Refrigeration and freezing are the most common forms of food preservation, because they slow down or stop the activity of disease-causing bacteria. And all foods contain bacteria unless they have been sterilized. That's why food kept in a refrigerator stays fresh longer; if milk is left on the kitchen counter, the bacteria in the milk will cause it to spoil within a few hours. Refrigerated, the milk will last for a week or two.
Temperature is how we measure how hot or cold an object is. All objects are made of atoms and molecules, which are constantly in motion. How fast those atoms and molecules jiggle about determines how hot or cold an object is. As a substance like water is cooled, its atoms and molecules move more slowly. At some point, the temperature drops low enough -- the atoms and molecules are moving slowly enough -- that the water changes from a liquid into a solid. It freezes into ice. This is called a "phase transition." The same thing happens in reverse when water is heated. The atoms and molecules begin moving faster, until their speed and temperature reach a point where the water evaporates into steam: a gas instead of a liquid.
In freezing, the aim is to stop bacterial activity altogether, so that food can be preserved for extended periods. Once the food is thawed, however, the microbes again become active and start multiplying. Freezing does not kill most types of bacteria. It is best to freeze foods as quickly as possible. Slow freezing causes ice crystals to form, which can damage cells during thawing, causing meat to lose its juiciness, for example.
The American Society for Microbiology contributed to the information in the TV version of this report.