Freeze-drying food

The freeze-drying process was originally developed for medical purposes during WWII and later used for preserving viruses for vaccine research. The technology was then applied to food preservation by NASA to develop meals for the astronauts traveling into space. The technology is used in food manufacturing and processing and is now available for at-home use. Freeze-drying can produce stable, high-quality food that retains its nutritional profile. This method also maintains the taste and smell of food.

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. There is no heat treatment step in the freeze-drying process. So it doesn’t kill illness-causing microorganisms such as bacteria and viruses. In fact, if present in the food, these microorganisms will be preserved and can start growing again if moisture is added back to the food.   

Raw foods contain a large amount of water — 80 to 95 percent. The water activity of foods also varies. Water activity is the measurement of the ability of available water molecules to interact with other food components (like sugar or salt) and microorganisms (bacteria, viruses, molds).

Freeze-drying lowers the water activity of the food because water molecules are 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 rapidly frozen under specific atmospheric pressure at a very low temperature between -30 and -50 degrees Fahrenheit.
• Primary drying or proper freeze-drying is the sublimation step. During sublimation, while at a certain pressure, a mild heat is passed over the food.
  • The ice crystals are vaporized to gas (water vapor).
  • The water vapor is pulled from the machine through a vacuum pump system.
  • The mild heat temperature is not sufficient to inactivate illness-causing microorganisms.
  • Sublimation can remove up to 90 percent of the water content from a food.
• Secondary drying or desorption drying removes moisture from the surface of the food. Desorption can remove an additional 1 to 2 percent of the water content from a food. 

When done correctly, freeze-drying does not allow the ice crystals to melt to the liquid state before being vaporized to gas. The ice crystals are removed quickly and in a way that the cell walls are not damaged. This promotes the quality and safety of the final food product. 

Preparing freeze-dried foods

Most foods can be freeze-dried. At-home freeze-drying equipment works well for preparing fruits, vegetables, dairy foods and proteins. Foods that are high in fat, high in sugar and baked goods do not freeze-dry well using home equipment.

• Thoroughly wash produce before peeling and cutting.
• Cut produce into uniform-size pieces to ensure even freeze-drying.
• Use a pre-treatment step to promote the quality of the final food product. This will limit darkening or browning by inactivating ripening enzymes. Pre-treatment options include:
  • Blanching: most vegetables and firm fruits can be blanched.
  • Ascorbic acid solution: Dip produce that cannot be blanched in a prepared solution of 1 tsp. ascorbic acid to one gallon of cool water.

Meat, poultry and seafood can be freeze-dried raw or cooked.

• Cut food into uniform-size pieces to ensure even freeze-drying.
• Seasonings can be added to both raw and cooked foods before freeze-drying.
• For cooked, ground beef: drain and thoroughly rinse with water to remove as much grease as possible before freeze-drying. Foods that are high in fat or oil may not freeze-dry well.
• To kill parasites in wild game meat, freeze the meat before preparing.
  • Cut the meat into pieces 6 inches thick or less and freeze at 0 degrees Fahrenheit or below for 30 days.

Thinner pieces of food will take less time to freeze dry. Freezing the prepared food in a household freezer for 48 hours before filling the trays may also speed up the freeze-drying process.

When filling the trays, evenly spread the food in a single layer. Follow the manufacturer's instructions for food capacity and freeze-drying times.

The National Center for Home Food Preservation has not published any research-based recipes yet. Validating freeze-drying recipes is a challenge due to the differences in at-home freeze-drying equipment. However, several universities are conducting research to develop safe recipe guidelines.

For freeze-dried foods you intend to rehydrate to eat, a best practice is to weigh the trays with the food before freeze-drying and then again after freeze-drying. The difference in weight is moisture removed from the food. This amount is a good starting point for how much liquid to use when rehydrating the food.

Testing the freeze-dried foods for doneness

At-home freeze-drying equipment is programmed by the manufacturer so the user can select from a list of options. However, it is important to test the freeze-dried food for doneness. To test for doneness:

• Take a piece of food from each tray in the freeze-dryer. The unit may process unevenly so it is important to verify multiple samples.
• Cut each piece in half. Inspect for any dark spots in the middle of the food. A dark spot would indicate moisture.
• Return all the trays to the freeze-dryer if a sample has a dark spot.

Packaging and storing freeze-dried foods

Freeze-dried foods absorb moisture easily, so it is important to package these foods quickly and in appropriate containers. Even the humidity in the home or kitchen can impact the freeze-dried food.

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, Mylar^Ⓡ bags, PETE resealable pouches, vacuumed sealed moisture barrier bags, or vacuum-sealed Mason-style jars.

Note: some vacuum sealer bags used for sous vide cooking or general food storage may not be the same as moisture barrier bags as they may allow air and moisture to pass through. A food-grade moisture absorber packet (desiccant) and an oxygen absorber can be used in these packaging options to minimize food safety risks.

Freeze-drying does not kill illness-causing microorganisms. If freeze-dried food is contaminated and stored incorrectly, microorganisms can resume growing and potentially cause foodborne illness when the food is consumed. The University of Ohio Extension provides more details on packaging and using oxygen absorbers to keep freeze-dried foods safe.

Store packaged freeze-dried foods in a cool, dry location. It is a best practice to label the container with the type of food, if the food is raw or cooked, and the date prepared.

Because the freeze-drying technology for home use is relatively new within the past 10 years, there is limited information on shelf life. Commercially prepared freeze-dried foods can have a shelf life of up to 25 years.

Using freeze-dried foods

Freeze-dried foods can be consumed in their dried state or be rehydrated depending on the food and intended use. For freeze-dried foods that are to be cooked before eating, like pasta, grains, meats, poultry, seafood or eggs, 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 illness-causing microorganisms that were present on the food product before freeze-drying may begin growing again. The process of freeze-drying does not kill illness-causing microorganisms but rather preserves them until they are exposed to water 

Equipment and resources

Freeze-drying food preservation uses specialized equipment that controls temperature and atmospheric pressure. There are currently four manufacturers of at-home freeze-drying equipment: Harvest Right, Stay Fresh, Prep4 Life, and Blue Alpine. The size, capacity, electrical equipment and cost vary between brands and models.

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.

• Use a thorough sanitation routine on freeze-drying equipment and countertops to prevent cross-contamination, the spread of illness-causing microorganisms from one surface to food or another surface.
• Wash and sanitize the food trays after each use, even if you are going to freeze-dry another batch of the same food. Unplug the equipment before cleaning.
• Use caution when cleaning around the condenser and heating elements to not dislodge any wiring.

Food safety reminders

Follow proper food handling procedures throughout the entire process to minimize the risk of foodborne illness.

• Use appropriate hand washing.
• Wear single-use disposable 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 cool 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.