**4. Microbial cultures used in the yogurt manufacture**

 Bacterial populations which are traditionally used in the manufacture of yogurt include species such as *Streptococcus thermophilus* (**Figure 1**) and *Lactobacillus bulgaricus* (**Figure 2**). The young cells of *S. thermophilus* are spherical in shape and occur in chains. In the dairy industry, they are often called "cocos". These cultures usually have weak milk clotting because of low acid production. Strains of *S. thermophilus* are commonly used in association with *Lactobacillus delbrueckii* ssp. *bulgaricus*. The latter is commonly referred as "rod" in the dairy industry, and the combination of the two bacterial populations is called the "coconut stick" [23].

 When a single population of *Lactobacillus delbrueckii* ssp. *bulgaricus* or *Streptococcus thermophilus* is used, the production of lactic acid and acetaldehyde was greatly reduced when compared to that of combined commercial cultures [24, 25].

 In the first fermentation stages of the yogurt, in which the action of those bacteria are evident, *S. thermophilus* grow much faster because of their greater aerotolerance, whereas at this stage, the populations of *L. bulgaricus* grow more slowly; however, and due to their greater proteolytic activity, these species provides enough peptides to stimulate and guarantee the growth of *S. thermophilus* [23]. The slow growth of *Lactobacillus* populations may be due to the fact that they are microaerophilic [26]. Thus, at the end of the first phase, the growth of *S. thermophilus* slows down because of the high concentration of lactic acid produced. Besides, in this phase the production of formic acid is high enough to stimulate the growth

**Figure 1.**  *Streptococcus thermophilus.* 

**Figure 2.**  *Lactobacillus bulgaricus.* 

of *Lactobacillus* [25]. Furthermore, due to this complementary action of the two species, the desirable acidity of the yogurt can be achieved. Sá and Barbosa (1990) [27] report that these two species of microorganisms develop in cooperation at an optimum temperature of 45°C; this temperature can, however, decrease to 42°C.

The optimum ratio between the two species that will enable the existence of the flavour and aroma that are characteristic of the product depends on the properties of the strains used. Nonetheless the most common ratio is approximately 1:1 [28]. The characteristic flavour of yogurt is also related to bacteria, as they produce acetaldehyde, acetone, acetoin and a small amount of diacetyl. Among all these, the best flavour compound of yogurt is acetaldehyde, and *L. bulgaricus*  is the bacteria that produce most of that compound. However, and in smaller quantities, *S. thermophilus* also produce acetaldehyde and support its conversation into threonine [29].

 It should also be noted that the predominance of any of these species may lead to defects in the final product. The main factors that can affect the proper balance between the two microorganisms are time, incubation temperature and the percentage of inoculum. For example, a shorter incubation time results in a product with lower coccus content and a poorer flavour; on the other hand, a longer incubation time would result in a product with bitter flavour [30].
