Under the anaerobic conditions found with brined vegetables, rapid fermentation by LAB and yeasts occurs, resulting in the destruction of most other microflora, usually within a few days of the onset of fermentation . In the U.S., cucumber pickles and sauerkraut represent the majority of fermented vegetable products. For pickles, fermentation was the primary means of preservation until the 1940s, when direct acidification and pasteurization of cucumber pickles was introduced (reviewed by Fleming et al. ). Currently, fermented cucumbers represent roughly 30% of commercial production of pickles, mostly for institutional markets (hamburger dill slices), with the majority of the retail market being nonfermented acidified pickles which are pasteurized to destroy vegetative microflora.
Vegetable fermentations typically begin with heterofermentative LAB, such as Leuconostoc mesenteroides and end with the most acid-resistant homo-fermentative LAB, usually Lactobacillus plantarum [1,52,53]. Lactobacillus plantarum is able to tolerate a lower internal pH than other LAB, and this feature may allow it to predominate in the terminal stages of most vegetable fermentations . During the fermentation of cucumbers and cabbage, hexose sugars, including glucose and fructose, are typically converted to lactic acid by homofermentative LAB via the Embden-Myerhof-Parnas pathway, while the heterofermentative LAB will produce a combination of lactic acid and acetic acid or ethanol, along with CO2 via the phospho-ketolase pathway . When fructose is present, LAB can use this sugar as an electron acceptor, producing mannitol, which subsequently can be converted anaerobically to lactic acid with an appropriate electron acceptor . In cucumber fermentation where malate is present, L. plantarum and other LAB have been found to carry out a decarboxylation of malate to produce lactic acid and CO2 . This one-step reaction occurs via malolactic enzyme, and is analogous to the amino acid decarboxylation reactions described below . During the reaction, a proton is taken up from the surrounding medium, which helps to buffer cellular pH and causes the pH in the surrounding medium to rise.
In the U.S., commercial cucumber fermentations are typically carried out with 5 to 6% NaCl, while cabbage fermentations are carried out with 2 to 3% NaCl . During the growth of LAB in vegetable fermentations, a variety of antimicrobial metabolic end products are produced, including organic acids, peroxides, amines, thiols, bacteriocins, and other enzymes and compounds [1,4,5,58-61]. These inhibitory compounds begin to accumulate in the initial stages of fermentation. A combination of several factors, including organic acids from the fermentation (up to 2 to 3% organic acids may be produced), complete fermentation of available sugar, terminal pH values around 3 to 3.5, and salt, can serve to destroy most vegetative bacterial cells, including human pathogens. Desirable textural and nutritional properties of the fermented vegetables may be maintained during storage in the fermentation brine for extended periods of time (a year or more) without refrigeration.
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