How Does Freeze Drying Work?

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Freeze drying is a method of food preservation that removes most of the water from food while the food is frozen. The result is a lightweight, dry food that can often retain much of its original shape, color, flavor and structure.

You may already be familiar with freeze-dried strawberries and other fruits in cereals and snack mixes, or the increasingly popular freeze-dried candies that become light, crisp and intensely flavored.

Freeze drying is also used for everything from coffee and herbs to meats, dairy products and complete meals. With a home freeze dryer, many of these same foods can be preserved right in your own kitchen.

The process is also called lyophilization. It is used commercially for foods, pharmaceuticals and other materials that benefit from having moisture removed without prolonged exposure to high heat. 

For home food preservers, freeze drying offers a very different option from canning, freezing or conventional dehydrating. Instead of storing food in liquid, keeping it continuously frozen, or drying it with warm moving air, a freeze dryer freezes the food and removes the frozen water under vacuum.

That unusual process is what gives freeze-dried food its characteristic light, porous texture.

What Is Freeze Drying?

Freeze drying removes water from food in three general stages:

1. Freezing
2. Primary drying
3. Secondary drying

The most important difference between freeze drying and ordinary dehydration happens during primary drying. Instead of melting the ice in the food and then evaporating the liquid water, a freeze dryer creates conditions that allow much of the ice to change directly into water vapor.

That change from solid ice directly to vapor is called sublimation. FDA describes freeze drying in essentially these terms: the food remains frozen while water is removed as vapor under vacuum rather than first becoming liquid water. 

How Does a Freeze Dryer Work?

Modern home freeze dryers automate most of the process, making a preservation method that once required specialized equipment practical for home kitchens.

The science happening inside the chamber is fairly straightforward, but what it can do for a home cook is remarkable. Garden produce, leftovers, meats, dairy products, fruits and even complete meals can become lightweight, shelf-stable food you can put away and use much later—with no freezer space or electricity required once it is properly packaged.

The Three Stage Process Inside Your Home Freeze Dryer:

1. The Food Freezes

The food is first brought to a very low temperature so that the water contained within it freezes. This freezing stage forms the ice inside the food that will later be removed during sublimation.

Freezing rate, food composition, thickness and other conditions can all influence the final structure and quality of freeze-dried foods. For the home cook, this makes good tray preparation important. At this freezing stage that means cutting foods into reasonably uniform pieces and spreading them evenly on the trays. This helps the batch freeze and dry more consistently.

When done consistently throughout a batch fewer pieces of food on the trays that need additional drying time.You will also have more predictable batch times, and food that is easier to portion and package when the freeze drying cycle is finished.

Home freeze dryers freeze food as the first stage of the freeze-drying cycle, so a separate freezer is not required. However, many home preservers choose to pre-freeze food before loading the machine.

If you don’t have the freezer space, simply load the prepared food and let the freeze dryer handle the complete freeze cycle. it will work very well. It just takes more time.

2. Primary Drying Removes Frozen Water

Once the food is thoroughly frozen, the freeze dryer lowers the pressure inside the chamber and carefully controls the temperature.

Under these conditions, the ice begins to sublimate.

Rather than becoming liquid water first, the ice changes directly into water vapor. That vapor is drawn away from the food and collected elsewhere inside the freeze-drying system.

Primary drying removes the majority of the frozen water and is generally the longest and most critical part of the freeze-drying process. 

3. Secondary Drying Removes More Tightly Held Moisture

After most of the ice has been removed, some water is still associated with the food.

During secondary drying, controlled conditions allow more of this remaining moisture to leave the product.

The finished food must be sufficiently dry before it is packaged. Food that contains too much residual moisture can lose quality and may not store safely as intended.

What Is Sublimation?

Sublimation is the key concept that makes freeze drying different from most other food-drying methods.

Normally, we think of ice melting into water and then evaporating:

ice → liquid water → water vapor

During sublimation, the middle step is largely bypassed:

ice → water vapor

The vacuum inside the freeze dryer lowers the pressure surrounding the frozen food. Carefully applied heat supplies the energy needed for the ice to leave the food as vapor without requiring the food to sit in liquid water first.

This helps explain why freeze-dried foods often retain so much of their original physical form. As you can see pictured below freeze dried beef steaks change very little in size and structure after freeze drying is complete.

Note also the ice in the sublimation chamber. That all came out of the beef.

What Happens to Food During Freeze Drying?

