Freeze-Drying

Freeze-drying (also known as "lyophilization") is a water removal process that involves freezing the material, then reducing the pressure and adding a low amount of heat to allow the ice in the material to change directly to a vapor (which is called "sublimation").

 

Phase One: Freezing
The food must be frozen below its triple point, which is the temperature and pressure at which solid, liquid, and vapor phases of a particular substance coexist in equilibrium.  It's not enough to get the food super cold, though, it is also important to freeze the material quickly enough to avoid bursting cell walls in materials that are susceptible to cell wall collapse (like lettuce!). 

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Phase Two: Drying

This phase can take a while.  Once Phase One has been completed, our machine then lowers the pressure in the chamber and warms up the trays just enough for the water to sublimate (the ice turns straight into vapor, skipping the entire liquid stage).  If it gets too warm, the food's structure (and nutrients) could be drastically altered, so it's a very slooooow and patient process.  A vacuum in the chamber speeds up the sublimation, and the cold condensor provides a surface for the vapor to stick to so it can turn back into ice.

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Phase Three: Packaging

Once the machine has dried the food, it's time to bag 'em up!  We operate as quickly as possible in a room that has been dehumidified to avoid letting the food soak up any moisture from the air (this is North Texas, after all).  Pieces that aren't fully dry will have a drastically reduced shelf life and can "share" moisture with others in the batch, which means they need to be thrown out (into the compost pile, so they can provide nutrients for more ).   

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