What is Freeze-Drying?

Freeze-drying (also known as lyophilisation) is a very delicate process where the food or substance is dehydrated by immediately freezing the material and extracting up to 99$ of the moisture in the food. This makes it resistant to bacteria and deterioration.

In this process, food is prepared, arranged onto drying racks, and placed inside of a freeze-drying machine. Rather than a chemical change or preservation, the machine subjects the food to a quick physical change that leaves the structure intact. The water in the food is quickly changed from a solid state to a gaseous state, skipping the liquid state.

History of Freeze-Drying Foods

During WWII, the United States started to use the modern day freeze-drying process when they started transporting serums, blood, and other medical supplies to Europe. This was developed because doctors found that medicines that needed to be refrigerated, like penicillin, would spoil before ever being used. The freeze-drying process allowed the materials to retain their chemical properties and drastically increase shelf-life. Shortly after, the process was adopted for other uses including food preservation and even book restoration.

The Freeze-drying Process

The process can be broken down into four stages that occur within the freeze-drying machine. A machine will consist of a freeze-drying chamber, shelves attached to heating units, a freezing coil connected to a refrigerator compressor, and a vacuum pump.

Pretreatment. Before the freeze-drying process begins, food is checked for bacteria and spoilage to ensure that the food will be safe to eat when reconstituted. Some food may also be concentrated or cooked and prepared before freezing. This is done to best preserve the structure of the food, as well as increase its quality and yield. This is to ensure that your food will be the same in look, taste, and nutrition after the process is completed.

Freezing. In this phase of the process, the food is placed onto the shelves in mass quantities, which is then put inside of the freeze-drying machine. The chamber is then sealed and the temperature is lowered down to between -58 to -112 degrees Fahrenheit. When the water is in a solid form, its molecules become separated from the food molecules and leave the structure of the food intact. That is why the freezing phase is so important. If any moisture is left in the food when the drying process begins, the structure may be ruined.

Primary Drying. Now that the food has been frozen in the chamber, the pressure is lowered to create a vacuum. This environment allows for a process known as sublimation, or the shift of a solid directly into a gas. Once the pressure is lowered, heat is slowly added to the trays. Because water cannot exist in a liquid form in a vacuum, the ice is converted directly into water vapor.

Sublimation leaves the structure of the food perfectly intact with its same taste, texture, and nutrients. The water vapor is filtered out of the freeze-drying chamber and condenses onto the refrigeration coils so it is away from the food and won’t risk any rehydration.

Secondary Drying. The drying process occurs over the course of multiple hours or even days as the water is gradually extracted from the food. This process takes so long so the food doesn't overheat and cause it to lose any nutrients or structure. In the final stages of drying, the temperature is raised higher in order to evaporate any moisture that might still be remaining in the food. This leaves the food with 1 to 4 percent of its original water content. Now it is ready to store!

Sealing. Once the food has gone through the freeze-drying process, it can be stored for 20 - 30 years. But there are two components that will cause our food to spoil more quickly: water and oxygen. Freeze dried food will last much longer if it is sealed in a #10 can or a Mylar bag. These containers will keep moisture out of the food so that it won’t reconstitute and will be ready to use when you need it. You can also add an oxygen absorber to the container to keep your food bacteria free for years to come.

The freeze-drying process is one of the best food preservation methods out there. It allows you to store all of your favorite fruits, vegetables, and complete meals for 20-30 years that have the same look and taste of fresh food. The process can be tedious, but it provides nutritious food that can be stored in your supplies and used in your everyday cooking.

Freeze-dried food vs dehydrated

A lot of people use the terms dehydrated and freeze-dried like they are the same thing. However, there are some major differences between dehydrated and freeze-dried food storage. Don’t worry, we’re going to help you understand those differences.

The Dehydration Process. With any type of food preservation, moisture needs to be removed from the food. The most common way to do this is by dehydrating.

Dehydrating has been a food preservation practice for thousands of years, dating back to at least 12,000 BC.^[1] The Romans and Middle Easterners would dry fruits and vegetables in “still houses” which would use a fire to dry out and smoke foods.

Modern-day dehydration isn’t that complex. Machines, like a SnackMaster Dehydrator, circulate hot and dry air across the food. This removes much of the water. The moist air is then dried so that water continues to be removed. The temperatures are high enough to remove water but not high enough to cook the food. Dehydrated food is usually withered and harder.

Moisture Content. The main objective with food preservation is to remove the moisture so that the food doesn’t decompose, grow mold, etc. Dehydration removes about 90-95 percent of the moisture content^[3] while freeze drying removes about 98-99 percent.^[4] Foods that you dehydrate at your home will typically have a 10 percent moisture content level while foods that are dehydrated professionally will have a lower moisture content - which increases the shelf life.

