MadSci Network: Microbiology

Re: What makes yogurt blow up?

Date: Tue Jul 25 18:11:31 2000
Posted By: James S. Dickson, Faculty, Microbiology, Iowa State University
Area of science: Microbiology
ID: 953740019.Mi

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.

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