Baking powder and bicarbonate of soda are not interchangeable

What follows is the working version of chemical raising agents in cakes: the decisions in the order you actually meet them, with the reasoning attached.

The chemistry in one paragraph

Bicarbonate of soda is sodium bicarbonate, an alkali that releases carbon dioxide when it meets an acid in the presence of moisture. Baking powder is bicarbonate already blended with one or more dry acids and a starch to keep them apart in the tin.

So baking powder works in any batter, while bicarbonate only works where the recipe supplies acid. Buttermilk, yoghurt, soured cream, brown sugar, honey, cocoa and lemon juice are the usual acid sources. Without one of those, bicarbonate sits there mostly unreacted and makes the cake taste of itself.

Why recipes use both

A recipe with buttermilk may include bicarbonate to neutralise that acidity, which affects flavour, colour and crumb. If the acid is more than the bicarbonate can consume, baking powder is added to supply the remaining lift independently.

The bicarbonate is therefore doing pH work as much as leavening work, which is why you cannot simply drop it. Raising the pH also encourages browning, which is why soda breads and some chocolate cakes go so dark. Read a dual recipe as two decisions rather than as redundancy.

Single and double acting

Most modern baking powders are double acting, meaning one acid reacts on contact with liquid and another only on heating. That second stage is what lets a batter sit for a few minutes without losing all its lift. Single acting powder, and bicarbonate with a fast acid such as lemon juice, react immediately and must go into the oven at once.

This is why some recipes insist on having the tin greased and the oven hot before mixing begins. If a cake rises well at the edges and poorly in the middle, a delay between mixing and baking is worth investigating.

Quantities and their limits

A common rule of thumb is around one teaspoon of baking powder per 125g of flour, adjusted for how much other lift the recipe has. Bicarbonate is roughly three to four times as powerful by weight, so it is used in much smaller amounts.

Weighed out, more raising agent does not mean more rise past a point, because the bubbles grow so large they merge and collapse. An overdosed cake rises dramatically then sinks, with a coarse, tunnelled crumb and a bitter aftertaste.

If a cake is not rising enough, the fault is usually aeration or oven temperature rather than insufficient powder.

Taste consequences

Unreacted bicarbonate tastes soapy and slightly metallic, and it is noticeable at surprisingly small excesses. It also raises the pH of the crumb, which can turn cocoa a deeper red-brown and give a slightly slippery mouthfeel. Baking powder in excess tends to taste sharp or chemical instead, from the acid salts left behind.

Both faults are more obvious the day after baking, once the crumb has cooled and set. Measure raising agents on scales if you can, because a heaped teaspoon can be half again a level one.

Freshness and storage

Baking powder loses potency over months, particularly once opened in a humid kitchen, and an old tin is a common hidden cause of flat cakes. Test it by stirring half a teaspoon into hot water, where it should fizz vigorously and immediately.

The dough tells you first: test bicarbonate with vinegar or lemon juice instead, since water alone will not react with it. Keep both sealed and dry, and write the opening date on the tin. Replacing a suspect tin costs very little compared with another failed cake.

The takeaway

Bicarbonate needs an acid, powder brings its own, and neither improves a cake you forgot to aerate.

Patience is an ingredient, and it is the one people leave out.