Dessicants and Moisture in Food Storage

WHY MOISTURE IS IMPORTANT

Moisture in inappropriate amounts and places is damaging to food.  Because of this, much effort is put into reducing the water content of dry foods in order to prolong their shelf lives.  Once it is reduced to the desired level the product can then be packaged for storage. Unfortunately, merely reducing moisture content is not always sufficient.  Environmental conditions can play a role as well.

There are four mechanisms by which environmental conditions may cause a moisture problem in your food storage:

1. – The air trapped in the container with the food may have held sufficient humidity to raise the moisture content of the food to undesirable levels.

2. – Even if the water vapor content wasn’t too high, a falling temperature level may cause the trapped humidity to reach its dew point causing water to be squeezed out of the air to condense on your food much the same way as dew forms on your lawn on cool mornings after a warm, humid night.  This can be a particular problem if the condensation is localized – say, only the portion of the food next to the walls of the container – resulting in excessive moisture in that local area even though the contents as a whole would be at a satisfactorily low moisture level.

3. – The seal of the container may not be sufficiently tight enough to prevent moisture laden air from leaking in.

4. – The packaging material itself may be porous to water vapor to one degree or another.  All paper, wood and cardboard has this fault.  Depending upon their particular physical properties some plastics do as well.  Metal and glass containers have excellent barrier properties though their seals may not.

The solution for moisture problems is multi-faceted.

1  – Make sure the product to be stored is at an appropriate water content for that particular foodstuff.  Beans and grains store well at a 10% moisture level, but milk powders, dried eggs and dehydrated or freeze dried foods should be lower for best results.  As a general rule, nearly any dry food will store well at moisture contents between 3%-10% with the lower the better.  Don’t get carried away with this though.  Extreme low moisture levels (below 3%) can make some foods difficult or impossible to reconstitute and damage the viability of seeds.

Ideally, the dry foodstuffs you have on hand will have no more than a 10% moisture content.  If they do not then you will need to reduce moisture to a level appropriate for the kind of food you are storing.

One of the following methods might be of use in lowering moisture content.

A – The least involved is to wait until the driest time of year for your location making sure there is plenty of free air circulation around the food product.  If this doesn’t suit, then turn your air conditioning on a little high.  Bring in your buckets, lids, and the storage food.  Let everything sit in a well-ventilated place where it’s going to get plenty of cool, dry air from the A/C (avoid anywhere near the kitchen or bathroom areas, as they put out a lot of moisture).  Stir the food frequently to maximize moisture loss.  A few days of cool, constant air flow and low humidity ought to dry things out. Due to its odor absorptive nature, I would not do this with any dried milk products or other powdered foods, flours or meals .  This method works best with coarse particles such as grain, legumes and dried foods.

B – Warm, dry air can also be used to lower moisture content and works well if you have large quantities of grains and legumes.  This is similar to what is used on farms for drying harvested grain.  You’ll need a source of forced, warm, not hot, air.  Place the grain in a drum or barrel and blow the heat from the bottom so that the warm and the moisture it will carry can exit from the top.  It’s important to not let the bottom product get too hot. You should also monitor the top, center of the drum to be certain the product there is not getting damp from the moisture escaping other areas.  Stirring occasionally may be necessary.  I’ve seen this done with an old, drum style vacuum cleaner that put off fairly warm exhaust air and it worked pretty well.  Do be sure to clean the vacuum thoroughly so you don’t blow the grain full of dust.

C – If the above methods won’t do or you have powdery foods to dry, you can put the food and a large quantity of desiccant (see below) in a storage container.  The desiccant should be in its own container placed on top of the food and the container lid sealed on.  After about a week, unseal and check the  desiccant.  If it’s saturated, change it out with dry desiccant and reseal.  Continue to do this until the contents are sufficiently dry.  If it doesn’t become saturated the first time, change it anyway before sealing the bucket permanently to deter saturation in storage.

If your food products are sufficiently dry you can pack them in storage containers using the packaging method of your choice and have a reasonable expectation of your food staying in good condition.  Whether you will need to use a desiccant will be dependent upon the conditions discussed below.

2  – Try to package your goods in a dry atmosphere and do not allow extreme temperature swings in storage areas.  Warm temperatures and a high relative humidity when a container is sealed means the air trapped inside the container will have a high dew point. This will lead to condensation should storage temperatures fall below that dew point.  An example of this would be a container sealed on a day that was 70º F and 40% relative humidity.  At that temperature the relative humidity would be quite reasonable for all but the most moisture sensitive food.  However, should the temperature fall to 44º F the capacity of the air to hold water vapor would have dropped to the point that it could not contain what was sealed in at 77º F and the excess would be squeezed out to condense on the food, i.e. – it will grow moister.  Possibly the food will be able to adsorb this moisture without harm and then again, it may not.

