Astronaut ice cream. How could anyone not want a treat that's so light-weight, so non-perishable, and so neapolitan? Today we shall take a peek at the physics behind the taste sensation.
Whatever food you happen to be eating for your next meal, chances are it will contain a lot of water. That's no judgment on the food. We contain a lot of water ourselves, so it's hardly our pace to cast the first stone. The difference between us and the food is simple; the water inside the food will encourage some very undesirable elements to grow, and the food doesn't have a functional immune system to control them. Fungus, bacteria, and all kinds of microbes will make the food their home and slowly spoil it. Find a way to remove the water and keep it out, and the food will keep for years without any damage.
There's a problem. Generally, removing the water means letting the liquid in the food turn to gas via heat. This takes a long time, and comes with a lot of structural changes that destroy the taste and texture of the food. At the very least, the ice cream would be melted. What's the only way to get out of the heat trap? Sublimation. It's not just a way to make your psychological problems barely socially acceptable! In physics, sublimation is the transition of matter from ice to gas, without becoming liquid between the two. To do this it needs to be exposed to a vacuum. In a vacuum, liquid water will boil at any temperature. We're told that, when people are briefly exposed to vacuum conditions, their saliva boils away. Put a block of ice in a vacuum, and it will never form a puddle. As soon as any molecules get the energy to break away from the ice, they'll zoom away as a gas. (This makes me want to do some variation on the Locked Room Murder Mystery, where a person is found dead in a locked room with a puddle of water. In a vacuum, the puddle would be gone and it would just be a slightly more humid locked room. Then again, in a vacuum, it's rarely going to be a mystery what killed someone.)
Freeze drying is a two-stage process. A freeze dryer works like a regular freezer to freeze food solid. Then a vacuum pump kicks in and lowers to pressure inside the freezer to 0.06 of Earth's atmospheric pressure. Left alone long enough like this, and the water molecules will drift away. (Some people have noticed that their freezers at home can freeze-dry food left exposed long enough even without the vacuum.) But to speed the process along, a tiny bit of heat is added to the chamber. As soon as the water molecules transition out of the ice stage, they immediately become gas, leaving the husk of the food behind with its structure mostly intact. This can be stored, without refrigeration, in a sealed bag for years without decaying or degrading. Since the water has been removed, it's also light enough to travel easily. It seems like the perfect space food, but nowadays astronauts chow down on regular ice cream. Only the Apollo 7 astronauts ate this kind of freeze-dried food.
Top Image: Evan Amos