Shelf Life. The moisture removal has a direct impact on the shelf life. Most dehydrated products like dried fruits, vegetables, powders, and TVP; have a shelf life of about 15-20 years. However, dehydrated items like honey, salt, sugar, hard wheat, and oats have a 30-year shelf life - sometimes longer.^[5] Freeze-dried foods will have a longer average shelf life. Freeze-dried fruits, vegetables, just-add-water meals and real meats will have a 25-30-year shelf life.^[6] Ideally, all of your food storage would be stored at a temperature of 60 degrees or lower.

Nutritional Content. According to research by the American Institute for Cancer Research^[7], freeze-dried foods retain the vast majority of the vitamins and minerals found in the original food. However, when compared to fresh fruits and vegetables, freeze-dried foods did lack in some vitamins - like Vitamin C - which breaks down very rapidly.

Dehydration doesn’t change the fiber or iron content of food. However, dehydration can break down vitamins and minerals during the preservation process and retain less of their nutritional value when compared to freeze-dried food. Dehydration tends to result in the loss of Vitamins A and C, thiamine, riboflavin and niacin.^[8]

Appearance & Composition. One of the main differences between dehydrated and freeze-dried food is how they look. Most people are familiar with banana chips (dehydrated) but not necessarily freeze-dried bananas (which become soft when you place them in your mouth). Weight is another difference. Freeze-dried foods are going to weigh a lot less than dehydrated foods. This makes them easier to haul or store.

Cooking. Dehydrated foods will require cooking. Many times, they will also require some type of seasoning. This means that you’ll need to spend time boiling the product in hot water and letting it cook. The preparation time for dehydrated products can take anywhere from 15 minutes to 4 hours depending. However, with freeze-dried foods, you just need to add water. Adding either hot water or cold water will get the job done depending on what you’re eating. Freeze-dried foods will usually be ready to eat in less than 5 minutes.

Cost. Obviously, the cost of food storage will depend on what you're buying. But usually, dehydrated foods are going to be cheaper than freeze-dried. Freeze-dried and dehydrated foods offer different benefits that might be worth the cost. However, if you're on a tight budget, dehydrated foods are definitely the way to go.

The Main Similarities

Packaging. Food storage cans are highly efficient. It’s not like the bag of potato chips that you open and half of it is air. Cans are filled as high as they can be no matter if they are dehydrated or freeze-dried. All of the cans are double-enameled and are sealed with an oxygen absorber to extend the shelf life.

Storage Requirements. There isn’t a difference in storing food that’s freeze-dried or dehydrated. The cans or buckets are all the same size.

Insurance for Your Family. Whether freeze-dried or dehydrated, the food will be insurance for you and your family against natural disasters, power outages, job losses and more. If you’re prepared, you don’t need to worry. Check out our complete listings of food storage and water storage items.

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[1] Nummer, Brian A. 2002. National Center for Home Food Preservation. May 2002 Edition. Accessed July 2013. https://nchfp.uga.edu/publications/nchfp/factsheets/food_pres_hist.html
[2] Bellis, Mary. 2013. Freeze-Drying & Freeze-Dried Food. About.com Money Inventors Article. Accessed July 2013. https://inventors.about.com/library/inventors/blfrdrfood.htm
[3] Andress and Harrison. 2006. “So Easy to Preserve” 5th ed. Bulletin 989, Cooperative Extension Service. The University of Georgia, Athens.
[4] Freeze Drying Process. 2007. Freeze-Dry Foods. Accessed July 2013. http://www.freeze-dry.com
[5] How Long Does Sugar Last? 2012. Eat By Date. Accessed July 2013. https://www.eatbydate.com/other/sweets/how-long-does-sugar-last/
[6] Product Shelf Life. 2007. Oregon Freeze Dry. Accessed July 2013. https://www.mountainhouse.com/shelf_lif.cfm
[7] Freeze-Dried Fruits Are a Good Health Choice? 2006. Ever Green, Ever Healthy May 2006 Edition. Accessed July 2013. https://preventcancer.aicr.org/site/News2?page=NewsArticle&id=9917&news_iv_ctrl=0&abbr=pr_
[8] Sessoms, Gail. 2011, May 25. Do Dehydrated Foods Lose Their Nutritional Value? LiveStrong.com Article. Accessed July 2013 https://www.livestrong.com/article/409547-do-dehydrated-foods-lose-their-nutritional-value/