Lactic Acid Bacteria

When raw milk is left at room temperature for some time, a microflora will develop in which lactic acid bacteria generally dominate. These bacteria acidify the milk and, as a consequence, inhibit the growth of other bacteria. Concomitantly, they give the milk a pleasant flavor. This spontaneous fermentation has been the basis of several fermented dairy products in history. The discovery of the role of lactic acid bacteria in milk fermentation paved the way for their isolation, characterization, and exploitation. This started more than a century ago and has resulted in the development of starter cultures for the manufacture of fermented dairy products. Both industrial and small-scale manufacture now almost always rely on industrially prepared starters (6).

The lactic acid bacteria used in dairy fermentations can roughly be divided into two groups on the basis of their growth optimum. Mesophilic lactic acid bacteria have an optimum growth temperature between 20° and 30°C; thermophilic strains, between 30° and 45°C. Traditional fermented products from subtropical countries harbor mainly thermo-philic lactic acid bacteria, whereas products with mesophilic bacteria originate mainly from western and northern European countries. Currently, most dairy industries use starter cultures for rapid acidification, because a spontaneous fermentation acidifies the milk too slowly. This method of working has reduced the number of strains present and is responsible for certain uniformity in dairy products. On the other hand, it has made production very reliable and efficient. The microorganisms present in natural niches still form a very interesting potential for product diversification, due to their large biodiversity (see below).

Industrial (dairy) starter cultures can be divided into two groups: undefined and defined starters. The undefined starters are a mixture of an unknown number of lactic acid bacteria types, which are originally derived from an artisanal production practice and further selected for the production of good-quality dairy products. Mesophilic undefined starter cultures are commonly used for the manufacture of Cheddar, Gouda, and other cheese types. Their use is based on their consistent performance, especially their well-recognized phage resistance (7,8).

Defined-strain starters are blends of two of more strains; in the case of Cheddar cheese, they are frequently used nowadays instead of the former undefined mixed-strain starters. The risk of phage attacks is greater here than with the use of undefined mixed-

strain cultures, so cultures with different phage sensitivity profiles are used in rotation (810). Another well-known example of a defined-strain starter is the mixture of thermophilic lactic acid bacteria in the manufacture of yogurt.

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