|MadSci Network: Microbiology|
When milk is fermented by bacteria, the bacteria grow by metabolizing (or eating) the sugar and the protein in the milk. The bacteria produce lactic acid as one byproduct of their metabolism. The lactic acid drops the pH of the milk, making it more acidic. This effect, in turn, denatures or changes proteins in the milk, and in a very complex process curd is formed. The curd or lumps represent complexes of protein complexes. One can achieve a similar result by adding specific enzymes such as rennet to milk. Some dairy products can be produced with the addition of acid alone.
Many microorganisms can ferment milk. In most cases we use lactobacilli. There are two classes of lactobacilli. Those that form gas (CO2) and those that do not. Both produce acid (lactic acid) from sugar fermentation. They can do this in the absence of oxygen (anaerobically).
Freezing food or water often kills approximately 90% of the bacteria. If you start with 1,000,000 bacteria then freeze & thaw one time you may still have 100,000 bacteria.
Some bacteria are heat sensitive and are killed with cooking while others may survive cooking. One bacteria which can survive milk pasteurization is Bacillus cereus, a cause of food poisoning. This bacterium can cause vomiting and diarrhea due to toxins which it produces
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