Baking powder vs. baking soda

Why

Baking soda (bicarbonate of soda in the UK) is sodium hydrogen carbonate; it releases the carbon dioxide that lifts the batter mainly together with an acid the batter itself has to supply – buttermilk, yoghurt, sour cream, lemon juice, vinegar, honey, brown sugar or natural, non-alkalised cocoa; without acid it breaks down only with heat, releases only half of its gas and leaves a bitter aftertaste. Baking powder is a ready-made mix of soda, an acidic component and starch; Czech baking powder (prášek do pečiva) usually contains diphosphates, and one 12 g sachet is meant for 500 g of flour. Czech baking powders with diphosphate react only slowly in the cold: they release a smaller part of the gas during mixing and most of it in the heat of the oven, so the batter can wait briefly, but not all afternoon. If there is more soda than the acid in the batter can neutralise, alkaline sodium carbonate is left over, the bake tastes bitter or even soapy and the crumb turns yellowish. Both leaveners weaken over time from moisture, and more leavener doesn't mean a lighter cake: the bubbles grow, merge and the batter collapses.

How to do it

• Use baking soda mainly in batters with an acidic ingredient; without one, only in the small amount a recipe deliberately calls for, otherwise reach for baking powder.
• Sift the leavener with the flour so it spreads evenly and leaves no bitter lumps in the batter.
• Measure doses exactly; soda is much stronger, and when substituting, roughly three times as much baking powder is usually used instead of soda.
• Don't leave batter with leavener standing for long; put it straight into a preheated oven.
• Check freshness: a little baking powder should fizz clearly in hot water, and soda should foam vigorously with vinegar at once.
• Store leaveners tightly closed in a dry place and use up opened packs soon.

If it goes wrong

A baked cake can't be fixed. A soapy taste means too much soda or too little acid; sour cream, yoghurt or tart fruit served alongside softens it, and next time weigh the soda exactly or replace part of it with baking powder. A batter that didn't rise most likely had old leavener or waited too long before going into the oven.