What is the difference between baking powder and bicarbonate of soda?

Both produce carbon dioxide to lift a bake, but by different routes. Understanding which one a recipe uses and why explains a surprising number of baking failures, including the metallic aftertaste people cannot place.

What you'll need

• Baking powder (optional)
• Bicarbonate of soda (optional)

Step by step

1. Understand bicarbonate of soda: it needs an acid.Buttermilk, yoghurt, lemon, vinegar, treacle, brown sugar, cocoa. Without one, it does not fully react and leaves a soapy metallic taste.
2. Understand baking powder: it contains its own acid.It is bicarbonate plus a dry acid plus a starch. It works in any recipe, with or without acidic ingredients.
3. Note the strength difference.Bicarbonate is roughly four times stronger. If substituting, use about a quarter as much bicarbonate as baking powder — and add an acid.
4. Use bicarbonate where browning matters.It raises the pH, which speeds browning. It is why cookies made with bicarbonate spread and brown more.
5. Bake promptly once liquid is added.Both start reacting on contact with liquid. Most baking powder is double-acting, giving a second reaction in the oven, but delay still costs lift.
6. Check they are still active.Bicarbonate should fizz vigorously in vinegar. Baking powder should fizz in hot water. Both lose potency, particularly once opened.
7. Measure them accurately.Level teaspoons, or weighed. A heaped teaspoon can be nearly double a level one, and excess raising agent tastes unpleasant and causes collapse.
8. Make your own baking powder if you run out.One part bicarbonate to two parts cream of tartar. It works and it is a useful thing to know.

Tips

• A soapy or metallic aftertaste almost always means too much bicarbonate, or bicarbonate without enough acid to react with.
• Both expire. Replace them every year or so — an old tub is a common cause of flat bakes with no obvious explanation.
• Self-raising flour already contains baking powder. Adding more on top is a common cause of over-risen, collapsed cakes.

Common mistakes

• Substituting bicarbonate for baking powder one-for-one — It is around four times stronger and needs an acid. The result is a bitter, soapy taste and often a collapse.
• Using bicarbonate in a recipe with no acid — It cannot fully react, so the unreacted alkali gives a metallic soapy flavour.
• Measuring with a heaped spoon — It can be nearly twice a level spoon. Excess raising agent over-inflates the bake, which then collapses.

Questions people ask