3  – Use appropriate packaging materials and make certain it is sealed correctly.  If you are going to consume them in four to five years, storing grains, beans and peas in unlined HDPE buckets at normal humidities is fine.  If you want to keep them at their best for many years beyond that, the plastic the pail is made of is too porous to water vapor for best results and should have an interior liner of a material with better barrier properties.  Dry milk powders should not be kept for more than a year in unlined HDPE, but can be kept for much longer in #10 metal cans, glass jars or Mylar bags.  Naturally, even the most highly resistant packaging material is useless if its seal isn’t good so be sure you use good technique when making closures.

Lastly, you may wish to consider using a desiccant if good humidity control at the time of packing is difficult or if the storage area is in a high humidity environment or if the packaging material does not have sufficiently high barrier properties.

NOTE: There has been some confusion in the past over the appropriate use of desiccants in food storage which I would like to address here.   Any desiccants you may seal in your storage containers (if you use them) are not for lowering the moisture content of the foods therein, but for moderating any shifts in moisture levels caused by those factors I mention above.  If the food you want to put up is too high in moisture for good storage this needs to be dealt with BEFORE you seal the packaging.  An example of what I’m trying to communicate here would be 10lbs of wheat with a 15% moisture content.  That’s too high for safe storage and needs to be lowered, preferably to 10% or less.  To lower the moisture content of that grain to 10% you need to remove the 5% excess.  5% of 10lbs is eight ounces of water.  Good dry silica gel (one of the most common desiccants) will hold 40% of its mass in moisture so to soak up that extra water you would need 20 ounces of silica gel – quite a large amount – all to remove that 5% excess moisture in ten pounds of grain.   Fifty pounds of grain at that same moisture level would require 100 ounces or six and a quarter pounds of silica gel.  Clearly no practical amount of desiccant you can put inside your storage packaging will do for you what should have been done before the food was put by.  Desiccants can be used for lowering food moisture content, but this will involve rotating packages of desiccant in and out of the foodstuff until the desired moisture content has been reached.  Once the package is sealed any desiccant you leave inside should be there to control moisture fluctuations or to guard against moisture infiltration from the outside.

WHAT IS A DESICCANT?

A desiccant is a substance with strong hygroscopic properties, meaning it will soak up water vapor from the surrounding air.  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 77º F (25º C).

The following table gives the amount of desiccant necessary per square area for flexible containers such as Mylar bags or per volume of area for rigid containers such five gallon pails or #10 metal cans.

Units of Desiccant Needed Per Given Container Volume.
FLEXIBLE CONTAINERS Desiccant RIGID CONTAINERS
Units of Volume in:
Area in Sq. Ft. Area in Sq. In. Units Required Gallons Cubic Feet Cubic Inches
0.1 30 1/6 1.1 0.14 237
0.3 45 1/3 2.1 0.28 476
0.6 90 1/2 3.2 0.42 714
1.3 180 1 6.2 0.83 1,428
1.9 270 2 12.5 1.67 2,856
2.5 360 3 18.7 2.50 4,284
3.1 450 4 25.0 3.33 5,712
Flexible containers would be Mylar and other plastic bags.  Rigid containers are buckets, jars, cans, etc.
Table adapted from “Moisture In Packaging: Selecting the Right Desiccant” ©, Multisorb Corp.  http://www.multisorb.com

This is all well and good so far as it goes but without knowing how much of a particular type of desiccant is needed to soak up that six grams of water it doesn’t do you much good.  The next table will reveal all:

Desiccant Needed to Adsorb 6 Grams of Water Vapor
Desiccant Type Mass (weight) of Desiccant Needed
Silica Gel 15 grams
Indicating Silica Gel 75 grams1
Montmorillonite Clay 24 grams
Calcium Oxide (quicklime) 21.5 grams
Calcium Sulfate (gypsum, Drierite) 60 grams
Wood 43 grams 1
1See desiccant descriptions for clarification.

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.

For coarse granular materials tightly woven fabrics might serve the purpose providing the seams are tightly stitched.

TYPES OF DESICCANTS

SILICA GEL

The most commonly known and used desiccant is silica gel which is a form of silica dioxide (SiO2), a naturally occurring mineral.  It will work from below freezing to past the boiling point of water, but performs best at room temperatures (70-90º F) and high humidity (60-90%). Its performance begins to drop off over 100º F, but will continue to work until approximately 220º F.  It will lower the relative humidity in a container to around 40% at any temperature in its range until it is saturated.  Silica gel will absorb up to 40% of its weight in moisture.  Some forms are approved by the FDA for direct food use (check with your supplier to be sure).  It recharges easily (see below in the indicating silica gel text) and does not swell in size as it adsorbs moisture.

INDICATING SILICA GEL

In the retail trade, the most common form of silica gel is indicating silica gel composed of small white crystals looking much like granulated sugar with pink or blue colored crystals scattered throughout.  This is ordinary silica gel with the colored specks being coated with cobalt chloride, a heavy metal salt.  When the gel has absorbed approximately eight percent of its weight in water the colored crystals will turn from blue to pink making an easy visual indicator of whether the gel has become saturated with moisture.  Because cobalt is a heavy metal, indicating silica gel is not food safe and should be kept from spilling into anything edible.

