I would wager that you're probably still using that same bag of all-purpose flour that you bought a long time ago, maybe when you first moved into your current home. But it's hard not to wonder as you pull that half-used, fully open bag of flour off the shelf: Does flour go bad? How long does flour last, just sitting there in your pantry? And if all-purpose flour does spoil, how do you know when it's time to replace your stash? Are there any risks to using expired flour? What is the best way to store flour so that it lasts as long as possible?
Let's start with the basics. Yes, all-purpose flour is shelf-stable, which, according to the US Department of Agriculture's Food Safety and Inspection Service, means that it does not need refrigeration after opening. However, the FSIS also notes that, "All foods eventually spoil if not preserved." The same is true of flour, which can go bad.
So how does flour spoil? It turns out it's not the starch in the flour that goes bad but the protein in wheat, also known as gluten. As Karin Allen, a food quality specialist, writes for Utah State University, "when the proteins that normally form gluten, are exposed to air, they can change significantly. These changes limit the amount of gluten that can be formed," making it harder to create those stretchy, starchy structures than it is with fresh flour and giving you a more crumbly final baked good. As Gwen Adams, marketing coordinator and food writer at King Arthur Flour, explains in an email to Extra Crispy, "The flour might also not perform at its best, meaning bread might not rise as high and cakes may sink."
The best way to tell if you're using old flour is also the most obvious: Look at the best-by date. "The typical shelf life of all-purpose flour is about one year or longer from when it’s milled," Adams says.
But if you've forgotten when you bough that bag of flour or you've transferred your flour from its original packaging into a separate container, the next best way to tell if your flour has gone bad is to smell it. "Old flour will have an off, old, musty smell which will come through in the final product," writes Sarah House, food innovation chef at Bob’s Red Mill, in an email. "Fresh flour should have little fragrance but will smell like fresh, clean grain."
And the best way to extend the life of the flour in your pantry by storing it correctly. House notes that the shelf-life of all-purpose flour can be up to 24 months if you do store flour properly, which means keeping your flour in "an airtight container in a cool, dry place." She adds, "For all-purpose white flours, the freezer is an excellent option, just make sure to let the flour come to room temperature before use."
Really, though, you don't have much to worry about when it comes to accidentally ruining your baked goods with rancid flour, especially if you're storing it correctly. "It's very rare for all-purpose flour to develop truly rancid flavors," explains Adams, "because while it has some fat, it has only a small amount, which means it will have a longer shelf life than whole wheat." But if you have any questions about how long that flour's been in your pantry, you're probably better off getting a new bag, just in case.