Ingredient

Baking powder

Baking powder is bicarbonate of soda with its acid already supplied, packaged together with a starch to keep them apart until you add liquid. It is the raising agent for any bake that has no acid of its own, which is most cakes, and its main failure mode is simply being too old.

What is in the tin

Three things. Bicarbonate of soda, the alkali that produces the gas. One or more powdered acids — cream of tartar, monocalcium phosphate, sodium acid pyrophosphate — to react with it. And a starch, usually cornflour or rice flour, which absorbs moisture and keeps the other two from reacting in the tin.

That starch is why baking powder eventually stops working: it cannot exclude moisture indefinitely, and a tin left open in a humid kitchen slowly reacts with itself until there is nothing left to give.

Single-acting baking powder reacts as soon as it is wet. Double-acting, which is most of what is sold, contains two acids — one that reacts with liquid and one that only reacts when heated. That second stage gives a further lift in the oven and makes the batter more forgiving if it has to wait.

Testing whether yours still works

Worth doing before you blame a recipe.

1. Put a teaspoon of baking powder in a cup.
2. Pour on a few tablespoons of hot water.
3. It should fizz vigorously and immediately.
4. A weak fizz, or a few bubbles, means it has lost most of its power. Replace it.
5. For bicarbonate of soda, use vinegar rather than water — it needs an acid.
6. As a rule of thumb, an opened tin is worth replacing after six months to a year. Write the opening date on it.

Getting the most from it

• Sift it with the flour, or at least whisk it through thoroughly. Pockets of undistributed baking powder produce uneven rise and bitter patches.
• Measure it properly. It is powerful, and the difference between a level and a heaping teaspoon is substantial. Too much gives a cake that rises fast, collapses, and tastes faintly bitter.
• Do not overmix once it is wet. Beyond losing the gas already produced, over-developing gluten in a cake batter works against the lift.
• Get it into the oven reasonably promptly, even with double-acting powder.
• Preheat properly. The second stage of a double-acting powder needs heat, and a batter going into a cool oven has a slower, weaker rise.
• Do not open the oven early. The structure has not set, and the collapse is real.

Substituting

Making your own

Roughly two parts cream of tartar to one part bicarbonate of soda, with one part cornflour if you are storing it. Single-acting, so use it immediately.

Bicarbonate plus an acid

Bicarbonate of soda with buttermilk, yoghurt, lemon juice or vinegar in the recipe. Requires adjusting the liquid, and getting the balance right is not trivial.

Self-raising flour

Plain flour with baking powder already in it, at a fixed proportion. Convenient, and it goes stale the same way — an old bag of self-raising flour raises poorly.

Whipped egg white

A mechanical rather than chemical route, and a completely different technique. It is what a genoise or a soufflé relies on.

Gluten-free note

Some baking powders use wheat starch. Check the tin if it matters.

Common questions