Go to the Article: Understanding the Role of Water in Cooking
Really interesting.
It would be nice to see how freeze drying works or how to collect the condensation. I have some ideas but I would enjoy to see some chef steps tricks.
Freeze drying is not all that much. It's a physical process invented by Mother nature not man. We just use it and show off talking about it. If water is in the solid state it cannot become a liquid if it's under sufficient vacuum. It can become a gas though. That's a physical property of our universe. In freeze drying we make use of it.
Freeze the item to be dried. Place it in a chamber that will maintain that frozen condition ( a chilled space ) that is vacuum tight and apply a strong vacuum to it. The water in the frozen item will be drawn off directly as a gas and not pass though the liquid or plastic state. It will then be drawn away as a gas by your vacuum device which is usually a pump. That gas will condense as ice if it contacts a hard frozen surface. Or it will turn to liquid it it enters the pump which is usually warm. A well made freeze dryer has a freezing chamber to freeze the object to be dried. That will be connected to a strong vacuum pump. The drafted water vapor will enter a chill box ( inline between the chamber and pump ) and be grabbed as ice on its surface so the pump does not pull it in. Or in cheaper units it will be drafted into the pump and contaminate the oil that seals the vacuum rotary piston. If the vapor is pure water this can be partially resolved by flowing the contaminated oil through a port that connects to the atmosphere and the oils heat will drive the water off as a gas through that port. That's "exhaust ballasting" and used on high end pumps that are not just pulling water vapor but other oil contaminates that can't be grabbed in a chill box.
Why do this? Because removing water via sublimation ( ice to steam without an intermediate phase ) leaves the molecular structure of the dried substance intact while simple dehydration often destroys it. That's why a freeze dried grape still looks a bit like a grape while a dehydrated one looks like a raisin. And because it's fun to do.
