We leave them kicking around the pantry for ages, but when should we really replace baking soda and baking powder?  
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When was the last time you flipped over that little canister of baking powder to look at its expiration date? The one on mine—August 2014—straight-up made me cringe.

In my defense, I’ve only recently gotten into baking in a big way, baking loaves of bread about once a month since the spring. And my biscuits have been pretty darn fluffy despite my, er, antique baking powder.

But August 2014! I pride myself on being on top of ensuring my fruits, veggies and meat are fresh and crisp, and try my darndest to get through all of it before it goes bad. My baking powder went somehow completely under my freshness radar. Same goes with those boxes of baking soda (which are, incredibly, up to date).

Knowing that the effects of this on my baking could be dire, I turned to the pros—my trusty Food Lover’s Companion and Francisco Migoya, head chef for Modernist Bread: The Art and Science, a 2600-page, multi-volume tome that will be available this fall. Migoya was kind enough to walk me through both of them on the phone.

The mixture of baking soda (aka sodium bicarbonate), acid and a moisture-absorber such as cornstarch—also called baking powder—is a vital part of the home baker’s pantry. According to Food Lover’s, one can easily test its viability by combining a teaspoon with 1/3 cup of hot water: “If it bubbles enthusiastically, it’s fine.”

I tested my own 2014-era powder using this method, and observed that it bubbled lazily, with little enthusiasm. I’ll definitely be replacing it in my next grocery store run. When I reached Migoya, however, he noted that baking powder and baking soda manufacturers “can’t say that it’ll last forever.” He asked me how often I replaced my salt. I admitted that I never think to do that (and I power through salt so fast it isn’t an issue) and he replied, “Exactly: Salt is a mineral.” So is sodium bicarbonate, he pointed out. “Minerals, for the most part, don’t really go bad; there’s nothing that bacteria could really deteriorate.”

Migoya pointed out that “mice won’t eat baking soda or powder, bugs don’t like them, [and] they don’t taste good. As long as you keep your baking soda in a cool, air-tight environment, keep water out, keep its average temperature around 70 degrees Fahrenheit, and keep it in a cool dry area,” he said, it’ll last a while.

I definitely failed when it came to stowing my baking powder consistently in a cool spot; it had somehow wended its way near a radiator, and apartments get hot—much warmer than 70 degrees— in New York City in the summertime. As for baking soda, I have two: I stash one in a cabinet, and the second, open, in the fridge for freshness. I won’t be using the latter for baking: Not only has it likely picked up odors from the fridge, warned Migoya, but “It’s come into contact with a lot of moisture, [and it] may not be as active or react as [normally] as when you add the water. It might not be as bubbly.”

He drew an interesting comparison to honey, sticky stuff that we wouldn’t necessarily think of as being extraordinarily shelf-stable. Because of how jealously honey guards its water content, creating “a hostile environment for whatever’s in the air and could make it go bad,” explained Migoya, “if you keep honey in an air-tight environment it’ll last for hundreds of years.” (In fact, scientists have found edible honey that’s thousands of years old.)

When I mentioned that my biscuits had puffed despite containing “expired” baking powder, he said that it made sense: “You saw it: The biscuits turned out fine. It’s not like how we keep spices in our cupboards for years … With minerals, rocks don’t go bad.”

Since baking powder is not purely sodium bicarbonate, it’s arguably more likely to lose its oomph, so do try that hot water test to make sure it’ll still work. And remember that the best proof of whether something is still working is in the pudding—or rather, the biscuits.

Alex Van Buren is a food and travel writer living in Brooklyn, New York whose work has appeared in Gourmet.com, Bon Appétit, Travel + Leisure, New York Magazine, Martha Stewart Living, and Epicurious. Follow her on Instagram and Twitter @alexvanburen.