Desiccant FAQ

What is a desiccant?

A desiccant is a substance with very hygroscopic (adsorbs moisture from the air) properties. There's any number of different substances that meet this description, but only some of them will serve our purposes.

The most commonly used desiccant is silica gel. This is an amorphous, highly adsorbent form of silica. It is most easily found in a form called "indicating silica gel" which are small white crystals looking much like granulated sugar with small colored specks scattered throughout.

Those specks are how we determine whether the gel is dry or has adsorbed all of the moisture it will hold. If the specks are blue, the gel is dry and capable of carrying out its moisture adsorbing mission. If the specks have turned pink, then the gel has adsorbed all it will and is now saturated. Part of what makes silica gel so useful is that it can be refreshed by driving out the adsorbed moisture so it can be used again. This is a simple as pouring the saturated desiccant into shallow pans and placing in a 250 F oven for no more than five hours until the colored crystals have once again turned blue. You can also do the same thing in a microwave. Stir thoroughly and repeat until dry.

Although I've never found anything that mentions this, apparently it is possible for silica gel to break down over time, or at least the colored crystals can. I had a five pound can stored in an outside shed here in Florida for several years before I opened it again to use some of it. Nearly all of the colored indicator specks had broken down and disappeared. I don't know if the gel itself was still good and with no way to reliably determine whether it was saturated or not, I discarded it. The can the gel was in was just cardboard and it gets very humid here in Florida so it really was very poorly stored. Under decent conditions it may not break down at all. (I've never heard of this occurring, anyway.)

There are other desiccants, but I am not familiar with any that can be used with foodstuffs. I know that Kearny recommends using a piece of gypsum wallboard as a desiccant in his expedient radiation meter in Nuclear War Survival Skills, does anyone know if this can be used with dry foodstuffs? How about other desiccants?

Where do I Find Dessiccants?

I get all the desiccant information from, all of my silica gel at 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 absorbent. All of the desiccant that I've found packaged this way has been rather expensive (to me) so shop carefully.

How do I Use Desiccants?

The key to storing many foodstuff for the long term is dry, dry, dry. Available oxygen and storage temperature also play roles, but it is moisture content that determines whether you get usable food out in five years or not.

Therefore, the idea here is to have the food you want to put into storage as dry as possible before it goes in and then take steps to deal with any moisture trapped, generated or leaked into your storage containers.

Ideally, the foodstuffs you have on hand will be no more than 10% moisture. If this is the case then you can go ahead and seal them into your storage containers using the packaging method of your choice and have a reasonable expectation of your food staying in good condition.

If your storage foods aren't sufficiently low in moisture content then you'll need to reduce the water they contain. Wheat has been found intact in Egyptian pyramids where it had lain for several thousand years. It was the bone dry desert air and the cool interior temperature of the pyramids which kept it from rotting away. We can approximate that Egyptian climate by several methods.

The least involved method is to wait until the driest time of year for your location. 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 from the a/c. I'd avoid anywhere near the kitchen or bathroom areas, as they put out a lot of moisture. Stir the food frequently to maximize moisture loss. About three days of cool, constant air flow and low humidity ought to dry things out a bit. Due to its highly 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.

If this won't do, you can place a large quantity of desiccant in your storage containers. Fill the remaining space with your food product and seal on the lid. 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. You'd hate to find later it saturated in storage.

I use silica gel for practically everything. My usual procedure is to save or scrounge clear plastic pill bottles such as 500ct aspirin bottles. 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. 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. This way whatever moisture does inadvertently get trapped inside can be safely absorbed. It won't dry out a lot of moisture -- you still need to take steps to get everything as dry as possible before you pack it -- but it will take care of what little is left.

IMPORTANT NOTE: The indicating form of silica gel (has small blue 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.

I've never found any certain rule of how much silica gel to use to how much dry goods. For my purposes, I use about four ounces of gel to a five gallon bucket of dry grain and beans. If I think the moisture content is over 10% when I seal them, I'll go as high as a half pound. This might be ridiculous overkill, but in Florida everything is high in moisture because of our ever present humidity. For a one-liter bottle of dry milk I'll use about one ounce of silica gel rolled up in a paper cartridge made from a coffee filter. If you're familiar with them, it looks like a paper cartridge such as you'd use for black powder weapons. They fit nicely into the bottle and keep the gel in.

What is a desiccant and what does it do?

A desiccant is a hydrating agent which attracts moisture from the atmosphere. It adsorbs and holds particles of water to itself by physical bonds or absorbs H2O to itself which is chemically integrated into another.

What are the major types of desiccants?

At present, there are five typical desiccants:

Silica gel
Silica gel is silicon dioxide. It is a naturally occurring mineral that is purified and processed into either granular or beaded form. As a desiccant, it has an average pore size of 2 angstroms and has a strong affinity for moisture molecules. Silica gel performs best at room temperature(20~32?) and high humidity(60~90%). It will drop the relative humidity in a container down around 40% RH.

Montmorillonite Clay
Its appearance is that of small gray pellets. Clay is a good basic desiccant that works satisfactorily below 50?. Above 50?, there is a possibility that the clay will give up moisture rather than pulling it in. The upside to clay is that it is normally the least expensive desiccant.

Molecular sieve
Molecular sieve is a synthetic desiccant that has a very strong affinity for moisture molecules. The pore size on the molecular sieve particles can be controlled by different manufacturing process. It can be used for picking up specific gases as well as moisture. Molecular sieve can hold moisture to temperatures well past 450°F (230°C). Because of its high affinity for moisture, molecular sieve is able to bring the relative humidity in packages down as low as 10% RH.

Mineral Desiccant
Hengyuan mineral desiccant is composed by several natural minerals. The appearance of it is offwhite globule. Mineral desiccant is degradable and environmentally-friendly product, the moisture adsorption rate is over 50%, twice of that of common silica gel.

Fiber Desiccant
Hengyuan fiber desiccant is refined by natural plant fiber through special technology. The fiber desiccant sheet is covered by PET film, convenient in use and not taking up space. Its moisture adsorption capacity reaches 100% of its own weight, far surpassing that of tradtional silica gel. In addition, the product is safe and secure, especially meeting requirements of bio-material, medical and health-care food.

What size desiccant do I need?

The answer to the size of desiccant needed depends on the size of the air space to be desiccated, the nature of the material in the package, the moisture barrier surrounding the package, the moisture barrier surrounding the package, the atmosphere conditions where the package is sealed, the desired shelf life, the type of seal on the package, etc. When we design the sizes of desiccant for customers, we will ask a number of questions to determine this information so we can calculate what size desiccant will work best in a particular application.

What is saturation and equilibrium capacity?

Although technically they are different situations, for most practical purposes these two terms cover the point at which a desiccant no longer adsorbs moisture. Saturation is when the desiccant is full and even if there were moisture molecules to pick up, the desiccant could not do it. Equilibrium capacity is when the desiccant has pulled so much moisture out of the air that the air retains a stronger hold on the moisture molecules than the desiccant can exert. At equilibrium capacity, adding more desiccant will not bring the Relative humidity any lower.

Who are our customers?

Pharmaceutical and Vitamin Packaging, Food packaging, Electronic Products, Metal Parts, Computers, Leather Products, Shipping Container and so on.

What does a unit refer to?

According to the adsorption capacity of desiccant products, a “Unit” is the quantity of desiccant which will adsorb 3.00 grams of water vapor at 20% relative humidity or 6.00 grams of water vapor at 40% relative humidity. More precisely, a “Unit” of Clay is typically 33 grams, and a “Unit” of Silica gel and Molecular sieve is typically 28 grams.