Thermophilic Lactic Acid Bacteria

The thermophilic lactic acid bacteria are best known as starters for fermented milks. Several varieties of fermented milks originate from countries in Asia Minor and the Balkans (e.g., Bulgarian yogurts). These yogurt products have emerged from spontaneous acidification of raw milk by indigenous organisms and have now been developed into mi-crobiologically well controlled industrial processes. The two most frequently used starter bacteria are classified as Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, generally shortened as Lb. bulgaricus and S. thermophilus, respectively.

Yogurt is usually made by inoculating an equal mixture of S. thermophilus and Lb. bulgaricus in milk and incubating it at 37-45°C. S. thermophilus grows optimally at these temperatures and hydrolyzes lactose via a h-galactosidase. S. thermophilus is nutritionally fastidious and requires a complex mixture of amino acids for growth. Because it is proteolytic negative, the coupling with the more proteolytic Lb. bulgaricus stimulates its growth in milk. The production of formic acid and carbon dioxide from lactose by S. thermophilus, on the other hand, stimulates the growth of Lb. bulgaricus (Fig. 2) (34).

Thermophilic lactic acid bacteria also play an essential role in the manufacture of some cheese types. The starters of Swiss-type and Italian cheeses consist mainly of S. thermophilus, Lb. helveticus, and Lb. bulgaricus (35,36). Also, in the ripening of Greek hard cheese types made from ewes' and goats' milk, thermophilic lactic acid bacteria play a dominant role (37). The specific high cooking temperature used in the manufacture creates their niche in these cheese types. They convert lactose to lactic acid as in all dairy fermentations. This acid plays its usual role as preservative and, for Swiss-type cheeses, is the substrate for the subsequent propionic acid fermentation, leading to large eye formation in the cheeses.

Protocoop Ration

lactic acid + acetatdehydv

Figure 2 Proto-cooperation between Streptrococcus thermophilus and Lactobacillus bulgaricus in a yogurt culture. This symbiosis results in an increased lactic acid and acetaldehyde (yogurt flavor) production.

lactic acid + acetatdehydv

Figure 2 Proto-cooperation between Streptrococcus thermophilus and Lactobacillus bulgaricus in a yogurt culture. This symbiosis results in an increased lactic acid and acetaldehyde (yogurt flavor) production.

Because thermophilic lactic acid bacteria were found to be very active, they are employed as adjunct cultures in the manufacture of various cheeses. In some cases they are specifically selected to address certain flavor aspects of cheese (e.g., bitterness). Smit et al. (38-40) reported on thermophilic lactic acid bacteria with high debittering activities, which were found to be very useful in debittering of cheeses made with a mesophilic starter culture that causes bitterness (Fig. 3). Various thermophilic adjunct cultures are also employed to enhance the formation of desired flavor notes in a given cheese.

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