Preserving by Home Freeze Drying

Canning, pickling, freezing, drying, and fermenting are well-known methods of preserving fruits and vegetables for future use. These processes have been used for generations and made simpler and safer over time with the help of science and innovation. Freeze drying (lyophilization) is a more recent option for home food preservation due to the advent of home freeze dry units. HarvestRight, a company in Salt Lake City, Utah, introduced a freeze drying unit for home use in 2018, opening new opportunities for home food preservation. Early in 2023, a second company, Prep4Life, introduced a slightly different freeze drying unit for home use known as THE CUBE; Prep4Life is also a Utah-based company.

Freeze drying unit on retail display
Freeze drying unit exhibit at retail location – Photo: mrgeiger

Freeze drying is not a new process. The process may date back to the 13th century, with the Incas using a simple process to preserve potatoes in the Andes. The first patent was issued in 1934. During World War II, it was used to safely transport blood serum and penicillin to the battlefield.  In the 1950s–1960s, freeze drying was viewed as a multi-purpose tool for pharmaceuticals and food processing and became a major component of space and military rations. Freeze drying has been widely used in the food industry for some time to extend the shelf-life of food while maintaining quality (think berries in commercial cereals that feature real berries) and offer consumers fast meal prep, emergency preparedness, and portable food. Freeze-dried foods also offer convenience as some foods can be eaten “as is” (except for raw meat, poultry, seafood, and eggs), added directly to recipes, or rehydrated and used as fresh food.

In a nutshell, freeze drying works by dropping the product temperature to <-40F, then reducing the pressure and adding heat to allow the frozen water in the product to change directly to a vapor (sublimate). Per HarvestRight, the process removes 98-99 percent of the moisture in food yet retains 97 percent of the nutrients, natural enzymes, and original flavor and color, making it a superior method for preserving food [1]. Additionally, freeze-dried foods are easy to use; food returns to its original pre-freeze dried state by just adding water. Since nearly all water has been removed, freeze-dried food is light, making it a favorite for camping and backpacking. A 10-pound bag of fresh apples weighs about one pound after freeze drying. Further, freeze dried foods supposedly have a 25-year shelf-life under proper storage conditions.

To date, very little university research has been done on in-home freeze drying; specifically research on how long the food retains quality and nutritive characteristics [3] as well as bacterial studies. Utah State Extension staff has been experimenting with the HarvestRight dryers. In a recent webinar, they stressed that freeze drying produces high quality foods that are safe as long as they are handled properly prior to freeze drying, dried thoroughly, packaged appropriately, and used or prepared correctly once the packaging is opened. It is important to note that freeze drying does not kill bacteria or other microorganisms; they remain viable, but dormant, despite the extreme conditions of freeze drying. Any bacteria or microorganism on raw foods prior to freeze drying will reactivate upon rehydration. Therefore, food items that are traditionally cooked before eating must also be cooked before eating as a freeze-dried food.

Nearly any food item can be freeze dried—fruits, vegetables, herbs, meats (cooked and raw), eggs, dairy, meals, casseroles, desserts. Utah State recommends that vegetables be blanched prior to freeze drying to prevent discoloration. Food high in fat content, high in sugar content, and baked goods such as breads, cakes, muffins, etc do not freeze dry well and should be avoided. Sugar causes foods to expand.

To ensure the safety and quality of freeze-dried foods, basic food safety principles must be used in preparation, product must be completely dried (crisp), and product must be stored properly. Proper packaging is crucial to extend the shelf life of freeze-dried foods and prevent contamination or spoilage. The storage container must eliminate oxygen, light, and moisture. In order of long-term to short-term storage, the following containers may be used: Mylar® bags, vacuum-sealed canning jars, #10 cans, vacuum sealed bags, and PETE re-sealable containers. An oxygen absorber must be enclosed in the container to remove or decrease the available oxygen in the package to help maintain product safety, quality, and extend shelf life. Foods should be stored in a cool, dark place. 

For long-term storage, PET or PETE (Polyethylene terephthalate) food grade, non-toxic plastic pouches, also known as “mylar bags” are excellent. The opaque (silver) Mylar® bags are preferred; they block out air and light during storage, can be resealed once opened and take up less space than glass jars or cans. Mylar® bags with a clear side are not long-term air tight [3]. Mason canning jars can be used if they are vacuum sealed with a vacuum sealing machine capable of using a jar sealing device. Metal cans have a zero oxygen transfer rate and are great for long-term storage [4]. However, a #10 can contains a large amount of dried food which must be used at the time of opening or resealed in another container. Vacuum bags and re-sealable containers have short-term oxygen barrier qualities. 

Oxygen absorbers do not have a long shelf life; as soon as they are exposed to air (oxygen), they start to absorb and are spent when they become hard. They are available in different sizes (measured in cc’s); contents and container size should be considered when purchasing absorbers. The smaller the container the less cc’s needed. There is no harm in using a larger than needed absorber and would be preferred to one that is too small [3]. When a container is opened, the absorber should be replaced before resealing.

A freeze dryer is not a fancy food dehydrator. While a freeze drying unit and a dehydrator both remove moisture from food so that microorganisms cannot grow and enzyme action is slowed down, a dehydrator uses low heat and a fan to remove 80-90 percent of the moisture content from food [5]. While dehydration is a very acceptable means of food preservation, it differs from freeze drying in several ways: 
– foods shrink up and develop a leathery feel and appearance;
– foods do not return to their natural state;  
– foods retain less of their nutritional value;
– foods have a 4 months to 1 year shelf life;   
– fewer foods are successfully dehydrated;
– foods rehydrate slowly.

There are many advantages to freeze drying. Besides holding nutritive value, it allows one to utilize garden produce at the peak of harvest, buy in bulk, save money over commercially prepared freeze-dried foods, offers a long shelf life, preserve foods that cannot be typically preserved, and offers compact, lightweight storage. Some disadvantages pointed out by Utah State Extension include unit size, noise, time for drying and allowing freezer to unthaw, cleaning, sanitation, and maintenance, small batch sizes, and cost—cost of the machine as well as machine accessories, packaging supplies, sealers [10], and electricity. In addition, reconstituting freeze-dried foods is somewhat experimental. Utah State Extension specialists suggest starting with a small amount of water and giving ample time to reabsorb; there is no need to rehydrate herbs, onions, or bell peppers as they can be added directly to foods and will absorb moisture from the food. Buying a Home Freeze-Dryer: What to Know Before You Go and Let’s Preserve:  Freeze Drying offer more information.  

The options for food preservation are many. Each method offers pros and cons to preservation and storage. If long-term food storage or portable food storage is the goal, freeze-drying is an option to consider. HarvestRight machines are available at several retail outlets. The Cube is available from the Prep4Life company. Imagine rehydrating lasagna on a camping trip!

Sources:

Reference to any commercial product, process, or service, or the use of any trade, firm, or corporate name is for general informational purposes only and does not constitute an endorsement, recommendation, or certification of any kind. Persons using such products assume responsibility for their use and should make their own assessment of the information and whether it is suitable for their intended use in accordance with current directions of the manufacturer.

Marlene Geiger

I am a graduate of the University of Nebraska-Lincoln with a BS in Home Economics Education and Extension and from Colorado State University with a MS in Textiles and Clothing. I enjoy spending time with family and friends, gardening, quilting, cooking, sewing, and sharing knowledge and experience with others.

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