Freeze Drying

Freeze drying, also called lyophilization, is a three-step process: Freezing the product, lowering the temperature, and then removing the ice by sublimation. Because of the low temperature used in processing, the quality of the rehydrated product is excellent. And the original shape of the product is maintained.

Freeze-driers have three major parts: ·

A pre-frozen product is placed in an air-tight drying chamber. A vacuum pump reduces the air pressure inside the chamber until a pressure of 1 to 0.5 mbar is reached. Under vacuum, the evaporation process of the frozen water already starts at -50 to -40° Celsius. The water vapor is then suctioned out of the drying chamber by the vacuum pump and directed into a downstream condenser. In this so-called ice trap, cooled to at least -70° Celsius, the water vapor deposits on a cooling coil as ice. Most of the moisture is removed from the product in this primary drying phase.

In the case of many foodstuffs, primary drying is followed by secondary drying. This process takes away more residual moisture by lowering the vacuum level to 0.01 mbar or below while raising the temperature above the freezing point. The drying chamber is then ventilated to atmospheric pressure. For ventilation, dry air or an inert gas is used, so that the dried product cannot absorb moisture from the ambient air. Finally, the dried product, with a water content of one to four percent, is removed for further processing.

Freeze drying is not possible without vacuum as it cannot be performed at ambient pressure. It usually requires pressure below 1 mbar. Vacuum is used during sublimation in primary drying. And it also plays a role during desorption in secondary drying. Vacuum pumps are furthermore used for the input and output air locks in continuous freeze-dryers. Primary applications of freeze drying include biological (e.g., bacteria and yeasts) and biomedical processes (e.g., surgical transplants), food processing (e.g., coffee) and preservation.

Freeze drying allows to gently remove water from products like foodstuffs or sensitive pharmaceutical substances. Without heating up the products to be dried. This brings several advantages: Important ingredients and aromas can be preserved, and product properties remain unchanged. Freeze-dried substances can thus easily be rehydrated later on. In ideal cases, you should not even be able to tell the difference between rehydrated freeze-dried food and fresh food.

Freeze drying usually requires pressure below 1mbar. For industrial applications, oil-lubricated rotary vane vacuum pumps combined with vacuum boosters are a cost-effective solution whenever non-corrosive solvents are being pumped. When it comes to pharmaceutical freeze drying, dry screw vacuum pumps are the solution of choice. They combine a dry operating principle with the possibility of Clean-in-Place (CIP). Oil-lubricated two-stage rotary vane vacuum pumps are proven for freeze drying in laboratories. Scroll vacuum pumps offer an excellent dry alternative.

R5 vacuum pumps combined with a vacuum booster are perfectly suited for industrial applications involving non-corrosive solvents. In the food industry, for example, this combination is a preferred solution. For pharmaceutical freeze drying, COBRA is the vacuum generator of choice due to the dry pumping principle and the possibility of Clean-in-Place (CIP). COBRA can be used with or without a vacuum booster, according to the specific needs. The oil-lubricated ZEBRA is preferred for laboratory applications. If a dry solution is needed, FOSSA is an excellent alternative.

Busch vacuum pumps are used worldwide for the freeze drying of foodstuffs like fruits, soups, and noodles. Our COBRA, R5, and vacuum boosters are employed in freeze drying of instant coffee, which is a harsh application due to the risk of carryover of coffee into the vacuum pumps. They are also used for the freeze drying of sensitive biologically active substances in the pharma and biopharma industries. A more exotic application is the drying of water-damaged old books and scrolls. Busch vacuum pumps have even been used to dry a longship. The latter was brought to the surface from the bottom of a lake where it had been buried for centuries.

On the following page, we have summarized all aspects that need to be considered when purchasing a vacuum system.

Freeze drying allows for example to extract water from foodstuffs. Without heating or lengthy drying processes, that would cause nutrients, aromatic substances and vitamins to get lost.

Using the technique of freeze drying under vacuum, these essential ingredients are largely unaffected. And the extraction of water with this method only slightly alters the internal structure of the food. In ideal cases, rehydrated freeze-dried food is almost indistinguishable from fresh food.

Typical examples of freeze-dried foods include coffee granules and other instant drinks, but also spices and fruit for muesli mixtures. Occasionally, freeze-dried products even reach outer space. As essential components of astronaut food.

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Freeze-drying is a particularly gentle process to remove water from sensitive products. It is therefore perfectly suited for drying pharmaceutical substances such as antibiotics, vaccines or bacteria. That, when dry, can be transported more easily and are at the same time more stable.

The water to be removed passes directly from the solid to the gaseous state. Allowing to extract it without heating up and damaging the substances. And without altering their product properties.

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