Freeze-drying, or lyophilization, is the drying of a material from the frozen state. This process is used in the manufacture of some brands of instant coffee; freeze-dried instant coffee has a more natural flavor than other kinds. Microbiologists use lyophilization for long-term preservation rather than destruction of cultures of microorganisms. Organisms are rapidly frozen in alcohol and dry ice or in liquid nitrogen and are then subjected to a high vacuum to remove all the water while in the frozen state. Rapid freezing allows only very tiny ice crystal to form in cells, so the organisms survive this process. Organism so treated can be kept alive for years, store under vacuum in the freeze-dried state.
Oct 4, 2009
Drying can be used to preserve foods because the absence of water inhibits the action of enzymes. Many foods, including peas, beans, raisins, and other fruits, are often preserved by drying. Yeast used in baking also can be preserved by drying. Endospores presents on such foods cab survive drying, but they do not produce toxins. Dried pepperoni sausage and smoked fish retain enough moisture for microorganism to grow. Because smoked fish is not cooked, eating it poises a risk of infection. Sealing such fish in plastic bags creates conditions that allow anaerobes such as Clostridium botulinum to grow.
Drying also naturally minimizes the spread o infectious agents. Some bacteria, such as Treponema pallidum, which causes syphilis, are extremely sensitive to drying and die almost immediately on a dry surface; thus they can be prevented from spreading by keeping toilet seats and other bathroom fixtures dry. Drying of laundry in dryers or in the sunshine also destroys pathogens.