The Leuconostoc belong to the Leuconosto-caceae Family, which also contains the closely related genera Weissella and Oenococcus (see below). Leuconostocs are mesophilic, with optimum growth temperatures ranging from 18°C to 25 °C. Some species are capable of growth at temperatures below 10°C. Microscopically, they appear coccoid or even somewhat rod-like, depending on the composition and form of the growth medium (liquid versus solid). The leuconostocs, in contrast to the obligate homofermenting lactococci and streptococci, are obligately heterofermentative. Accordingly, they are missing an intact glycolytic pathway, and instead rely on the phosphoketo-lase pathway for metabolism of sugars (see below). Although leuconostocs can grow in an ambient atmosphere, a reduced or anaerobic environment generally enhances growth. Plas-mids are common in Leuconostoc species and, when present, may encode for important functions including lactose and citrate metabolism and bacteriocin production.
The genus Leuconostoc has also been subject to taxonomical revision, because several species have been moved to other existing or newly-formed genera. For example, Oenococ-cus oeni was formerly classified as Leuconos-toc oenos, and other Leuconostoc species have been reclassified as Weisella. The genus currently consists of thirteen species (Figure 2-4).
Speciation is based on both genetic and phenotypic characteristics. The latter include carbohydrate fermentation profiles, the ability to produce the polysaccharide dextran, resistance to the antibiotic vancomycin, and various physiological properties (Table 2-3). Most species are associated with particular habitats, including plant and vegetable material, milk and dairy environments, and meat products. In addition, some species are involved in food spoilage (e.g., Leuconostoc gasicomitatum), whereas others are used in food fermentations. The latter include Leuconostoc mesenteroides subsp. cremoris and Leuconostoc lactis, which are used in dairy fermentations, and Leuconos-
toc mesenteroides subsp. mesenteroides, Leuconostoc kimchii, and Leuconostoc fallax, that are used in vegetable fermentations. In general, the habitats occupied by these strains are reflected by the particular carbohydrates they ferment. Plant-associated strains, such as L. mesenteroides subsp. mesenteroides, ferment plant sugars (i.e., fructose, sucrose, arabinose, trehalose), whereas dairy strains (L. mesenteroides subsp. cremoris and L. lactis) are more likely to ferment lactose, galactose, and glucose. It is also possible for otherwise useful species to cause spoilage, particularly in the case of polysaccharide-producing strains that form slime.
Heterofermentative metabolism of sugars by leuconostocs results in formation of a mixture of end-products, including lactic acid, acetic acid, ethanol, and carbon dioxide.The presence of CO2 is readily detected and can be used di-agnostically to separate these bacteria from ho-mofermentative streptococci, lactococci, and pediococci. Also, while leuconostocs generally decrease the pH in the growth medium to between 4.5 and 5.0, acid production by some species may be relatively modest, especially when compared to homofermentative lacto-bacilli and other lactic acid bacteria.Thus, acidification is not necessarily the major function of these bacteria during fermentations.
In sauerkraut and other vegetable fermentations, for example, they lower the pH during the very early manufacturing stages and produce enough CO2 to reduce the redox potential in the food environment. These metabolic events then create an atmosphere conducive for growth of other microorganisms that are responsible for more significant acid development. In addition, the heterofermentative end-products result in a more diverse flavor and aroma profile in fermented foods. The Leu-conostoc species used in dairy fermentations are particularly important for flavor, in part, for the same reasons as the vegetable-associated strains. However, dairy leucononstocs also produce the four-carbon volatile, diacetyl, which imparts a desirable buttery aroma to cultured dairy products (discussed below).
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