The indicating silica gel will still adsorb up to 40% of its weight in water vapor like the non-indicating type will but once it has gone past the 8% level and the crystals have turned pink there is no way to tell how close it is to saturation.  This isn’t necessarily a problem, you’ll just have to treat like the other non-indicating desiccants and either weigh it to determine adsorption or use a humidity indicator card.  These cards are made to show various humidity ranges and can be had from many desiccant and packaging suppliers.

When saturated, both varieties of silica gel can be dried out and used again.  This is done by heating the crystals in an oven at a temperature of no more than 300° F (149° C) for approximately three hours or until the crystals turn blue.  Dehydrating the desiccant may also be accomplished by heating in a microwave oven.  Using a 900 watt oven heat the crystals for three minute intervals until the color change occurs.  The exact amount of time necessary will depend upon the oven wattage.  Spreading the desiccant in a broad pan in a shallow layer will speed the process.  Heating to 325° F (149° C) or more, or using a microwave oven over 900 watts can damage the gel and render it unable to adsorb moisture.

If your desiccant is packaged in Tyvek, do not heat above 250° F (121° C) or you could damage the material.  This leaves a fairly narrow temperature window since silica gel will not begin to desorb moisture below 220° F (104° C).  It’s a good idea to use a reliable oven thermometer to check your oven temperature as the thermostats in home ovens are often off by more than twenty five degrees.  Start with the packets in a cold oven and raise the temperature to 245° F (118° C), keeping it there for twenty four hours.  Spread the packets so they are not touching and keep them at least 16 inches from any heating elements or flames so that radiant heat does not damage the packaging.  Tyvek should not be microwaved.

CLAY DESICCANT

Although not generally found in the retail market, clay desiccant is fairly common in commercial and industrial use. The primary reason for this seems to be that it is inexpensive compared to any other form of desiccant.  Some mail order suppliers offer it for retail sale.

The desiccant material is Montmorillonite clay, composed primarily of magnesium aluminum silicate, a naturally occurring mineral. After mining it is purified, reduced to granules and subjected to a controlled dehydration process to increase its sorbent porosity.  It recharges easily and does not swell as it adsorbs water vapor.  It works well at low and room temperatures, but has a rather low ceiling temperature. At 120º F it will begin to desorb or shed the moisture it has adsorbed. This is an important consideration for storage in hot areas.

Subject to a degree of variability for being a natural material, clay desiccant will adsorb approximately 25% of its weight in water vapor at 77º F and 40% relative humidity.

CALCIUM OXIDE

Also known as “quicklime” or “unslaked lime”, calcium oxide is a slow, but strong adsorbent.  It is efficient at low humidities and can drop moisture vapor to below 10% relative humidity.  Qucklime is caustic so must be carefully handled, particularly with regards to dust inhalation and exposure to skin and eyes.  It expands as it soaks up water vapor and this must be taken into account when packaging. It will adsorb up to about 28% of its weight in moisture, but does so slowly over a period of several days rather than a matter of hours like other desiccants.  It is most effective when used in high humidity environment where a very low humidity level is desired.  It will release a fair amount of heat if exposed to direct (liquid) moisture or extreme humidities.

Calcium oxide can be recharged, but I do not have any details on how to go about this other than roasting at fire temperatures.

For expedient use, quicklime can be manufactured from clean lime stone (calcium carbonate) or pickling lime (calcium hydroxide) available in the canning sections of many grocery and hardware stores.

CALCIUM SULFATE

Also known as gypsum and commercially as Drierite, calcium sulfate is another naturally occurring mineral. It is produced by the controlled dehydration of gypsum CaSO4).  It is chemically stable and does not readily release its adsorbed moisture.  It has a low adsorbency capacity, only approximately 10% of it weight.  It can be regenerated, but apparently not easily so.

For expedient use, gypsum is commonly used in household drywall and Kearny mentions using this source in his Nuclear War Survival Skills.  This makes only a so-so desiccant and you’d be much better off to use a more suitable choice but in an emergency it can get the job done.

OTHER DESICCANTS

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From: Pyotr Filipivich pyotr@coho.halcyon.com

Simple trick is to dry a piece of wood in the oven and once it is bone dry (more than usual) then put it in your container and seal it.  The wood will suck up any available moisture.

Editors note:  Wood can soak up to 14% of its weight in moisture, depending on species.  Woods with coarse, open grains work the best. I’m not aware at what temperature it will begin to “desorb” or shed its stored water which might be fairly low. Some empirical experimentation would be in order before relying heavily on it.

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.  This is to keep the desiccants from robbing needed moisture from your oxygen absorbers which will hinder their operation.

If your desiccant is pre-packaged, that’s all there is to it, 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 from leaking out (remember the indicating type of silica gel is not food safe).  The paper is permeable to moisture, but it’s tight enough not to let the crystals out. I use several winds of plain cotton string for this as both adhesive tapes and rubber bands have a way of going bad over time which might allow the cap to come off spilling the desiccant 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 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.  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 twice 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 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 at Wal-Mart 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.