The key to storing grain is keeping it dry. Whenever crushed grain is left to sit around, it begins to take up moisture from the air. Most malts have a moisture content between 4 percent and 6 percent, and there are very few climates in the world that do not cause malt to absorb water over time. My rule with crushed malt is to use it as soon as possible, which means don’t mill your malt until brew day. This, of course, requires a mill.
If you buy pre-crushed grain and want to store it for some time, you have a few options. One option is to measure the grain into convenient quantities, such as five- or 10-pound lots, and bag it in sealable plastic bags. You can use a fancy vacuum packer, but large freezer bags will do the trick, too. You also can store malt in a sealable container like one used for flour.
Some malts are even sold in woven nylon bags that have plastic liners. These bags work pretty well for extended storage if you roll up the open part of the bag and secure it to reseal the bag.
Malt can be stored at room temperature for extended periods without harm. As long as it stays dry and free of bugs, malt will keep for about a year — although like anything else it will lose its fresh flavor the longer you keep it. Because malt does contain a small amount of oil, it can go rancid if left unused for too long. Rancidity is more likely to occur if the malt is stored in a very hot environment.
I would not recommend storing malt in the freezer, mainly because it is not necessary. The other problem with freezers is their amazing ability to make things taste like they were stored in a freezer! I can’t imagine a beer that ended up tasting like a freezer, but I would guess that it wouldn’t be all that great.