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Thermophilic cultures originate from south and east Europe. They consist mainly of Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus and lactis, and Lactobacillus helveticus. The thermophilic LAB are used for rapid acidification or as adjuncts in cheeses. Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, and some Lactobacillus delbrueckii subsp. lactis catabolize lactose into lactate and galactose, which are secreted. The residual galactose can create problems (e.g., growth of undesirable bacteria) in cheese and influences the browning of pizza cheese. Lactobacillus helveticus can use galactose as a carbon source and thereby remove the residual galactose. Furthermore, some strains of Lactobacillus helveticus are very proteolytic, thereby influencing the taste and texture of cheese (25,26). Thermophilic LAB can also produce acetaldehyde, which is the characteristic flavor of yogurt. The composition of these starter cultures varies. Both defined single or multiple starter strain cultures and mixed undefined cultures are used. Mozzarella and yogurt are commonly produced by single or multiple strain starters that contain one or more S. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus strains, but many cooked cheeses are produced with mixed cultures. They are used either as milk cultures or whey cultures, with or without rennet. Thermophilic cultures and some examples of their products are described in more detail in Chapter 16.

Other cheeses or fermented milk products are made with both mesophilic or thermo-philic LAB starter cultures, or by addition of other LAB such as Enterococcus faecium, Lactobacillus acidophilus, or other genera of bacteria (e.g., Bifidobacterium). Yeast and mold can also be added. Table 6 gives examples of dairy products produced with different combinations of microorganisms.

One of the disadvantages of using pure bacterial starter cultures is that they are more sensitive to bacteriophages than undefined mixed starters (23) or indigenous flora, where there will always be strains present that can survive phage attack and continue fermentation. Lactic acid fermentations of milk for cheese production are especially susceptible to phages, and special precautions have to be taken in order to exclude them. The use of phage-resistant starter cultures, a high level of cleaning and sanitation, use of closed vats equipped with filters, specially designed pipelines, equipment and facilities, minimal access of persons, and high personal hygiene are all recommended to achieve this. In this way, it was possible to use a mixed O-culture, TK5, for production of Cheddar cheese in Denmark for 11 years before bacteriophages able to inhibit acidification appeared (27,28). Unfortunately, the phages became so virulent that after 12 years of production, it was not possible to use the TK5 starter culture any longer in the dairy (23).

Table 6 Examples of Fermented Milk Products and the Composition of their Added Starter Cultures

Fermented milk products

Microorganisms

Acidophilus milk

DL starter, Lb. acidophilus

Viili

DL starter, Geotricium candidum

Langfil

D, L, or DL culture

Cultura, AB-milk

Lb. acidophilus, Lb. johnsonii, Lb. gasseri,

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