How Does Freeze-Drying Work and Are Freeze-Dried Foods Healthy?

What is freeze-drying?

Freeze-drying or lyophilization is a dehydration technique based on the sublimation of water in a product (1, 2).

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 (1).

Freeze-drying is considered a high quality dehydration method for a couple of reasons, including (1, 2):

• it operates at low temperatures, which contributes to preserving a product’s nutritional value, taste, appearance, and heat-sensitive compounds
• freezing inhibits chemical and microbiological processes, which significantly delays the product’s deterioration (thereby extending its shelf life)

How does it work?

Raw foods are comprised of about 80%–95% water, which can be divided into “free” and “bound” water. While free water freezes, bound water doesn’t (1).

In the freeze-drying process, all free water and some bound water must be removed, and free water must remain frozen at all times. In fact, this is the primary difference between freeze- and vacuum-drying — and the most challenging step (1).

The freeze-drying process can be divided into three steps (1, 3, 4):

1. Freezing: The product is often frozen under atmospheric pressure.
2. Primary drying: Also known as proper freeze-drying, this is the sublimation step in which frozen free water is removed.
3. Secondary drying: Also known as desorption drying, it’s the drying of products to their desired humidity by removing the remaining bound water.

Freeze-drying should be performed in controlled conditions that do not melt the water, seeing that the presence of liquid water may change the product’s composition, shape, and physical properties (1).

That would likely lead to a significant reduction of the product’s shelf life (1).

Once frozen water is removed via sublimation, the remaining product develops a highly porous structure. However, adding water rehydrates the product again almost immediately (1).

Summary

Freeze-drying is a form of dehydration that removes a product’s water content by turning it from ice to vapor. The three-step process preserves a product’s nutritional value, taste, and appearance while extending shelf life.

Are freeze-dried foods healthy?

Freeze-dried foods are a healthy food choice. In fact, freeze-drying is one of the most common dehydration methods due to its numerous benefits.

Freeze-drying is one of the best ways to retain the activity of beneficial plant compounds, such as phytochemicals, and nutrients while preserving color, flavor, and structure. That’s why it’s widely used to produce high-value food products (3, 5, 6).

For instance, studies show that compared with other drying methods, freeze-drying is the most effective at retaining antioxidants, such as anthocyanins, flavonoids, and ascorbic acid or vitamin C (3, 7).

Antioxidants are beneficial compounds that help fight off the damaging effects of oxidative stress in your body. They’re also the compounds behind most fruit and vegetables’ health benefits (7, 8).

However, while freeze-drying may sometimes even increase a fruit’s phytochemical concentration, the opposite may also be true, depending on the fruit (3).

Additionally, given that decreased water activity inhibits the growth of most bacteria, yeasts, and molds, by removing a product’s water content, freeze-drying helps prolong a food’s shelf life (2, 4, 5).

This is especially important for fresh plant-based foods, which may not be available year-round (3).

Lastly, removing a product’s water content leads to a reduced volume and weight, making it easier to handle, storage, and transport (2, 3, 4).

Summary

Freeze-drying helps retain nutrients and phytochemicals in foods. Therefore, freeze-dried foods are a healthy food choice. It also prolongs a product’s shelf life and facilitates storage, handling, and transportation.

Foods that can be freeze-dried

Freeze-dried foods are often used for hiking, camping, space exploration, emergency and survival applications, and military rations. They are preferred over regular foods due to their light weight, long shelf life, portability, and ease of preparation (9).

While plant-based foods are the most popular freeze-dried products, a wide range of foods can be preserved using this method.

Here’s a list of some of the foods that can be freeze-dried (3, 5, 9):

• Fruits: strawberries, apples, blackberries, bananas, pears, oranges, and fruit puree
• Vegetables: almost all vegetables, such as carrots, asparagus, mushrooms, peppers, pumpkin, and tomatoes
• Meats: beef, fish, chicken, eggs, pork, turkey, and shrimp
• Grains: rice, beans, pasta, quinoa, and polenta
• Frozen meals: whole meals like Pad Thai, stews, chili, and snacks
• Beverages: milk, juices, coffee, and tea for instant drinks
• Spices: ginger, oregano, mint, basil, and garlic
• Sweeteners: maple syrup for sugar powder

Summary

A wide range of foods can be preserved by freeze-drying, including fruits, vegetables, meats, whole meals, grains, beverages, spices, and sweeteners.

Potential downsides of freeze-drying

Freeze-drying makes an excellent food preservation method. However, there are some potential downsides to consider.

First, while reducing a product’s water availability diminishes microbial growth, disease-causing microorganisms in raw foods can survive the drying process and remain present during storage. They can cause foodborne illness when eaten (4).

Therefore, foods that need to be cooked before consumption must also be cooked before being freeze-dried.

Second, despite preserving foods’ antioxidant content, the high porosity of freeze-dried products allows easy access to oxygen, which may lead to higher levels of oxidation or degradation of bioactive compounds (2, 7).

Summary

Freeze-drying doesn’t kill bacteria present in raw food. Thus, you must cook raw foods before freeze-drying to avoid foodborne illness. The high porosity of freeze-dried foods may lead to greater degradation of nutrients.

Can you freeze-dry foods at home?

Freeze-drying requires specialized and often expensive equipment that works under a specific temperature and pressure.

However, you can still freeze-dry foods at home by following the freezer method if you don’t want to invest in a home freeze-drying machine.

Here are the steps you need to follow for the freezer method. It doesn’t require special equipment, yet it does take the longest.

1. If necessary, wash, dry, and cook the food.
2. Cut it into 1-inch squares, or as small and even as you’d like. It is best to avoid large chunks of food.
3. Place the food into a single layer on a tray and place the tray in your freezer. It’ll take roughly one week to freeze-dry foods in the freezer properly.
4. Test your food by removing a piece from the freezer and letting it thaw. If it returns to its natural or standard color, the process is complete. Yet, if the food turns black or dark brown upon thawing, it hasn’t freeze-dried completely.
5. Store your freeze-dried foods in airtight containers.

Summary

You can easily freeze-dry foods at home without purchasing specialized equipment by following the freezer method.

The bottom line

Freeze-drying is a way of dehydrating frozen food via a process that transforms ice into vapor, also known as sublimation.

It is a healthy food preservation method, as it retains most of the beneficial plant compounds and nutrients in foods, as well as their color, taste, and appearance.

However, it doesn’t kill harmful bacteria. Therefore, you must cook raw foods before freeze-drying them.

You can easily freeze-dry your own foods at home using your freezer if you do not have a home freeze-drying machine.