When ice leaves the food during sublimation, it can leave behind tiny open spaces in the food’s structure.

That porous structure is one reason many freeze-dried foods become extremely lightweight and crisp when dry. It is also one reason many freeze-dried foods rehydrate relatively well: water, or other liquid, can move back into those spaces when the food is reconstituted.

Recent reviews of freeze-drying technology describe good retention of food structure, color, flavor and rehydration ability compared with many other drying processes, although results vary considerably with the food and the conditions used during processing. 

The texture of the finished dry food is not the same as fresh food. Fruits may become crisp, meats become dry and brittle, and sauces or liquids become light, porous solids that can be powdered.

Some freeze-dried foods are enjoyable eaten dry. Others are best rehydrated before use.

Why Does Removing Water Preserve Food?

Microorganisms and many chemical and enzymatic reactions depend on available water.

Food scientists describe this using the term water activity. Water activity is not simply the total amount of water in a food; it describes how available that water is for microbial growth and other reactions.

Reducing water activity is one of the established ways food can be made more stable during storage. FDA notes that water activity strongly affects food stability and microbial growth. 

Freeze drying removes enough water that properly processed food can become stable at room temperature when it is also packaged and stored correctly.

That does not mean freeze drying sterilizes food.

This is an important food-safety distinction. Utah State University Extension specifically warns home freeze dryers that freeze drying does not kill bacteria. Microorganisms can survive in a dormant state and may become active again when moisture returns. 

For that reason, food should be handled safely before freeze drying and again after rehydration.

Does Freeze Drying Preserve Nutrients?

Freeze drying is well regarded for preserving food quality, and research shows it can retain nutrients and other heat-sensitive compounds very well compared with some higher-temperature drying processes. 

However, there is not one universal percentage of nutrients that every freeze-dried food retains. Read the Pubmed study here.

Nutrient retention depends on several factors, including:

• the food itself
• the nutrient being measured
• preparation and pretreatment
• freezing conditions
• drying temperature and pressure
• exposure to oxygen and light
• packaging
• storage time and temperature

So rather than saying freeze drying always preserves 97 percent of nutrients, it is more accurate to say that freeze drying can preserve many nutrients exceptionally well, but the amount retained varies.

That is still an important advantage, particularly for foods that contain compounds sensitive to prolonged heat.

Freeze Drying Compared With Other Food Preservation Methods

Freeze drying is one of several useful ways to preserve food at home. Freezing, dehydrating, water-bath canning and pressure canning all have advantages, and an experienced home preserver may use several of these methods depending on the food and how it will eventually be used.

Freezing is inexpensive and excellent for preserving food quality and nutrients, but frozen food requires freezer space and continuous electricity.

Dehydrating is also relatively inexpensive and produces shelf-stable food, but warm-air drying can substantially change the size, texture and structure of foods.

Canning provides dependable shelf-stable food without continued electricity. Water-bath and pressure canning are well-established preservation methods, but they use heat and require foods to be prepared and processed according to safe, tested methods.

So where does a freeze dryer fit?

Freeze drying combines several advantages that are difficult to get from one other preservation method. It can preserve a remarkably wide variety of raw and cooked foods using relatively low processing temperatures. The finished food is lightweight, shelf stable and requires no electricity once properly packaged. Many freeze-dried foods retain their shape and flavor well, can be eaten dry, and readily absorb water when rehydrated. Complete meals and ingredients can also be preserved in forms that aren’t practical with traditional home canning.

That versatility comes at a price. Home freeze dryers remain considerably more expensive than most other food-preservation equipment, although smaller machines and increasing competition are beginning to give home cooks more choices. For a freeze dryer to be worth the investment, those unique advantages need to fit the way you actually cook, preserve and store food.

For a gardener processing a large harvest, a cook trying to reduce food waste, a household with limited freezer space, or someone building a shelf-stable pantry, freeze drying can fill an important place alongside the other preservation methods rather than replacing them.

We compare Freeze Drying vs. Dehydrating in our existing guide. [Future link: Our Freeze Drying vs. Freezing vs. Canning guide will help you decide which preservation method makes the most sense for different foods and storage goals.]

Can You Freeze Dry Food at Home?

Yes. Home freeze dryers are designed to automate the freezing, vacuum and drying stages needed for freeze drying food.

That has made a process once associated mainly with commercial and laboratory equipment accessible to home gardeners, cooks, hunters and food preservers.

