Although lactic acid is the major compound produced during the fermentation, other metabolic end products are also formed. Importantly, many of these products contribute to the overall flavor of sauerkraut. In particular, end products produced by Leuconostoc sp. and other heterofermentative lactic acid bacteria are essential for good-tasting sauerkraut.As much as 0.3% acetic acid and 0.5% ethanol can be present in the finished sauerkraut. In addition, these bacteria may also synthesize small amounts of diacetyl, acetaldehyde, and other volatile flavor compounds. Finally, the CO2 that accumulated during the initiation stage of the fermentation provides carbonation and enhances mouth feel.
Mannitol is another end product that accumulates during the sauerkraut fermentation. It is formed directly from fructose by heterofer-mentative leuconostocs and lactobacilli via the NADH-dependent enzyme, mannitol dehydro-genase.As shown in Figure 7-3, regeneration of NAD in the heterofermentative pathway ordinarily occurs twice, first when acetyl CoA is reduced to acetaldehyde and then when ac-etaldehyde is reduced to ethanol. However, fructose, when available in excess, can serve as an alternative electron acceptor.This allows the cell to use acetyl phosphate as a phosphoryl group donor in the acetate-generating reaction catalyzed by acetate kinase.As a consequence, one additional molecule of ATP is synthesized by substrate level phosphorylation.
More than 100 mM mannitol can be formed in sauerkraut by this pathway (with a commensurate amount of ATP made for the cell). Eventually, some of this mannitol may be consumed by lactobacilli that emerge later in the fermentation.Although most of the mannitol is produced directly from fructose, the appearance of mannitol even when fructose has been glucose fructose mannitol dehydrogenase
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