Freeze-drying food

Freeze-drying is used in food processing to produce stable, high-quality food that retains its nutritional profile. The taste and smell of the food are also maintained.

Freeze-drying or lyophilization is a dehydration technique based on the sublimation of water in a product. This means that the product’s water content transitions from a solid to a gaseous state — or from ice to vapor — without going through the liquid state.

Freeze drying is different from vacuum drying and hot air drying (dehydration) and should not be confused with these methods of food preservation.

Raw foods contain a large amount of water — 80 to 95 percent. The water activity of foods varies. During the process of freeze-drying, both water molecules that are bound to other molecules (sugars, salts, proteins) and water molecules that are free must be removed.

The significant reduction of the water activity of the final food product contributes to its shelf-life and food safety. The water activity for most properly freeze-dried food products ranges from 0.08 to 0.330.

How to freeze-dry

The process of freeze-drying (lyophilization) is a complex, multi-step process that requires specific equipment.

The three-step freeze-drying process:

• Freezing: The food product is frozen under specific atmospheric pressure.
• Primary drying or proper freeze-drying is the sublimation step at a certain pressure and heat that removes free frozen water.
• Secondary drying releases ice water from the food product (desorption drying ) and dries the food product at a required humidity.

The process must be done to ensure that the ice water does not melt before entering the gas phase as this can cause food safety and quality concerns.

The freezing process must result in the optimal size of ice crystals as this impacts sublimation and the visual and textural quality of the end product.

Preparing freeze-dried foods

Freeze-dried foods can be consumed dried or rehydrated depending on the food and intended use. For foods that are cooked before eating, like pasta, grains or meats, when they are freeze-dried they should be cooked to the safe minimum internal cooking temperature for that food after being rehydrated.

It is important to note that, when rehydrated, any bacteria that were present on the raw food product prior to freeze-drying may become active again. The process of freeze-drying does not kill bacteria but rather slows bacterial growth by reducing its ability to access water.

Storing freeze-dried foods

The USDA notes that properly freeze-dried foods are shelf-stable and must be packaged in moisture-proof, hermetically sealed containers. Packaging options may include moisture barrier bags, Mylar bags or glass jars. A food-grade moisture absorber packet (desiccant) can be used in these packaging options.

Vacuum sealer bags used for sous vide cooking or general food storage are not the same as moisture barrier bags because vacuum sealer bags are permeable (allowing air and moisture to pass through). Proper storage is needed to prevent bacterial growth by controlling the oxygen and water available to the bacteria.

Use proper food handling

Following proper food handling procedures are essential throughout the entire process.

• Appropriate handwashing.
• Use single-use gloves when handling foods that will not be cooked before eating (ready-to-eat foods).
• Store ingredients at safe temperatures and free from contamination.
• Cook foods to minimum internal temperatures and cooling per guidelines.
• Prevent cross-contact of allergens.
• Maintain a clean and sanitized kitchen space and equipment.

Learn more about sanitation and preventing food-borne illness.

Equipment and resources

Freeze-drying food preservation uses specialized equipment that controls temperature and atmospheric pressure.

Utah State University Extension has written helpful articles to assist individuals in selecting a freeze-drying unit and related supplies.

The Iowa State University Extension AnswerLine also has equipment recommendations.