Does Flour Go Bad? Here’s How to Tell If It’s Still Safe to Use

It's not an uncommon experience to be cleaning out your pantry and find a bag of flour you forgot you had. A quick peek at the package shows a date that has unfortunately already passed. Do you use it anyway or toss it? Does flour go bad, and is it safe to use expired flour? To find out, we consulted experts in flour milling, food safety, and food science. 

What the Date on the Bag Means

Flour leaves the mill, and typically, when it reaches the store shelf, it has been stamped with a best-by date of a year or so. When looking at that bag of flour, it's important to distinguish between best-by and expiration dates. "Manufacturers set 'use by' and 'best if used by' dates to tell consumers the optimal time for flour’s taste and performance; they aren’t food‑safety deadlines,” says Holly Markovitz, director of R&D at King Arthur Baking Company, who make flour, along with other baking ingredients. “Sell-by dates are time stamps directed to the retailer to stop displaying the products,” she notes. What you're likely to find on flour are best-by dates, not expiration dates. So the date stamp is an indication of age rather than a direction to throw out unsafe food.

How Flour Is Stored Matters

Our experts say that whether the flour lasts as long as that date or longer, the actual best usage window depends on the conditions under which it is stored. Flour is a shelf-stable product, but how and where it is stored impacts its quality. 

"'Best used by' dates are helpful rough guidelines, but storage conditions make a big difference,” says Harold McGee, authority on the science of food and cooking, and author of On Food and Cooking and The Curious Cook. “Flours kept tightly wrapped in the fridge or freezer will retain their quality much longer than they will in a half-filled glass canister on a sunny kitchen counter,” he explains. 

When the industry gives flour a best-by date, they assume you will store the flour correctly. “It's not super humid, it's not super dry, and you're probably storing it somewhere dark," says Gevork Kazanchyan, MS, REHS, CP-FS, professor of environmental health at California State University, Northridge, and former public health deputy. 

So if the date is somewhat arbitrary, what about safety and quality? 

Wheat Flour Safety 

Wheat flour is generally safe. "With flour, your risks are the presentation of food insects, because eggs are in a good amount of grains. There have been some foodborne outbreaks involving flour, such as Salmonella, but those have been due to contaminated products. So the storage length is not part of the risk," says Kazanchyan. Overall, it’s more about quality than safety, he explains. 

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Wheat Flour Quality

Flour can go rancid. McGee explains what that means: "In general, it’s a stale, off flavor caused by the breakdown of fat and oil molecules into small smellable fragments. Exposure to oxygen, light, and heat can cause that breakdown. The oils in grains are concentrated in the outer bran and the inner germ, so flours made from the whole grain go rancid more quickly and noticeably than 'refined' white flours that are bran- and germ-free.”

How Can You Tell if Your Flour Is Rancid? 

“Your nose knows! Rancidity occurs when naturally occurring fats in the flour oxidize, like how olive oil can spoil. If you smell a musty, off note, then it’s time to buy a new bag," says Markovitz. Kazanchyan describes the smell of rancid flour as "sour, funky, musty, barnyardy, and maybe a solvent-like, chemical-like, or a paint smell. It's going to have the fragrance of a consumer product you would never eat." As unappetizing as that sounds, Kazanchyan says it’s a good thing. "It’s obvious when flour goes rancid, but with botulism and E. coli in foods—things that can actually be injurious, if not fatal—we don't taste the difference, and that's the problem." So giving your flour a sniff is a good way to test its quality—and if in doubt, toss it out!