In response to an Instructables thread from 2009 that I ran across, I decided to test if any idea for an at home lyophilizer (freeze dryer) could work? I didn't have a vacuum pump or any sophisticated condensing or cooling equipment. I've worked with desiccators in the lab and I thought that if I could get a vacuum chamber at the vapor point of water, I might be able to keep it under that point with anhydrous salts.
Step 1: The Science Behind The Suck
Through my research I learned two interesting facts that led me toward my final project design:
1. Freeze drying is the result of both negative pressure and temperature.The rate of drying depends on both.
2. Freezer burn, and that annoying build up of ice in the back of your freezer, are the result of the same process--namely water vapor leaving or forming solid ice. This freezer-burn vaporization is called sublimation and its opposite freezer ice buildup is called deposition.
Now this led me to think that if I could lower the pressure in a sealed container to the vapor pressure point and keep it there, I could achieve freeze dried fruit goodness in the back of my freezer.
Scientifically, the problem is vapor pressure. Vapor pressure relies on temperature and pressure. The vapor pressure of our berries will tend to equalize with the pressure and temperature inside the chamber. This means roughly that a food being passively dried will only get as dry as its environment.This makes it more difficult to dry something further. (The water keeps coming out of the fruit and raises the pressure.)
This can be solved at home by putting a desiccant in the low-pressure container to absorb the vapor and to actively pull it from the food you want to dry.
However, this means that the rate at which the vapor leaves the food and enters the desiccant is much slower than an active vacuum system (traditional vacuum lyophilzer).
After testing how much vacuum it could pull, I chose to use a wine saver hand pump for my first experiments (see picture below). There are many other similar systems on the market--usually labeled as instant marination containers or vacuum bag systems.