Propionic acid fermentation is carried out by several bacteria that belong to the genus Propionibacterium and to the species Clostridium propionicum. During propionic acid fermentation, both sugar and lactate can be used as the initial substrate. When sugar is available, these bacteria use the EMP pathway to produce pyruvate; the pyruvate is carboxylated to oxalacetate by methyl malonyl coen-zyme-A (CoA) and then reduced to propionate via malate, fumarate, and succinate. The other end products of propionic fermentation are acetic acid and CO2 (Figure 3). In particular, the propionic acid fermentation of 3 mol of glucose produces 4 mol of propionic acid, 2 mol of acetic acid, 2 mol of CO2, and 12 mol of ATP. When lactate is the initial substrate, propionic fermentation results in the production of 2 mol of propionic acid, 1 mol of acetic acid, and 1 mol of CO2. In this process, 1 mol of ATP is generated per nine carbons, and because of this, propionic bacteria generally grow very slowly.
The typical natural habitats of Propionibacterium are the rumen, the intestinal tract of animals and the skin of mammalians. Propionibacterium also colonize cheese during maturation. While the metabolic activity of Propionibacterium should be avoided during the maturation of the vast majority of cheese, it is required for the production of some typical products, such as Emmental cheese. In this Swiss-type cheese, two successive fermentations occur. During its manufacture, lactic acid bacteria convert lactose into lactate, and then during
Propionate i4 mo1 2 mo'
Figure 3 Propionic fermentation and different yields of the final products coming from glucose or lactate as the initial substrate.
ripening, propionic acid bacteria convert the lactate into propionic acid, acetic acid, and CO2. The CO2 is responsible for 'eye' formation and the propionic acid promotes the typical nutty flavor of this Swiss-type cheese. The ability to use lactate is particularly relevant for the ecological distribution of propionic acid fermenting bacteria, which can use the final product of lactic acid fermentation.
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