How Does The Freeze Drying Process Work?

how does freeze drying work intro text


Ever wonder how does freeze-drying work for food preservation? We too, which is why we got a freeze dryer, and it’s a gorgeous piece of equipment. If you’re on the fence about whether freeze-drying is for you, or wondering how it can help you preserve food, read on!

What is Freeze-Drying?

Freeze-drying is when a food item is completely frozen inside a vacuum environment to remove water. Technically known as lyophilization, this preservation method was first used during World War II. The main purpose was to preserve sensitive biological materials like antibiotics. This helped improve transportation without the need for cold storage.

According to history, coffee was the first food to get freeze-dried. Eventually, as the process and technology developed, people can now freeze-dry vegetables, fruits, meats, fish, dairy products, herbs, and even candies.

Unlike canning and dehydrating, freeze drying can help keep your food safe and nutritious to eat for 25-30 years.

How Does Freeze Drying Work?

The process has three phases

Freezing Phase

This is the most critical phase of the freeze-drying process. In this phase, food is frozen at -30 to -50 degrees Fahrenheit. Ideally, this is done in a vacuum, high-pressure environment.

If you’re using a freeze dryer, the machine is going to freeze your food below its triple point. This ensures that the drying or sublimation phase happens instead of melting.

What this phase does is preserve the food’s physical form which is the color and shape. There are several ways to freeze your food.


To do this, you must line the freeze dryer tray with parchment paper. This keeps the food from sticking to the tray. Afterward, prepare the food by cutting them into pieces, then place them on the tray.

Once the food is placed on the tray, put them in the freezer and let it sit there to freeze. A deep freezer is best suited for this phase, but a regular freezer can do just as fine.

how does freeze drying work freezing phase

Chilled bath

This is usually done with dry ice instead of regular ice to prevent adding in more water. The initial principle here was the shell bath, where a liquid product was placed inside a shell and continuously rolled in a freezing environment to maximize surface area.

If you don’t have a freezer, then a cooler and a load of dry ice would do the trick. Place your prepared food inside a zip lock bag and take out all the air. Seal the bag but leave an opening for the gas to escape. Lay the bags of food flat on the bottom of a chest cooler and cover them completely with dry ice.

Don’t put any cover on. Or if there’s a need to cover it, don’t cover it too tightly because the gas will cause the container to explode. You have the option of just freezing the food or letting it sit under dry ice for 24 hours to completely freeze dry.

Shelf In The Freeze Dryer

You can also do the first phase in the freeze dryer, but this will take longer. It’s also a challenge when the outside temperature is too hot.

Primary Drying (Sublimation) Phase

This phase is when all the water goes out of the food in the form of gas. What happens is that as the pressure is lowered, heat is added for water to sublimate.

Sublimation is the process where ice (water in solid form) changes directly into gas or vapor, totally skipping the liquid phase.  One great example of this is dry ice. Ever notice the “cloud of smoke” coming out of it? It’s because of carbon dioxide.

What causes the water to sublimate? It’s a pressurized and vacuum environment. If you keep ice in a vacuum with a 0.6 kPa, it will sublime when introduced to a temperature above 32 degrees Fahrenheit.

The vacuum environment inside the freeze dryer helps speed up this process. The condenser in the machine also provides a space for water to stick and solidify.

At this phase, up to 95% of the food’s water content is takeout. This is where the fun part begins, especially if you’re freeze-drying gummy bears. As water is taken out of the food in the form of gas, it causes the food to expand and pop. You’ll see the bubbles forming as if the food is boiling.

It can be a slow process that should be done with caution. If you add in too much heat, it’s going to change the structure of the food.

how does freeze drying work phases and process

Secondary Drying (Adsorption) Phase

It’s the final or what you could call the finishing stage of the freeze-drying process. Others call this the desorption phase. As the temperature continues to increase, any ionically-bound water molecules are removed from the food. This leaves your food completely moisture-free and good for years and years of storage.

How Do the Phases Happen in A Freeze Dryer?

What makes a freeze dryer unique is that it can provide two essential things for preserving food. One is that it’s capable of freezing food, and the other is provide a vacuum environment. And it all happens inside the chamber.

The chamber is where you place the food for processing. It’s created to give your food a sealed environment to control the whole process.

It gets its freezing temperature from a liquid coolant that travels through coiled pipes. Because of the low pressure inside the freezer, the liquid then turns to gas. Of course, this change requires energy, which comes from any source of warmth inside the chamber.

Now what happens when the liquid leaves the sealed part of the dryer? It turns back into liquid due to the change in pressure and in turn, produces heat. Now you have the answer to the puzzling question of why the back of your freezer is warm. So in effect, the cycle is like drawing out heat from the chamber instead of cooling it down.

And now for the fun part. We have the vacuum pump that’s the boss behind the low pressure inside the chamber. It functions by drawing out gas molecules from the chamber. Imagine what would happen if all that gas from the sublimation process had nowhere to go.

Tips, Tricks, Best Practices

Now that you know how freeze drying works, you might either be excited to try out your freeze dryer… or itching to get one if you haven’t yet. Here are some tips to help make your freeze dryer work like a charm every time.

Keep the food no more than the height of the tray. The thicker the food, the harder it is to freeze dry. Either the process will take longer than usual, or won’t freeze-dry properly. You might end up with food that’s dry on the top but still moist in the middle.

Meat should be kept under a half-inch thick. Same thing as above. You want to keep the meat at this level of thickness so that the meat is fully processed inside and out.

You can place two types of food on the tray. If you’ve got a big tray, and you have a lot of food to freeze dry but in small quantities, they can share the same tray. However, keep in mind the next tip.

Only freeze dry food that has a similar taste and smell. Although freeze dryers can take care of different types of food at once, there’s a condition. They have to have the same flavor and smell profile. Aromatics should go with aromatics, spicy food should go with spicy food, sweet with sweet, you get the idea. Because of the enclosed chamber, the taste and smell can mix together. You wouldn’t want to end up with Skittles smelling and tasting like onion.


How does freeze drying work? It preserves your food in 3 phases:

The freezing phase, which is the most critical phase, is when you freeze food at -30 to -50 degrees Fahrenheit. This can be done through a:

  • Freezer
  • Chilled bath
  • Shelf in the freeze dryer

The next phase is the primary drying or sublimation phase. This is where water goes out of the food in the form of gas

The third and last phase is the secondary drying or adsorption phase. What happens here is that as the temperature increase, all the residual water that’s ionically bound to the food is removed.

All of these happen under the food’s triple point temperature. It’s the only temperature in which water can exist in three states of matter.

Got questions about your freeze-drying journey? Hit us up in the comment section and let’s exchange thoughts!

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