From: FoodPhD@aol.com
Date: Wed, 8 Oct 1997 01:00:09 -0400 (EDT)
To: pinhole@exploratorium.edu
Subject: Re: basic chemistry questions from new teacher (sublimation)
A bit more on sublimation:
Ice cubes in a freezer do shrink because of sublimation. The vapor pressure
of the water is much less at the walls where the coolant is circulating than
in the "warmer" center of the freezer. Therefore, the vapor migrates to
there. This is also the cause of freezer burn in foods stored in a freezer
for a long time.
Commercially, this is the basis for freeze drying. However, in this case,
pressure is controlled to stay below water's triple point (where all three
phases of matter are present simultaneously). The food can then be heated to
around 120 F without melting it to speed the removal of the water.
Additionally, as the water migrates to the food's surface to sublime, it
leaves microscopic channels since the food maintains it's original shape
because it doesn't melt. This allows freeze dried foods to rehydrate more
thoroughly and more quickly.
Rob Kravets
Benicia HS
Food PhD@aol.com