One ingredient's shelf life is much shorter than you think.
When I saw an Instagram post showing the difference between biscuits made with older versus newer baking powder, I knew I had to try the experiment myself. In all my years of baking, it had never occurred to me that baking powder could lose efficacy over time and result in smaller, flatter baked goods.
My own tin of baking powder was opened about a month ago, so I knew it was fresh. I contacted a friend who does not bake often, and she gave me a container of baking powder that she estimated was at least a year old (her partner said two years). Regardless, it was quite hard and lumpy and had to be pushed through a sieve to measure and use.
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I made two separate batches of scones, side by side, each using a different baking powder. I noticed an immediate difference upon adding the buttermilk-egg mixture to the dry ingredients. The dough with newer baking powder felt spongy and airy right away, while the dough with older baking powder felt dense and dry.
This makes sense, according to how double-action baking powder is supposed to work. Arm & Hammer explains that it has a two-phase effect: "The first surge of air bubbles is created when the alkaline baking soda and the acidic cream of tartar are combined with the recipe's milk or water. The second leavening period happens when the ingredients reach a certain temperature, as occurs during baking in your oven."
Working with the light, airy dough was easier. I folded it a few times and patted it into a rough square before cutting it into pieces. The dense dough felt like more of a battle. It was tougher to fold, shape, and cut.
The difference was even more noticeable after baking. The scones with newer baking powder emerged higher, larger, and fluffier than the others. They were the ones to which my hungry children gravitated immediately, without knowing the motive behind my experiment. Meanwhile, the flatter ones lingered on the rack, despite still tasting great.
The results surprised and impressed me. It made me realize that freshness does matter when it comes to baking powder, and buying it in small quantities if you're not a regular baker makes sense.
Cookbook author David Lebovitz explains that baking powder is sensitive to moisture and humidity, with a six- to 12-month shelf life. "Baking powder should be kept in a cool, dry place, such as inside a cabinet, and should be discarded when it is no longer active." He recommends testing how active it is by spooning a 1/2 teaspoon of baking powder into a bowl and pouring a 1/4 cup of boiling water over top. If it bubbles vigorously, you know it's still good. Baking soda, on the other hand, has "an indefinite shelf life," though manufacturers do recommend replacing it every three years.
America's Test Kitchen, who posted the original comparison picture, recommends writing the "opened date" right on the baking powder can and discarding it after six months. Even unopened, you can only expect it to last 18 months, so this is not an item to buy in bulk.