How do they make freeze dried ice cream (like for astronauts)? How does freeze-drying work?
It just seems like we should be able to make it at home, or at least buy it in the supermarket or a specialty market.
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Freeze drying (or lyophilization) removes water from the ice cream by lowering the air pressure to a point where ice sublimates from a solid to a gas. The ice cream is placed in a vacuum chamber and frozen until the water crystallizes. The air pressure is lowered, creating a partial vacuum, forcing air out of the chamber; next heat is applied, sublimating the ice; finally a freezing coil traps the vaporized water. This process continues for hours, resulting in a freeze-dried ice cream slice.
Summarizing without the technical language: by using a vacuum chamber and controlling the temperature carefully, you can get the water out of the ice cream without it melting (the water goes straight from ice to vapour) and ruining the structure.
Unless you have access to specialist equipment you will not be able to make it at home. You can buy it online pretty easily but it's mostly interesting as a novelty; it's not actually that great as food (certainly much less enjoyable than regular ice cream), so I doubt demand is high enough to sell outside science museum gift shops.