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How Freeze Drying Works

Freeze drying is the process by which 98% of the moisture is removed in order to preserve the food for much longer. It also allows the food to retain 98% of its appearance, texture and nutritional composition. The process of freeze drying was invented in 1906, but it was not until the 1950’s that freeze drying food was tried. 

To understand how a freeze dryer 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. With the Harvest Right at home freeze dryer, there is also 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 involves multiple smaller processes and has specific vocabulary, such as temperature, atmosphere, pressure(I’m still not sure what a militorr is), and sublimation.  I will do my best to simplify it for you. 

Freeze Drying is like a 3 act play. 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: the trays of food are loaded on to trays into the machine and frozen. I mean frozen the regular way,  not "born of cold and winter rain and mountain air combining". Frozen 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 is important for the process to work properly that the ice crystals be not too big and not too small so that they can evaporate with out 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 word used here to describe when H2O skips the liquid phase and goes straight from solid ice to steam vapor). The machine ever so slowly warms 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. 

The third and final act of the play, desorption drying, begins. At this point the food is only mostly dry - about 95%. There is a big difference between mostly dry and all dry. 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 F.  Now, because the bulk of the ice crystals are gone, the texture of the food  remains the same no matter the temperature ( up to 150deg!). 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 marvel of modern science using multiple machines in concert to create the best preservation of food in history. Beginning with a quick deep freeze, then sublimation, and final drying to create a product that, as long as it stays airtight, keeps for 25 years. The applications are endless! People are even freeze drying candy! All Different Kinds of Candies!