Halfway through a baking project and realize you're out of baking powder? Make your own with these substitutions.

After busily cleaning out the pantry, I discovered two open, but only partially used, containers that I didn't know I had. One was baking powder and one was baking soda. It's always useful to have extra on hand in case of a baking emergency, but I figured that it would be a good idea to test them to see if they still worked, since chemical leaveners can lose their potency over time for various reasons, including poor storage conditions.

The procedure for testing these two products is simple. Get out two small glasses or bowls and fill one with 3-4 tablespoons of white vinegar and fill the other with 3-4 tablespoons of room temperature water. Add about 1/2 teaspoon baking soda to the bowl containing the vinegar and the same amount of baking powder to the water bowl. Each mixture should fizz up, with the soda/vinegar producing a bigger reaction. If they both work, mark the containers with the date and check them again in 6-12 months (if you haven't used them up by then) to make sure they're still active. And if they don't fizz up, you better stop by the store before the next time you want to whip up a batch of cookies.

Baking soda is also known as bicarbonate of soda. It reacts with acidic ingredients, such as buttermilk, yogurt and molasses to create bubbles of carbon dioxide, which causes batter to rise. It starts to work immediately when exposed to the acidic ingredient, so a batter made with baking soda should generally be baked as soon after mixing as possible. Baking soda can also aid in browning during baking.

Baking powder is actually a combination of baking soda, cream of tartar and a bit of cornstarch. Cream of tartar is an acidic ingredient, so it prompts the baking soda to work even when no other acidic ingredients are present in a batter. The cornstarch absorbs moisture and prevents the baking powder from being activated too quickly, or before it makes it into the batter in the first place. It works by releasing CO2 bubbles, just as baking soda does. Many baking powders, and most of those used in the US, are "double acting," meaning that they work once when exposed to moisture and again when exposed to heat. This gives the impression that baking powder doughs and batters will rise higher than those made with baking soda alone, but this is not necessarily the case.

Cream of tartar is potassium hydrogen tartrate, which is an acidic salt. Grapes are the most common natural source of tartaric acid. During the wine production process, a sediment forms as a result of combining potassium hydroxide with the tartaric acid, leaving a white sediment: cream of tartar.

A combination of cream of tartar and baking soda was the original baking powder. Baking soda reacts almost instantly when exposed to wet/acidic ingredients, but the addition of cream of tartar tempers the effect of the baking soda and delays the rising reaction – which will produce a higher rise in the oven. Unlike baking powder, however, cream of tartar will not lose it potency over time, so you can buy a jar and keep it in your kitchen cupboard forever.