Quick caveat: I answered this question in the context of food preservation because it came to mind when I was making instant coffee this week. Freeze drying also has other applications, which have slightly different methods.

The short answer

Freeze drying is a food preservation process in which food is flash frozen and dehydrated using two methods in order to remove as much water as possible.

The long answer

Freeze drying is a method of food preservation that freezes food before removing its water content. The purpose of freeze drying is to preserve a food for a long time while retaining high nutritional value.

But before we get into what freeze drying is, you need to know that there's two types of water in food: free water and bound water.

Free water vs. bound water

Free water is what you can remove by squeezing, cutting, or pressing (e.g. juice from an orange). Whereas bound water can't be removed through these methods. Its molecules are bound with other food molecules so tightly that they do not behave like normal liquid water and thus can't be removed through traditional dehydrating. An example of bound water is the water that is found in cactus needles (who knew there was water in cactus needles?).

Freeze drying: A 3-step process

Put simply, freeze drying is when food is frozen and then dehydrated two ways to remove its free water and then its bound water. So maybe it should be called "freeze drying drying" Here's how it works:

Step 1: Freezing

Freeze drying is best executed when the frozen item has large ice crystals, since this allows for more efficient dehydration. However, the formation of large ice crystals tends to break down cell membranes and impact the quality, texture, and nutritional value of food. So when freeze drying food, the freezing step is done rapidly to form small ice crystals.

Step 2: Removing free water through sublimation

Sublimation is when a substance transitions from a solid state (ice) to a gaseous state (water vapor) without going through a liquid state (water). To achieve this, the completely frozen item is placed under low pressure and heat is added in order for the water content to sublimate. The vacuum speeds up this process. At this stage, about 80-95% of the water is removed from the food item.

Step 3: Removing bound water through adsorption

The food item is nearly fully dehydrated at this point, but the bound water content still remains. To extend the shelf life of the food even further, the extra step of adsorption is completed to remove the bound water. In adsorption, atoms, ions, or molecules of gases, liquids, or dissolved solids stick to a surface, creating a film on the outside.

To achieve adsorption in freeze drying, the item is placed under even lower pressure and the temperature is raised slightly to break the bonds between bound water molecules and frozen material. At the end of this process, only 1-5% of the food item's water content remains, leading to a long shelf life and high quality product.

Sources

Encyclopædia Britannica, inc. (2023, June 28). Sublimation. Encyclopædia Britannica. https://www.britannica.com/science/sublimation-phase-change

Lang, A. (2022, July 20). How Does Freeze-Drying Work and Are Freeze-Dried Foods Healthy?. Healthline. https://www.healthline.com/nutrition/freeze-drying

Meirowitz, R. (2018, December 5). Microencapsulation technology for coating and laminating. Smart Textile Coatings and Laminates (Second Edition). https://www.sciencedirect.com/science/article/abs/pii/B9780081024287000055?via%3Dihub

Nowak, D., & Jakubczyk, E. (2020, October 18). The Freeze-Drying of Foods—The Characteristic of the Process Course and the Effect of Its Parameters on the Physical Properties of Food Materials. Foods (Basel, Switzerland). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7603155/

Rosenberg, D. (2023, January 25). What is Adsorption in Freeze Drying?. Empire Freezing & Drying. https://empiredrying.com/what-is-adsorption-in-freeze-drying/

Vaclavik, V. A., & Christian, E. W. (2021). Water. In Essentials of Food Science (3rd ed., pp. 27–28). essay, Springer.

What is Freeze Drying?. Millrock Technology, Inc. (2023, July 17). https://www.millrocktech.com/lyosight/lyobrary/what-is-freeze-drying/