Some things in life come and go, while others are solid and dependable over the long haul. That's true in your kitchen too, where – unlike quick-spoiling perishables – long-lasting pantry staples are always there for you when you need them. That doesn't mean they'll last forever, unfortunately, just that their shelf life is measured in months or years instead of days or weeks. With cornmeal, for example, you can keep it on hand for a year or more before it needs to be replaced.
Learn the Difference
There are actually two different kinds of cornmeal you could be buying, and their shelf lives are different. Stone-ground cornmeal contains everything that makes up the entire grain of corn: the bran and the germ, as well as the starchy endosperm that makes up most of the grain's weight. That's important, because the germ adds a lot of nutrition, but it's also perishable because of the oils it contains. Regular mass-market cornmeal is made from corn that's had the germ milled out to improve its shelf life among other things. It's less healthy but a lot more durable.
Storing Commercial Cornmeal
Commercial, degermed cornmeal isn't an especially perishable product. The U.S. Agency for International Development estimates that it's best if used within 18 months of its packaging date, but that's got nothing to do with food safety. In longer storage, it just loses flavor and nutrients. At home, it's safe to assume you can keep it for up to a year in a dark cupboard or pantry without any significant loss of quality. If you don't use cornmeal much but like to always have it on hand – or if you buy it in bulk to save money – you can extend your shelf life by keeping it in the fridge for 18 months or in the freezer for up to a couple of years.
Storing Stone-Ground Cornmeal
If you're a serious corn lover or if you make a real effort to buy healthy whole grains, you might prefer to use stone-ground cornmeal. The trade-off is that your corn will only really be at its best for about a month. After that its flavor will deteriorate pretty quickly, and its oils will develop a musty, rancid odor. That's good news and bad news: It's not great that it goes off so quickly, but at least your nose will tell you whether you should still use it. You can extend the life of stone-ground cornmeal to two to three months by keeping it in your fridge or four to six months by keeping it in the freezer.
Store Your Cornmeal Properly
Those numbers all assume that your cornmeal was reasonably fresh when you opened it and that you've stored it properly. A glass jar on a sunny counter next to your stove is probably the worst place you could keep it. An airtight, critter-proof jar or canister is much better, kept in a dark place where the temperature doesn't fluctuate too much through the course of the year. An airtight bag or container is important in the fridge or freezer too. In your freezer or for long-term storage, the best option of all is a vacuum-sealed bag. Extracting most of the air slows the corn's oxidation to a crawl and helps keep it from absorbing any off-flavors while it's in storage.