect has been scarcely studied. Our aim was to investigate how the cutting and small variation of salt concentration impact the microbial and biochemical changes during spontaneously fermentation of vegetables. A 23 experimental design was designed to evaluate the type of cutting and salt concentration effect, for a root vegetable, carrot, and a leafy vegetable, cabbage, either thinly or roughly cut (grated vs sliced carrot and shredded vs leaf pieces of cabbage). Vegetables were pressed down in 500 mL-jars and filled up with brine to reach final NaCl concentrations of 0.8% or 1.0% (w/w). Jars were incubated at 19°C for 7 months and two independent jars characterised at 4 stages by culturomics, 16S rRNA V5-V7 and gyrB metataxonomics, and targeted metabolomics. The cutting type of cabbage significantly influenced microbial counts, taxonomic profiles, metabolite contents, and pH, while the NaCl concentration did not. A great variability from jar-to-jar was also observed. Finely-cut vegetables showed a quicker fermentation, in particular for cabbage, and, at a lower significance level, for carrot, as demonstrated by results of ANOVA and principal component analysis. Bile-tolerant Enterobacteriaceae grew first in all cases and reached higher maximal counts in cabbage than in carrot (8 vs 7 logCFU.g-1), followed by lactic acid bacteria, which reached higher counts in carrot than in cabbage (9 vs 8 logCFU.g-1). Consequently, the pH decreased faster in carrot than in cabbage, down to 3.8 in 40 h in all carrot vs, in 86 h, 4.8 in shredded cabbage and 5.3 in leaf cabbage. Viable bile-tolerant Enterobacteriaceae were not detected anymore after 2 weeks fermentation, except in some jars of roughly-cut cabbage. No pathogenic bacteria were detected. The gyrB marker, though rarely used in metataxonomics, provided resolution at the species-level in these ecosystems (i.e. for 97% of ASV vs 20% for 16S). Among the 17 Lactobacillales species identified by gyrB, 10 matched bacterial strains isolated from the products. Moreover, taxonomic profiles varied according to the marker, with for example, Leuconostoc nearly undetectable with the 16S marker used. This study highlights the tight competition between bile-tolerant Enterobacteriaceae and LAB in the colonisation of fermented vegetables, illustrates the diversity of their microbiota, and documents the effect of the type of cutting on the fermentation rate.