Freeze Drying

Freeze drying is a process for removing water to preserve food. Freeze drying does not require adding preservatives, avoids degrading flavor, retains texture, and maintains high nutritional content in the final product.

Look at the Water Phase Diagram below. Water can change into steam (vapor) at room temperature if pressure is low enough (i.e., below 3 kPa). Freeze drying removes water content from a product by lowering pressure until water can be boiled off without necessarily cooking or freezing the product. The product can be held steady at it's optimum normal temperature, allowing as much of its original structure to be preserved as possible.

Watch this YouTube video of water freezing in a vacuum:
Water starts off at room temperature and pressure. At 0:36 the water begins to boil because pressure has fallen below the line where water changes to vapor at room temperature. Since no heat is being added to the water, any water boiling off as steam ends up removing heat from the remaining body of liquid water. Around 0:55, the liquid water cools enough that we are now traveling along the line between liquid & vapor. The water continues to vaporize, cool, and depressurize until 2:58 where the triple point is hit. Once pressure falls below the triple point, you are suddenly left with solid and vapor -- no liquid water is allowed in this region of the Water Phase Diagram. Thus, the liquid water instantaneously flash freezes into solid ice.

Freeze drying can also be applied to products that must be maintained in a frozen state, such as Ice Cream. Pressure is lowered below the triple point, then the ice cream is gently heated to make the frozen water change directly into steam. This YouTube video explains the process:

When the product is heated, water and ice inside of the product will turn into steam, which is then free to escape out of the product into the surrounding vacuum. To keep removing all water/ice content from the product, it is necessary to re-condense this evacuated steam back into ice to keep it from being reabsorbed back into the product or getting inside of the vacuum pump. The astronaut ice cream video describes using dry ice in a cold trap section of the vacuum pipe. 

In my opinion, it might make more sense to use a peltier (aka thermoelectric cooler) device in this application. Normally, peltiers are not very efficient, especially when operated at maximum temperature differential. But in this application, if we can maintain vacuum pressure right around the phase diagram's triple point, we should not need to change the temperature very much in order to move through the different phase states of water.

A peltier acts as a heat pump -- apply electricity and heat will be pumped from the cold side of the peltier to the warm side. If heat is then further removed from the warm side of the peltier, the cold side of the peltier should get very cold. Since the product being freeze dried will naturally cool down as heat is removed by steam escaping from the product, we already have an inherit heat removal mechanism. We need to continuously apply a little heat to the product, to continue driving off water content, so heat sinking the warm side of the peltier to the product should work rather nicely. Assuming the cold side gets cold enough, any steam hitting the cold side should re-condense as ice, preventing the steam from becoming reabsorbed into the product.
K9spud LLC,
Sep 15, 2013, 1:35 AM