|MadSci Network: Microbiology|
The main question is what is it that bacteria do to make milk thick, especially in the case of yogurt.
To answer this question let's look at the composition of milk. The major components in milk, besides water, are proteins and a form of sugar called lactose. When bacteria grow in milk they use the lactose and proteins to supply the necessary energy and biological material they need to increase in number. In the process of using the lactose for energy the bacteria produce an acid called lactic acid and thus, these bacteria are referred to as the Lactic Acid Bacteria.
When lactic acid is produced the pH of the milk begins to drop. The term "pH" is used to describe how much acid is present. The more acid that is present the lower the pH. This drop in pH is what causes milk to become thick. The proteins in milk exist in sort of a globular form (or folded up into a ball). As the pH drops the proteins begin to unfold so that they are not shaped like a ball anymore. This process of unfolding causes the milk to become thicker. This process is called precipitation or coagulation. You can mimic this process by adding lemon juice to milk slowly. As you stir you can see it getting thicker and forming lumps.
There are quite a few different Lactic Acid Bacteria that are used in the diary industry. They have different patterns of growth, which have been taken advantage of for making the various milk products. The bacterium used to make yogurt, Streptococcus thermophilus, grows best at higher temperatures. This higher temperature used for making yogurt favors them over other bacteria present in the milk. The bacteria really do not expand or compress, as metals do, in response to the temperature. It is difficult to answer the question about the role of vitamins in making yogurt from milk. Many of the processes that bacteria use to grow require vitamins at one point or another but to specifically answer this question I would have to cover a large amount of college level biochemistry.
Try the links in the MadSci Library for more information on Microbiology.