Milk is fermented to make yogurt.   To set up yogurt production milk is heated, this helps kill other microorganisms that would compete with the starter culture.  Lactobacillus bulgaricus interacts with  Streptococcus thermophilus to form a symbiotic starter culture to ferment milk. By creating the starter culture it increases the production of lactic acid in the fermentation process. Fermentation of the milk usually happens when the milk is around 45 degrees Celsius.  Streptococcus thermophilus starts to grow first dropping the milks pH from 6.6 to 5.0, and produces carbon dioxide and lactic acid. These products stimulate Lactobacillus bulgaricus to grow and further drop the pH to 4.2. The sugar found in milk is lactose, when the starter culture is added to milk it breaks the lactose into glucose.  After the sugar glucose is formed it then is fermented into lactic acid by the microorganisms.  Acetaldehyde is also produced, this chemical compound creates the tart and sour taste of plain yogurt. The lactic acid decreases the pH of the milk and causes the casein, protein found in milk, molecules to denature and stick together.  The milk then curdles to produce yogurt. To enhance the flavor a wide variety of fruits are mixed in or are found at the bottom of the finished product.