Ingredient

Bicarbonate of soda

Bicarbonate of soda is an alkali that releases carbon dioxide when it meets an acid. That is its entire job, and almost every question about it — why a cake tasted soapy, why soda bread did not rise, why a recipe calls for buttermilk — comes back to whether there was an acid in the bowl and whether the quantities matched.

The reaction, and why the acid is not optional

Bicarbonate of soda is sodium bicarbonate. Bring it into contact with an acid in the presence of moisture and it reacts, producing carbon dioxide gas. Those bubbles are caught in the batter, expand in the oven, and lift the bake.

Without an acid, very little happens at room temperature. Bicarbonate does decompose with heat alone, but slowly and incompletely, and — crucially — whatever does not react remains in the food as an alkaline salt. That is the soapy, metallic, faintly bitter taste of a bake with too much bicarbonate or not enough acid. It is unmistakable and it cannot be fixed afterwards.

It also affects colour. Alkaline conditions accelerate browning reactions, which is why a cookie made with bicarbonate spreads and browns more than one made with baking powder, and why bicarbonate turns cocoa darker and redder.

The reaction begins immediately. Once wet, the gas starts being produced, and a batter left standing loses much of its lift before it reaches the oven. Soda-raised mixtures go straight in.

Bicarbonate against baking powder

The commonest confusion in baking, and the difference is simple.

Bicarbonate of soda

An alkali alone. Needs an acid in the recipe. Reacts immediately on contact with liquid. Roughly three to four times as powerful as baking powder by weight.

Baking powder

Bicarbonate of soda with a powdered acid and a starch already mixed in. Self-contained — needs no acid in the recipe, only moisture and usually heat.

Double-acting baking powder

Contains two acids: one reacting on contact with liquid, one only on heating. Gives a second lift in the oven and is more forgiving of a batter that waits.

Can I swap them?

Not directly. Baking powder for bicarbonate needs roughly three to four times as much and will change the flavour and the browning. Bicarbonate for baking powder needs an acid added, and getting the balance right is guesswork.

Why some recipes use both

Where there is more acid in the recipe than the bicarbonate can neutralise, bicarbonate deals with the acid and adjusts the pH, and baking powder supplies the rest of the lift.

The acids that partner it

• Buttermilk — the classic, in soda bread, scones and pancakes.
• Yoghurt, soured cream, kefir — all work, and all bring fat as well.
• Lemon juice or vinegar — direct, cheap, and what a milk-plus-acid substitute relies on.
• Brown sugar, treacle, golden syrup and honey — all mildly acidic, which is why they turn up in gingerbread and flapjacks alongside bicarbonate.
• Natural cocoa powder — acidic, and it reacts. Dutch-processed cocoa has been alkalised and does not, which is why swapping one for the other breaks a recipe.
• Cream of tartar — a dry acid, and mixing it with bicarbonate is essentially how you make baking powder.
• Fruit purée — apple, banana, and most fruit.

Its other uses in cooking

Not all of them are about raising.

• Tenderising pulses. A pinch in the water speeds the softening of chickpeas and beans considerably, because alkaline conditions break down pectin. It is the trick behind very smooth hummus. Too much gives a soapy taste and a mushy result.
• Silkier onions, faster. A small pinch when caramelising onions speeds the browning noticeably, for the same reason.
• Crisper skin. A little rubbed into chicken skin or tossed with potatoes raises the surface pH, which speeds browning and gives a crisper result.
• Greener vegetables — but do not. Alkali preserves chlorophyll, so bicarbonate keeps greens bright. It also destroys their texture and some of their nutrients, and the result is vivid mush. This is a technique to know about and avoid.
• Skinning hazelnuts and almonds, where a bicarbonate blanch loosens the skins.
• Cleaning. Which is what most boxes are eventually used for.

Common questions