An In-Depth Look at the Freeze-Drying Process
Freeze-drying has been around for decades as a reliable source of food preservation. In a nut shell, the material is subjected to very low pressures that extract up to 98 percent of the water from the food. This allows for very long shelf-life – many times 30 years or more.
What is it?
Freeze-drying, also known as lyophilisation, is a dehydration process that extracts the majority of water out of a meal or food. The items are prepared and then placed on racks and placed inside the freeze-drying machine.
The machine subjects the food to a quick physical change – it is not a chemical process. The water in the food is quickly changed from a solid state to a gaseous state, skipping the liquid state.
Most consider Clarence Frank Birdseye II to be the father of the freeze-drying process (Sounds like a fake name, I know, but it’s true!). He started creating the freeze-drying process in the early 1900s.
The freeze-drying process really took off during WWII as a way to transport serums and other medical supplies. Doctors found that medicines that required refrigeration were spoiling by the time they were transported to other parts of the world. The freeze-dried process was invented and allowed for materials to retain their chemical properties and drastically increasing the shelf-life.
In a short matter of time, the process was adopted for other areas including food preservation.
The process is typically broken down into four stages:
1. Pretreatment. Sometimes items need to be prepared before the process. There are professionals that spend days and hours making sure that the physical and chemical components of the food will be the same after the process.
Sometimes the pretreatment process can include making the food more concentrated, increasing the surface area or adding components to increase the food’s stability during the process.
This stage of the process is not always used. However, this stage is in place to ensure that your food will return to its original taste, texture, and physical state.
2. Freezing. This is probably the most important step in the process. If this is not done correctly, the food will spoil.
In mass quantities, the food is typically placed on sheets and put inside the freeze-drying machine. The food is then quickly cooled to freezing temperatures between -58 to -112 degrees Fahrenheit.
If the process is done too slowly, crystals will form on the food and destroy the food’s texture and nutritional value.
3. Primary drying. Now that the food has been brought to a very cold temperature, and heat is added to bring the frozen moisture to a gaseous state. This is typically done in a partial vacuum to speed up the process.
Sometimes this process can take a few days. If too much heat is added to quickly, it could ruin the structure of the food.
4. Secondary drying. Sometimes, even after the primary drying stage, the food can still have moisture in it. The secondary drying stage tries to remove the unfrozen water molecules. Even thought this stage isn’t always necessary, the food is brought to a higher temperature (sometimes above 32 F).
This ensures that by the end of the four stages the food typically has 1 – 4 percent of its original water levels.
The freeze-dried food is typically stored in #10 cans or pouches to ensure that moisture doesn’t enter back in to the food. There are a lot of factors to help freeze-dried food maintain a 20-30 year shelf-life. Read our article about some of those factors.
Freeze-drying is a great way to preserve food that has been tested and proven. The freeze-drying process causes less damage to foods then other dehyrdration methods that use higher temperatures.
Freeze-drying is a great, reliable method to create food storage that lasts for 30 years or more.Updated February 10, 2012