Factor #1: The Temperature
Temperature has more to do with how long well dried foods store than anything else. The USDA states, "Each 5.6oC. (10.08oF) drop in temperature doubles the storage life of the seeds". Obviously, there is a limit as to how far this statement can be taken. However I expect it basically holds true from room temperature down to freezing. No doubt, the inverse could also be considered true. "Each 5.6oC. (10.08oF) rise in temperature halves the storage life of seeds." This theory holds true for non-garden seeds as well. Factor #2: Product Moisture Content
By looking at the USDA nutritional tables, dry beans, grains, and flours contain an average of 10% moisture. Although it is very difficult and unnecessary to remove all moisture from dry foods, it is imperative that any food be stored as dry as possible. Foods with excess moisture can spoil right in their containers. This is an important consideration when packing food with dry ice as moisture condenses and freezes on the outer surface of the dry ice. For long term storage, grains should have a moisture content of 10% or less. It is difficult to accurately measure this without special equipment. Factor #3: Atmosphere the product is stored in
Foods packed in air don't store as well as in oxygen free gasses. This is because air contains oxygen which oxidizes many of the compounds in food. Food storage companies have a couple of different processes for removing the oxygen:
Displacing the oxygen: This is done by purging out all the air in the product with an inert gas. Nitrogen is almost always used because it is the most inert gas known. People doing their own packing occasionally use dry ice which gives off carbon dioxide gas, and probably works just about as well.
Absorb the oxygen: Oxygen absorber packets do just that. Air contains about 78% nitrogen and 21% oxygen, leaving about 1% for the other gasses. If the oxygen is absorbed, what remains is 99% pure nitrogen in a partial vacuum.
If oxygen absorber packets are used, care must be taken to use a storage container that can stand some vacuum. As air is sucked into your container as the oxygen is absorbed, it reintroduces more oxygen that must be absorbed. Before long, the oxygen absorbers will have absorbed all the oxygen they can. Obviously, your product won't be oxygen free under these circumstances. Walton Feed gets around this problem with their plastic Super Pail buckets by purging the product first with nitrogen before tossing in the two oxygen absorber packets. This way the absorbers have little or no oxygen to absorb and don't create a vacuum within the pail. As cans work well under a partial vacuum, purging them with nitrogen isn't necessary before inserting the oxygen absorber packet and sealing the lid. Large seeds store better in nitrogen. On the other hand, small seeds, like many garden seeds store better in air. For this reason Walton cans their garden seed packs in air. Factor #4: The container the product is stored in
To get the best storage life out of your product it must have a hermetic (air tight) seal. Containers that do this well are:
Sealable food storage buckets
Sealable food quality metal or plastic drums
Whatever container you use, be sure it is food grade as your product can be tainted with whatever the container is made from. Plastic sacks are not good air tight containers, for even if they are sealed, the relatively thin plastic 'breathes,' allowing air to pass through. Paper sacks are of course even worse.
There is some concern as to how good a seal is made by the lids on plastic buckets used by food storage companies. Manufacturer studies show an extremely small amount of air transfer. This amount is so small, however, that it can be considered a hermetic seal. It has also been found that the lids can be re-used several times without dramatically degrading the performance of the seal. http://standeyo.com/News_Files/Hollys.html