An In Depth Look At the Freeze Drying Process

When it comes to food storage, freeze dried food has become the select choice since its ability to stay fresh for 20 – 30 years is revolutionary. Now it is possible to freeze dry fruit, vegetables, meats, dairy products and whole meals. Because it is so light, preppers, backpackers, and campers love keeping freeze dried meals on hand. So how is this even possible?

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.