Two common methods of preserving microbial cultures are deep freezing and lyophilization or freeze drying). Deep freezing is a process in which a pure culture of microorganisms is placed in a suspending liquid and frozen quickly at temperatures ranging from -50 to -95 degrees Celsius. With this type of freezing method, cultures can usually be thawed and used after several years. Lyophilization, or freeze drying, quick freezes suspended microorganisms at temperatures from -54 to -72 degrees Celsius while water is removed by using a high-pressure vacuum. While under the vacuum the container is sealed with a torch. The surrounding microbes in the sealed container can last for years. The organisms can be retained and revived by hydrating them and placing them into a liquid nutrient medium.
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