A home freeze dryer is still a significant appliance investment, however. Machine size, capacity, operating costs, noise, space, electrical requirements and maintenance all matter.

If you are deciding whether one belongs in your home, see our guide to Considering a Freeze Dryer for Home Use. That article focuses specifically on what ownership involves and whether a home freeze dryer makes sense for your lifestyle.

What Foods Can Be Freeze Dried?

A remarkably wide variety of foods can be freeze dried.

Fruits, vegetables, cooked meats, eggs, dairy products, herbs, sauces and complete cooked meals are among the possibilities.

But that does not mean every food freeze dries equally well. Foods that contain very high amounts of fat or sugar can present special problems, and food-safety considerations differ depending on what you are processing.

Our guide What Is Good to Freeze Dry? takes a closer look at which foods work well and which ones require more thought.

How Long Does Freeze-Dried Food Last?

One of the major attractions of freeze drying is the possibility of long-term room-temperature storage.

But there is no responsible single shelf-life number that applies to every home freeze-dried food.

Storage life depends on the food, how completely it was dried, fat content, packaging, oxygen exposure, humidity, light and storage temperature.

Proper packaging is therefore part of the preservation system—not an optional last step.

For detailed information about Mylar bags, oxygen absorbers, containers and storage conditions, see our guide to Freeze-Dried Food Storage.

Is Freeze Drying Only Used for Food?

No.

Freeze drying has long been used for pharmaceuticals, biological materials and other products that need moisture removed while maintaining delicate structures or compounds.

The same principles—freezing, reducing pressure and removing ice through sublimation—apply across many freeze-drying applications. 

For most home users, however, food preservation is what makes the technology so interesting.

Why Freeze Drying Is Different

Freeze drying is unusual because it allows us to remove most of the water from food without relying primarily on prolonged high-temperature drying.

That combination can produce foods that are:

lightweight,
dry and shelf stable when properly processed and packaged,
often close to their original shape,
concentrated in flavor,
and capable of rehydrating surprisingly well.

For home preservers, that opens possibilities that are difficult to achieve with canning, conventional dehydration or freezer storage alone.

The machine may do most of the technical work, but successful home freeze drying still depends on understanding the food being preserved, preparing it properly, making sure it is completely dry and packaging it correctly.

And that is where the rest of our home freeze-drying series comes in.

Freeze drying is a remarkable method of food preservation that dramatically extends the shelf life of your favorite foods in a highly nutritious, light weight form that is easy to use in many ways. With a few exceptions, the freeze drying process will process almost any food in your kitchen, garden or pantry into nutritious, flavorful shelf stable dried foods.

What is the purpose of freeze-drying?

The purpose of freeze drying, in general, is to remove moisture from foods, pharmaceuticals, and other materials while preserving their quality and extending their shelf life. The Freeze-drying process, also known as lyophilization, involves removing moisture from a product through sublimation while preserving its texture, flavor, and nutritional content.

What is Freeze-Drying? The complete freeze drying process consists of three main stages: freezing, primary drying( my freeze dryer calls this stage vacuum freezing), and secondary drying.

1. Freezing: Once you place the food in the freeze-drying machine, it is frozen to a very low temperature, which solidifies the water content within it.
2. Primary Drying: Once frozen, the food dryer will create a vacuum. Under reduced pressure and controlled temperature, the ice within the product sublimates, transforming directly from a solid to a vapor. This phase removes the majority of the moisture.
3. Secondary Drying: After primary drying, the food is further dried at slightly higher temperatures to remove any remaining moisture.

Importance of Food Preservation:

All methods of food preservation at home have something to offer your family. However, freeze drying food at home is especially good at maximizing the benefits of your investment.

Not invested in a freeze dryer yet? With your oven, air dryer or regular food dryer you can make these and so much more…

Learn More About Home Freeze Drying

Ready to explore home freeze drying further? These guides will help you decide whether it’s right for you and get started.

• Considering a Freeze Dryer for Home Use
  What to consider before investing in a home freeze dryer.
• Essential Home Freeze Drying Equipment
  The machine, trays, packaging supplies and other equipment you’ll need.
• What Is Good to Freeze Dry?
  Foods that work particularly well—and foods that don’t.
• Freeze Drying vs. Dehydrating
  Compare two very different ways of making shelf-stable dried food.
• Tips for Freeze-Dried Food Storage
  How to package and protect food after the drying cycle is complete.

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