Thing I Learned Today: Cryodesiccation

by Brian Shourd on November 13, 2012

Tags: engineering, science

My daughter Ada currently is in love with these wierd little toddler snacks. They’re tiny lumps of freeze-dried yogurt, which got me wondering what exactly freeze-drying is. It’s actually a pretty clever process.

The goal, of course, is to take all (or most of) the moisture out of a biological product, generally with the intention of preserving it. Normally, one might use a dehydrator - a combination of heat and dry air in a contained environment to remove all of the water from fruits and/or meat. This has the problem that the food inside has to be heated to some extent in order to give the water enough energy to make the transition from a liquid to a gas.

However, we know that the state of matter (liquid, solid, gas) is affected not just by temperature, but also by pressure. These phases are represented by a phase diagram that looks something like:

A Phase Diagram

A Phase Diagram

At a specific pressure and temperature, water is in the phase plotted on this chart. Notice that at low temperatures and low pressures, water can transition directly from a solid to a gas, without being a liquid in between. This is called sublimation, and you may be familiar with it if you’ve ever looked at Dry Ice. Dry Ice is just solid carbon dioxide, and carbon dioxide’s phase diagram is such that normal atmospheric pressure is below the triple point, so that it sublimates when it heats up and we don’t get to see liquid carbon dioxide - the ice just smokes.

If we make an object cold enough so that all the water inside freezes, then reduce the pressure below the triple point, we can heat the object back up enough for the frozen water inside to sublimate and escape. In fact, this whole thing can happen at cold temperatures (relative to room temperature), so the object itself is still frozen. Hence the name - freeze drying. And that’s how we get astronaut ice cream.