Q: I love quantity discounts on malt, but do malts have limited shelf life? I assume cool, dry storage is best, but is this the same for dry malt extract? Liquid extracts? Should I refrigerate or freeze?
DM: Malt, like almost any food product, is perishable. Whole grain malt, however, will keep for six months or more if it is stored in cool, dry conditions. Preground malt is less stable, because crushing strips away the tough protective husk and exposes the interior of the grain to airborne moisture and mold spores. Nonetheless, even preground malt will keep for a couple of months if it is stored in airtight bags. By the way, the bags in which preground malt is packed are very good these days.

Malt extract is likewise fairly stable, but not everlasting. Dry malt extract will show deterioration by absorbing moisture from the air and hardening. I have seen it keep for a year when stored in a very dry room packed in a double layer of airtight plastic. Liquid malt extracts, on the other hand, seem to slowly deteriorate in the can or drum from the day they are packaged. Very old malt syrups become dark and acquire a coarse, molasses-like flavor that is obvious in beers made from them. You can do little to extend the life of malt syrup by your own packaging or repackaging. About all you can do is keep it at moderate temperatures (60-80 °F) and use it within six months.

I do not recommend freezing or refrigerating malt in any form. Unlike hops, they do not keep better under cold storage. I do, however, recommend storing opened packages of malt grain or dry malt extract in a tightly sealed container and with a few packets of silica gel desiccant. You can get these things in the notions department of department stores, and they last forever if you occasionally redry them in the oven. Dry air and moderate temperatures will give your malt the longest possible shelf life.

(from:brewingtechniques.com)