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How Does Freeze-Drying Food Work?
With its unique preservation properties, freeze-drying is a popular technique in the food industry. It helps maintain foods’ freshness and nutritional value for many decades! It is able to do this because freeze-drying is known to preserve 97% of nutrients and can be used to store food for up to 30 years.
But how does freeze-drying food work? It works by following a process called lyophilization. Freeze-drying machines are able to simulate this by following a three-staged approach: freezing, primary drying, and secondary drying.
In this article, we’ll explore the different stages and scientific concepts of freeze-drying. We’ll also highlight the pros and cons of this preservation method. So, stick around.
What Is Freeze Drying?
As the name implies, freeze-drying is a preservation process using temperature and pressure to remove moisture, producing a porous structure.
The process involves placing a completely frozen sample in a vacuum chamber and then applying heat to convert the frozen water directly into vapor.
In the case of food, freeze-drying retains the nutritional content, flavor, color, and texture. So, the food maintains its freshness even after being stored for a long time.
However, lyophilization isn’t food-exclusive. The pharmaceutical industry uses it to preserve many biological samples, vaccines, microbial cells, antibiotics, and drugs.
Shelf Life of Freeze-Dried Foods
Generally, most freeze-dried foods can survive 20-30 years without spoiling. However, the shelf life depends on the food type, pre-freezing preparation, and storage conditions.
For instance, raw meat is highly nutritious and can harbor numerous bacteria. As a result, it can have a shelf life of 10 years when freeze-dried. For that reason, it’s best to cook the steak before freeze-drying to extend its expiration date.
Similarly, you should freeze vegetables, fruits, and beverages before the freeze-drying process for best results.
Regardless of the food type, you need to ensure optimal storage conditions that are cool, dry, and dark. Otherwise, you risk spoiling the food.
Nutritional Benefits of Freeze-Dried Foods
Freeze-drying is one of the best preservation methods for maintaining food quality. As you might know, freeze-drying only involves low processing temperatures and pressure—no chemical reactions take place.
That helps retain vitamins, minerals, antioxidants, and enzymes—all of which are heat-sensitive compounds. Sure, high temperatures help destroy food pathogens, slowing down spoilage.
However, once exposed to high cooking temperatures, those nutrients change in structure and lose their benefits.
Freeze-drying helps you avoid all this hassle. For instance, the former preserves 93% more plant pigments (carotenoids) than hot air drying. It also retains plant phenolics and other beneficial nutrients.
How Does Freeze-Drying Food Work?
As the name implies, freeze-drying, or lyophilization, works by freezing the water content of food. The complex process involves three stages: freezing, primary drying (sublimation), and secondary drying.
Pro Tip: To perform the lyophilization process, we encourage a home freeze-dryer machine. They recreate the correct environment to allow for freeze-drying to occur, as well as are safe to use.
Before we explain each step, let’s discuss water molecules in food.
Generally, raw food is around 80% to 90% water. However, not all water molecules are the same. Some molecules are bound to the cells, while others are free.
During freeze-drying, all the loose water converts to ice, while some bound moisture evaporates. However, that’s easier said than done.
Note: Freeze-dried food is different from dehydrated food. Dehydrated food loses much more nutritional value and flavor and has a much shorter shelf life than freeze-dried foods.
Here’s a detailed explanation of food freeze-drying:
Step 1: Freezing
Freezing is always the first and most critical step in lyophilization. This stage involves cooling food materials below their triple point under atmospheric pressure.
For those wondering, the former describes the temperature and pressure at which the substance’s gas, solid, and liquid phases exist together at equilibrium.
Temperatures below the triple point cause water molecules to transform from water vapor to ice. Lowering the pressure, on the other hand, converts solid ice into a gas.
For the freezing step, you want to rapidly lower the temperature to around -112ºF (-80°C) to prevent large ice crystals from forming. Such structures can damage food cells, reducing their quality.
For that reason, most lyophilization devices use liquid nitrogen, which has a temperature of -320ºF (-196°C).
Step 2: Primary Drying (Sublimation)
In the second stage, ice sublimation occurs, which converts solid water molecules into gas without entering the liquid phase. This process requires lowering the pressure while adding heat to allow the water to vaporize.
Most lyophilization devices use a vacuum pump to lower the pressure and a cold condenser to trap the water vapor and let it solidify.
However, heat can be supplied through different methods, including conduction, radiation, or microwaving. Of course, each food material requires different pressure and temperature conditions to sublimate.
This process is complex and slow. In some cases, it can take several days to get rid of all the frozen water.
The good news is that if done correctly, around 93-95% of the food’s water is removed. As a result, the product develops a porous structure.
Step 3: Secondary Drying
Secondary drying, or desorption, is the last phase of freeze-drying. During this stage, the remaining moisture is removed to fully dehydrate the product.
As you know, food has bound water, which can make up around 7-8%. Since those molecules bond to inorganic surfaces through several forces, they don’t evaporate during sublimation.
That’s when the adsorption phase enters. The goal is to break the water bonds, releasing them. This can be achieved by exposing sublimed food to temperatures around 50ºF – 122ºF (10°C-50°C) for several hours.
Such a range is higher than the primary drying phase. As a result, the food dehydrates, reaching a moisture content of 1-5%.
Pros and Cons of Freeze-Dried Food
From the above, you can see that lyophilization offers numerous benefits. For starters, it extends the shelf life of food. Additionally, it allows food to rehydrate completely. Not to mention, freeze-dried products are travel-friendly!
However, that doesn’t mean this preservation method is flawless. The process is complex, and any errors can affect the product’s quality. Aside from that, it doesn’t eliminate microorganisms. Plus, it’s costly.
Let’s discuss the pros and cons of freeze-drying food in further detail!
- Shelf-life extension: Most microorganisms require a water activity of 0.95 to grow. Freeze-dried food can have a water activity below 0.6, preventing food spoilage.
- Quick rehydration: Since the removed water leaves behind a porous structure, freeze-dried food rehydrates in a few minutes, returning to its original, fresh state.
- Versatility: You can freeze dry fruits and vegetables in large or small chunks. You can even turn them into powder and use them in smoothies.
- Travel-friendly: Freeze-dried food is light thanks to removing all the moisture. As long as you keep them in a cool, dry place, you can travel with them anywhere!
- Complex processes: Overdrying, too much heating, condenser overloading, and vapor choking are all common problems with freeze-drying and can affect the food’s quality.
- Microbial growth: Freeze-drying also preserves microbial cells. Once rehydrated, those pathogens can multiply, causing food-borne illnesses.
- Costly: Lyophilization is expensive since it consumes a lot of energy and time. It can cost around five times as much as other drying methods.
Common Items that Can Be Freeze-Dried
One of the reasons freeze-drying is so popular is that all sorts of items can be freeze-dried. Since freeze-drying helps you preserve items for a long time, people often use this method to preserve food, as well as decorations, and for arts and crafts. Some examples of foods that are commonly freeze-dried are:
As you can see, freeze-drying is a sophisticated yet highly effective food preservation method. The process involves stripping food of its moisture content through sublimation and desorption. This allows for more nutrients and flavor to be preserved after completing the proper freeze-drying process. Nutrients and flavor are often lost over time when using more traditional food preservation methods.
Still, freeze-drying poses some challenges, such as the potential for microbial growth. Not to mention, it can be costly!
Despite these drawbacks, this process remains a popular choice for food preservation since it extends its shelf life and maintains its nutritional value!
If you would like to learn more about freeze drying, see the related articles on Soulful Prepper.
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