Freezing foods at –20°C is a common method for significantly slowing microbial growth, although it does not sterilize foods. However, microbial growth is sufficiently controlled in this way as to extend storage. The microbial cells do not grow while frozen, but when they are thawed, the cells can begin reproducing again. Even refrigeration at 4°C slows microbial growth to an extent, although many microbes can and do grow at this temperature. The use of ultra-low temperatures of –70° C and below are used for preserving bacterial cultures and medical specimens requiring long-term storage or transport.
Desiccation, drying or dehydration, is the removal of water from a material. Desiccation has been used for thousands of years to preserve food. Drying does not sterilize food, but the removal of water inactivates many proteins in microbes, slowing their growth. Some vegetative bacteria, such as Treponema pallidum, are very sensitive to desiccation and will die within minutes on a completely dry surface. Drying prevents endospores from germinating, although they can survive and may reproduce if water is reintroduced.Drying is often combined with freezing. This is called lyophilization, or freeze-drying, the desiccation of material while frozen. During lyophilization, foods are flash-frozen and then placed in a vacuum so that the water sublimates away. Lyophilization better preserves the flavors of foods than desiccation alone. Freeze-dried materials must be stored so that they do not reabsorb moisture in order to remain unspoiled.