|MadSci Network: Microbiology|
What make yogurt blow up? Yougurt is a fermented dairy product, as are cheeses and buttermilk. Bacteria belonging to the general group known as lactic acid bacteria are added to milk and allowed to grow. These bacteria produce lactic acid as a primary byproduct, which lowers the pH of the milk (increases the acidity). The lower pH coagulates the protein in the milk, resulting in the thickening that we associate with yogurt. The fermentation is stopped by placing the yogurt in the refrigerator, which stops the bacteria from growing. For safety purposes, most yogurts are sealed with a plastic film to prevent contamination just prior to refrigeration Now, why does it blow up? When yogurt is removed from the refrigerator, the bacteria begin growing again. The bacteria produce not only lactic acid, but also carbon dioxide (which is a harmless gas). The bacteria begin to grow and produce gas, but now the gas is sealed inside the container with the safety seal. When you pull the seal loose, the gas rushes out. If you let the yogurt stay out of the refrigerator long enough, the out rushing gas can be quite dramatic. In fact, if you were to simply leave the container of yogurt on the counter top for a day or two, the pressure from the gas would eventually break the safety seal.
Try the links in the MadSci Library for more information on Microbiology.