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Name: Jay S.
Status: student
Age: 16
Location: N/A 
Country: N/A
Date: 10/31/2004


Question:
I was just curious about how freeze drying works. I know that you have answered questions similar. Now I'm not meaning commercial dryers or things. I would like to know the reaction taking place when a person places plant matter in a box with dry ice and the organic material after a while is dried out. Commercial dryers use vacuums, this is not required with dry ice. I understand how commercial systems work (a little). I think the Carbon Dioxide makes the air capable of holding more water vapour than normal at a given temperature and then this water saturated air is released from the container slowly as the plant dries. My second question is how much moisture can Carbon Dioxide absorb from a plant. How could I determine the right amount of dry ice for how much water I think is stored in the plant matter?


Replies:
I think what actually happens is that carbon dioxide completely displaces the air in the container (CO2 is heavier than air), and pure CO2 contains no dissolved water. The air that it displaced did contain some dissolved water, and that dissolved water was entering the plant matter at the same rate that water from the plant matter was leaving. In other words, there was an equilibrium. With only CO2, the equilibrium point will be shifted, and the plant matter will dry until, again, water is leaving at the same rate that it is re-entering from the CO2/water solution.

But, in fact, the water that leaves the plant matter might not stay dissolved in the CO2. Because the CO2 is very cold, the water might condense, freeze, and precipitate out as snow flakes. Then it clearly would not be able to re-enter the plant matter. So the limit might not be about how much water can dissolve in CO2, but rather how much energy the CO2 can absorb and still be cold enough to freeze water.

Also, I do not know how rapidly water will be leaving the plant matter when the plant matter has cooled to the temperature of the CO2.

Tim Mooney


What CO2 does is to act like a pump to remove water from the air surrounding the material being "freeze dried". The Assuming the material is a plant, the plant tries to maintain an equilibrium amount of water vapor in the vicinity of its leaves and stems. If the amount of water vapor decreases, the plant releases water to try to re-establish that equilibrium. As long as the dry ice keeps freezing the water vapor, it is removed from the vicinity of the plant and eventually the plant "runs out of water" and is freeze dried. It is not a chemical reaction that is dehydrating the plant it is "simply" a losing race with trying to keep the proper amount of water vapor around its foliage. So, as long as any dry ice remains, it will continue to freeze the water out and the plant will continue to try to supply water in the form of vapor evaporated from its leaves to maintain the proper amount of moisture in its surroundings. In commercial processes the basic mechanism is increased by using auxiliary vacuum pumps (this greatly speeds transport of the water vapor from the "hot" region around the plant to the "cold" region of CO2 or even more effective freeze drying agents such as liquid nitrogen, but the principles are the same. Also in commercial applications the temperature of the substance being "freeze dried" is also decreased to reduce the amount of other components that might be frozen out. Specifically, in the case of drying coffee the temperature is reduced so that flavoring components that are less volatile than water are not remove by the process.

Vince Calder



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