Freeze drying, also known as lyophilization, is a drying process that preserves delicate structures and is used in the biotech industry to preserve vaccines and pharmaceuticals. It is also used to preserve specialty food products and flower specimens. The process is expensive and time-consuming, but it results in a sample whose structure is largely preserved and can be stored at room temperature and pressure. The main mechanism is sublimation, which converts ice directly into water vapor without passing through the intermediate stage of a liquid.
Freeze drying is a process more commonly known as lyophilization. The word comes from the Greek and means “solvent-loving product”. This process is a way of drying something that minimizes damage to its internal structure. Because it is a relatively complex and expensive form of drying, it is limited to those materials that are sensitive to heat and have delicate structures and of substantial value. One of the only substances that cannot be preserved effectively with freeze-drying are mammalian cells, which are too fragile.
The preferred preservation method in the biotech industry, freeze-drying, is routinely used to preserve vaccines, pharmaceuticals and other proteins. Freeze drying is also used to preserve specialty food products, eliminating the need for refrigeration. Freeze-dried food is eaten by mountain climbers and astronauts. Freeze drying is used by botanists to preserve flower specimens indefinitely. Because the freeze-drying process removes most of the water from the sample, freeze-dried materials become highly absorbent, and simply adding water can return the sample to something close to its original state.
The energy and equipment costs of freeze drying are about 2-3 times higher than other drying methods. The drying cycle is also longer, around 24 hours. First, the temperature of the sample is lowered to near freezing. Then, the sample is placed in a vacuum chamber. More energetic molecules escape, further lowering the temperature, while the extremely low pressure causes water molecules to be sucked out of the sample. Attached to the vacuum chamber is a condenser, which converts the moisture in the air into liquid and removes it.
Great care is taken throughout the process to ensure that the structure of the sample remains constant. For example, the sample could simply be frozen by vacuum rather than being frozen at atmospheric pressure, but this would cause the sample to shrink, irreversibly damaging its structure.
The main mechanism that allows freeze-drying is sublimation, whereby ice is converted directly into water vapor, without passing through the intermediate stage of a liquid. Rather than through heating, this is done by removing pressure so that the ice boils without melting. The result is a sample whose structure is largely preserved, which can be stored at room temperature and pressure.
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