Have you ever wondered how food can be preserved for longer? Well, freeze drying is a process that removes 98% of the moisture from food, which helps to keep it fresh for longer. It also means that the food can retain 98% of its appearance, texture and nutritional composition. The process of freeze drying was invented in 1906, but it wasn't until the 1950s that freeze drying food was tried.
Let's dive into the world of freeze drying together! To understand how it all works, we first need to go through the components (or sections) of the machines. Well, actually it is several machines all connected. First, we have the cylindrical chamber with an airtight sealing door that holds the shelves. The shelves are connected to a warmer. Next, we have the compressor (basically the same thing that is in a regular freezer that keeps it, you know, freezing). Also, there is a vacuum pump connected to the cylinder chamber. The Harvest Right at home freeze dryer is pretty amazing! It has a set of sensors connected to a small computer that allows it to run the cycles automatically. Scientists in labs have to read the sensors and adjust temperature and pressure manually at different points of a cycle.
Freeze drying is definitely a scientific process. It seems so simple, you “freeze” and “dry” the food, but when I actually looked into what goes on, it felt so complicated! It's a pretty complex process, with lots of different steps and technical terms like temperature, atmosphere, pressure (I'm still not sure what a millitorr is!), and sublimation. I'll do my best to explain it in a way that's easy to understand.
Freeze drying is like a three-act play. The first act is where the food is frozen. In the second act, the food is basically sent to outer space (no air pressure, no heat). Act three finishes drying and gets it ready for packaging. Act one is where the trays of food are loaded onto trays into the machine and frozen. I mean frozen the regular way, not "born of cold and winter rain and mountain air combining." It's important to freeze the food down to minus 50 degrees. That's 50 degrees below ZERO or 82 degrees below "freezing." Once the food is that cold, all of the water, liquid, fluid, moisture, H2O, whatever... has become ice crystals. It's really important for the process to work properly that the ice crystals be not too big and not too small so that they can evaporate without affecting the texture of the food.
In the second act, the machine uses a vacuum pump to pull almost all of the atmosphere out of the chamber. So, now it's like we've sent the food into outer space! No warmth, no atmospheric pressure. Conditions are right for what scientists call "sublimation drying," which is a fancy way of describing when H2O skips the liquid phase and goes straight from solid ice to steam vapor. The machine does its thing ever so slowly, warming just the trays to around -10 degrees F. This is the tricky part. Because there is no (or very little) atmospheric pressure, the temperature needed to turn the ice crystals into vapor (sublimate) is also very low, only about -20 up to +0. As the temperature comes up a little, the ice crystals become vapor and condense onto the still frozen chamber walls.
Now, it's time for the third and final act of the play, desorption drying! At this point, the food is only mostly dry, about 95% so. There's a big difference between mostly dry and all dry, so the machine now warms the trays further while still maintaining the vacuum inside the chamber. It slowly warms the shelves up to over 120 degrees Fahrenheit. Now, because the bulk of the ice crystals are gone, the texture of the food remains the same no matter the temperature (up to 150 degrees Fahrenheit). Once the sensors read no more vapor in the chamber, the play is over. The food is warm and dry, and the chamber walls are covered in condensation ice.
Freeze drying is a truly amazing process that uses multiple machines to create the best food preservation ever! It starts with a quick deep freeze, then sublimation, and finally drying to create a product that, as long as it stays airtight, lasts for 25 years. The possibilities are endless! People are even freeze drying candy!