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If you were asked:"What is the boiling point of water?" You are likely to answer 212 degrees Fahrenheit or 100 degrees Celsius and be pretty sure you were right. The truth is you are only kind of right if this is your answer. Water, like all liquids, boils when the vapor pressure of the liquid (the pressure of evaporation) is equal to the pressure of the gas above the liquid, this is without regard to temperature.

All liquids evaporate, so they have a vapor pressure, and generally the easiest way to increase a liquid’s vapor pressure is to heat it, thereby increasing its rate of evaporation. This is the way we generally speed up drying by heating the molecules of a liquid and increasing their rate of evaporation. This idea is used when we dry something that is wet or cook food by boiling.

We heat things to dry them out but sometimes it we want to dry something without heating because the heat causes changes we might not want. Say we wanted to store food by removing water, which we could add later to reconstitute it. This would be ideal in cases where food was already cooked and where you had food that you would want to heat because of what heating would do to freshness or nutritional value. Well there is a method that is used to remove water without heating to the level of cooking, it is called freeze drying.

In freeze drying, liquid is removed from foods by a process that dries and simultaneously freezes. This is explained best by use of a demonstration experiment. What is needed is:

-A vacuum chamber contained in glass (demonstration vacuum table found in most school science labs)
-A small hand held hair dryer
-A solution of coffee, or kool-aid (100 to 150 mL)
-A small flat pan or pie tin

The solution is place in the pan or pie tin which is then placed under the glass of the vacuum chamber and sealed in place. The vacuum pump is then turned on and the pressure is reduced. As the pressure goes down the outsides of the chamber will begin to get cold and the liquid will begin to boil. At the same time there will be ice forming on the liquid!

As the pump continues to operate the chamber will get colder and colder. As this happens ice will begin to form on the glass. This is when the hair drier is used to defrost the sides of the chamber to increase visibility and aid the evaporation going on inside the chamber. After a few minutes the pressure inside the chamber will be near a vacuum and the liquid will be all gone with only a little ice left.

As a result of the lowering of the pressure inside the chamber the vapor pressure of the liquid became equal to the pressure above it. The reason the chamber became cold is that the process takes heat from the liquid and the surrounding environment to get the energy for the boiling of the liquid. The remaining liquid, because of its loss of energy, begins the process of freezing. The lost of water (Drying) and the lost of heat (freezing) both happen because of the lowering of air pressure above the liquid.

When the vacuum seal is broken and the pan is examine dry solid crystals remain that can be reconstituted by simply adding water. This experiment demonstrates on a small scale what happens on a commercial basis. Take for example Freeze Dried Coffee.

Liquid fresh brewed coffee is pumped into a large vacuum chamber and spread out over a long series of trays which have electrical heating elements on the bottom. The chamber is sealed and evacuation begins, the heating elements are turned on and set to maintain temperatures just above freezing. This means ice is never allowed to form and the coffee will be perfectly dried without the worry of “over cooking”. This same process is used to Freeze Dry and dehydrate other foods for preservation without fear of the use of heat.