Why does my cake taste of bicarbonate?

That distinctive soapy, slightly bitter aftertaste is unreacted bicarbonate of soda. It is a chemistry problem with a straightforward cause and it cannot be masked once baked.

Step by step

1. Understand what bicarb needs.It is alkaline and only produces gas when it meets an acid and liquid. Buttermilk, yoghurt, soured cream, lemon juice, vinegar, treacle, golden syrup, brown sugar and natural cocoa all supply that acid.
2. Know the ratio.Roughly a quarter teaspoon of bicarb per 125g of flour, and it needs about 120ml of an acidic liquid to react fully. More bicarb than the acid can neutralise is what you taste.
3. Do not swap it for baking powder by volume.Bicarb is about three times stronger. Using bicarb where a recipe says baking powder, spoon for spoon, guarantees the soapy taste. Baking powder already contains its own acid.
4. Check your cocoa.Natural cocoa is acidic and works with bicarb. Dutch-processed cocoa has been alkalised and does not, so a recipe written for one and made with the other leaves bicarb unreacted.
5. Measure properly.Level teaspoons, not heaped. A heaped teaspoon can be nearly double, and at these quantities that is the difference between fine and inedible.
6. Sift it in.Bicarb clumps. An unbroken lump leaves a concentrated bitter pocket and often a greenish or brownish spot. Sift it with the flour rather than adding it separately.
7. Mix it evenly.Distribute it through the dry ingredients before any liquid goes in. Uneven distribution gives you a cake that tastes fine in some mouthfuls and soapy in others.
8. Get it into the oven promptly.Bicarb reacts as soon as it meets acid and liquid. A batter left standing has already used its lift, and the taste problem is not improved by waiting either.
9. Watch for the visual signs.Too much bicarb darkens a cake, coarsens the crumb, and can give a slightly green tinge to anything with cocoa or certain berries. If it looks wrong it usually tastes wrong.
10. Check the recipe is balanced.Recipes converted between countries or scaled up are a common source. If a recipe has bicarb but no obvious acid, it is probably wrong — or assumes an acidic ingredient you substituted out.
11. Substitute correctly if you must.To replace bicarb with baking powder, use about three times as much and expect a slightly different texture. To go the other way, you need bicarb plus an acid, which is harder to get right.
12. Accept it cannot be rescued.The taste is a chemical residue through the whole crumb. Strong icing masks it briefly and it comes back in the aftertaste. Better to make another.

Tips

• Bicarb is three times stronger than baking powder. Almost every soapy cake is that swap made by volume.
• If a recipe has bicarb, find the acid. No buttermilk, yoghurt, lemon, treacle or natural cocoa means something is missing.
• Level the spoon. A heaped teaspoon of bicarb is nearly two.

Common mistakes

• Substituting bicarb for baking powder spoon for spoon — Bicarb is roughly three times as strong and has no built-in acid, so most of it is left unreacted and tastes soapy and metallic.
• Using Dutch-processed cocoa in a recipe written for natural cocoa — Dutch cocoa is alkalised, so it does not supply the acid the bicarb needs to react with.

If it doesn't work