If you've ever been to the Air and Space Museum in Washington DC, perhaps you've sampled the kind of freeze-dried ice cream astronauts eat during space missions. A stiff, dry square, the freeze-dried version tastes just like ice cream, except that it isn't cold, and has no moisture.
That's because when you freeze dry something, you're essentially removing all the water from it. The scientific term for this process is called lyophilization. If you look up lyophilization in the dictionary, chances are you'll come across another scientific term: sublimation.
When heat is applied, a substance normally changes from a solid to a liquid, and finally to a gas, or vapor. Sublimation is a process whereby a substance changes from a solid to a vapor without first becoming a liquid. This process is the key to freeze-drying.
Here's how it works. Let's say you wanted to freeze-dry a tomato. Using a freeze drying machine, you'd first freeze the tomato in order to change its water into ice. The machine would then create a vacuum, thereby lowering the pressure inside. Adding heat to this low pressure environment allows the ice to sublimate into vapor. After drawing off the water vapor, the process is complete. You've got a freeze-dried, waterless tomato.
As for why you'd want to do this to a tomato, let alone anything else, freeze-drying has many practical applications. Since water is what causes foods to spoil, removing water from food allows them to last much longer. The pharmaceutical industry relies on freeze drying to give its products a long shelf life. And, last but not least, freeze-drying allows astronauts to eat ice cream.