Lactic acid bacteria

Background

Lactic acid bacteria are a group of related bacteria that produce lactic acid as a result of carbohydrate fermentation. These microbes are broadly used by us in the production of fermented food products, such as yogurt (Streptococcus spp. and Lactobacillus spp.), cheeses (Lactococcus spp.), sauerkraut (Leuconostoc spp.) and sausage.

These organisms are heterotrophic and generally have complex nutritional requirements because they lack many biosynthetic capabilities. Most species have multiple requirements for amino acids and vitamins. Because of this, lactic acid bacteria are generally abundant only in communities where these requirements can be provided. They are often associated with animal oral cavities and intestines (eg. Enterococcus faecalis), plant leaves (Lactobacillus, Leuconostoc) as well as decaying plant or animal matter such as rotting vegetables, fecal matter, compost, etc.

Lactic acid bacteria are used in the food industry for several reasons. Their growth lowers both the carbohydrate content of the foods that they ferment, and the pH due to lactic acid production. It is this acidification process which is one of the most desirable side-effects of their growth. The pH may drop to as low as 4.0, low enough to inhibit the growth of most other microorganisms including the most common human pathogens, thus allowing these foods prolonged shelf life. The acidity also changes the texture of the foods due to precipitation of some proteins, and the biochemical conversions involved in growth enhance the flavor. The fermentation (and growth of the bacteria) is self-limiting due to the sensitivity of lactic acid bacteria to such acidic pH.

Propionic acid bacteria

Background

The category of propionic acid bacteria comprises a numver of species of varying appearance. They do not form spores, their optimum temperature is about 30¡É, and several species suvive HTST pasteurisation. They fement lactate to propionic acid, carbon dioxide and other products. Pure cultures of propionic acid bacteria are used(together with certain lactobacilli and lactococci) in the manufacture of Emmenthal, Gruyere, Jarlsberg, Greve and Maasdam cheese, where they are responsible for the formation of eye and contribute to the characteristic flavour.

Acetic acid bacteria
Background

Acetic acid bacteria are bacteria that derive their energy from the oxidation of ethanol to acetic acid during respiration. They are Gram-negative rod-shaped bacteria that require oxygen. The acetic acid bacteria are found in nature where ethanol is being formed as a result of yeast fermentation of sugars and plant carbohydrates. They can be isolated from the nectar of flowers and from damaged fruit. Other good sources are fresh apple cider and unpasteurized beer which has not been filter sterilized. In these liquids the acetic acid bacteria grow as a surface film due to their aerobic nature and active motility. Vinegar is produced when acetic acid bacteria act on alcoholic beverages such as wine. Some genera, such as Acetobacter, can eventually oxidize acetic acid to carbon dioxide and water using Krebs cycle enzymes. Other genera, such as Gluconobacter, doesn't further oxidize acetic acid, as it does not have a full set of Krebs cycle enzymes. As these bacteria produce acid, they are unusually acid tolerant, growing well below pH 5.0, although the pH optimum for growth is

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