Frequently Asked Questions On Desiccant
What Is A Desiccant?
A desiccant is a substance with very hygroscopic properties, meaning
it will soak up water vapor from the air surrounding it. A number
of different substances are capable of doing this, but only a relative
few of them are of practical use and fewer still are going to be
readily available to the average person. Before elaborating on the
different types that might be useful for our purposes it's necessary
to explain how to choose a desiccant.
The U.S. military has done much of the best research on the use
of desiccants in packaging and have largely set the standards by
which they are judged. Each type of desiccant has temperature and
humidity ranges where it performs best and particular physical and
chemical characteristics that may need to be considered in relation
to what you propose to do with them.
The most applicable standard for home food storage defines a unit
of desiccant as the amount of desiccant that will adsorb at least
6 grams of water vapor at 40% relative humidity at 77F (25C).
In order to maximize surface area to obtain optimal adsorption,
desiccants are manufactured in granular or powder forms. This presents
a problem of keeping the desiccant, which may not be safe for direct
contact with food, out of the product while still allowing sufficient
air flow for it to carry out its task. Manufacturers call this "dusting"
and deal with it by packaging the adsorbent in materials such as
uncoated Tyvek, a spunbonded high-density polyethylene material
produced by the Dupont corporation. Unfortunately, I have not yet
been able to locate a retail source of uncoated Tyvek, just the
coated variety such as is used in postal envelopes. Second best,
and what I use, is two or more layers of coffee filter paper securely
sealed over the mouth of the container holding the desiccant. I've
also made "cartridges" of filter paper for use in narrow
necked containers such as two-liter bottles. For this I used ordinary
white glue. Getting a good seal all the way around requires some
care in execution. Brown Kraft (butcher paper) may be used as well.
HOW DO I USE DESICCANTS?
Before you get to this point you should have already used the charts
above and determined how much of the particular desiccant you're
interested in you need for the size of the storage containers you'll
be using. Once you know that you're ready to put them it into use.
Although they perform different functions, desiccants and oxygen
absorbers are used in a similar fashion. They both begin to adsorb
their respective targets as soon as they are exposed to them so
you want to only keep out in the open air as much desiccant as you
are going to use up in fifteen minutes or so. If you'll be using
oxygen absorbers in the same package, place the desiccant on the
bottom of the package and the oxygen absorber on the top.
If your desiccant is pre-packaged, that's all there is to it,
just put it in the package and seal it up. If you have purchased
bulk desiccant you'll first need to make your own containers.
I use indicating silica gel for practically everything. My usual
procedure is to save or scrounge clear plastic pill bottles, such
as aspirin bottles or small plastic jars. Fill the bottle with the
desiccant (remember to dry the gel first) and then use a double
thickness of coffee filter paper carefully and securely tied around
the neck of the bottle to keep any of it from leaking out (remember
the indicating type of silica gel is not food safe). The paper is
very permeable to moisture so the gel can do its adsorbing, but
it's tight enough not to let the crystals out. I use plain cotton
string for this as both adhesive tapes and rubber bands have a way
of going bad over time which could allow the cap to come off and
the desiccant to spill into the food.
For containers that have openings too narrow to use a desiccant
container such as described above you can make desiccant packets
with the same filter paper. The easiest way I've found to do this
is to wrap at least a double layer of paper around the barrel of
a marker pen and use a thin bead of white glue to seal it with.
Slide the packet off the pen and allow to dry. When ready, fill
with the necessary amount of desiccant. You can then fold the top
over and tie with string or staple closed. Take care that the top
is closed securely enough not to allow any desiccant to leak out.
Virgin (not recycled) brown Kraft paper can be used to make the
packets with as well.
The above method will also work for the other desiccants, subject
to whatever precautions the individual type may have.
IMPORTANT NOTE: The indicating form of silica gel (has small blue
or pink specks in it) is not edible so you want to use care when
putting together your desiccant package to insure that is does not
spill into your food.
WHERE DO I FIND DESICCANTS?
I buy indicating silica gel on internet in their dry flower section
where it is sold in one and five pound cans for flower drying. I've
seen it sold the same way in crafts stores and other department
type stores that carry flower-arranging supplies. You can also buy
it from many other businesses already prepackaged in one form or
another to be used as an adsorbent. All of the desiccant that I've
found packaged this way has been rather expensive (to me) so shop
carefully. There are a number of Internet sources available which
will probably provide your best route for finding what you want.
Businesses carrying packaging supplies sometimes also sell desiccants.
Some businesses commonly receive packets or bags of desiccants packaged
along with the products they receive. I've seen Montmorillonite
clay in bags as large as a pound shipped with pianos coming in from
Japan. Small packets of silica gel seem to be packed in nearly everything.
Naturally, any salvaged or recycled desiccant should be of a type
appropriate for use with the product you want to package.
It is possible to make your own desiccants using gypsum from drywall
and maybe Plaster of Paris. Calcium oxide can also be produced from
limestone (calcium carbonate) or slaked or pickling lime (calcium
hydroxide) by roasting to drive off the adsorbed water and carbon
dioxide. I don't have any clear instructions, as of yet, on how
to go about this. Please do keep in mind that calcium oxide (quicklime)
is caustic in nature and is hazardous if handled incorrectly.
What is a desiccant?
The dictionary defines desiccant as a substance that has a high
affinity for water and is used as a drying agent such as calcium
oxide and silica gel.
How does a desiccant act as a drying agent?
Desiccant attracts moisture from the air by creating a low vapor
pressure at the surface of the desiccant. Since the vapor pressure
of the humid air is higher than the vapor pressure of the desiccant,
the water molecules move from the air to the desiccant.