What Does Baking Soda Do in Baking?

Baking soda is sodium bicarbonate, an alkaline powder. When it comes into contact with an acid in a wet mixture, it reacts to produce carbon dioxide, water and a sodium salt.

The carbon dioxide forms bubbles that expand in the oven, lifting the batter before the structure sets. Unlike yeast, this is a chemical reaction rather than a living process, so it happens immediately and does not need hours of resting.

Without acid, baking soda does very little at room temperature and leaves a soapy metallic taste. Recipes using it therefore include an acidic ingredient:

• •Buttermilk, yogurt or sour cream
• •Lemon juice or vinegar
• •Brown sugar, molasses or honey
• •Cocoa powder that has not been Dutch processed

Baking powder is baking soda with a powdered acid already mixed in, plus a starch to keep them apart while dry. It only needs moisture and heat, so it works in recipes with no acidic ingredient.

Most modern baking powder is double acting: one acid reacts on contact with liquid, another only when heated. That gives some lift when mixed and a second lift in the oven, which is more forgiving of a batter sitting on the counter.

Rising is not all it does. Baking soda raises the pH of a dough, and alkaline conditions accelerate the Maillard reaction, so cookies made with it brown faster and spread more.

That is why a chewy dark cookie recipe often calls for baking soda while a pale cakey one calls for powder. A pinch also speeds up caramelising onions and helps dried beans soften, both for the same reason: alkalinity changes how the chemistry proceeds.

Too much baking soda is a common fault. Excess soda that finds no acid to react with leaves a bitter soapy flavour and can turn baked goods oddly dark or coarse.

A rough guideline is around a quarter teaspoon of baking soda per cup of flour, but the acid content of the recipe matters more than the flour quantity. If a recipe seems unbalanced, adding a little baking powder is usually safer than adding more soda.

Baking soda and baking powder both lose effectiveness over time, and stale leavening is a common cause of flat results. Both can be tested in seconds.

For baking soda, add a spoonful of vinegar or lemon juice to a small amount and look for immediate vigorous fizzing. For baking powder, add hot water instead, since it carries its own acid and only needs moisture. In both cases a weak or slow reaction means it should be replaced. Store both sealed and dry, since humidity gradually triggers the reaction in the container.

Baking soda can release some gas even without an acid, but only when heated. In a hot oven, sodium bicarbonate breaks down into sodium carbonate, water and carbon dioxide. That gives a little lift, but it leaves behind sodium carbonate, a stronger alkali than the soda you started with. Sodium carbonate is the same chemical sold as washing soda, and it is responsible for the soapy, slightly chemical taste and yellowish crumb of baked goods made with too much baking soda and not enough acid to use it up.

Soda bread is a good example of the chemistry working well. Traditional Irish soda bread uses flour, salt, baking soda and buttermilk, with the acid in buttermilk reacting with the soda to produce carbon dioxide as soon as they are mixed. Because the gas starts forming straight away, the dough should go into the oven quickly. It also needs little kneading, since it does not rely on a strong gluten network to trap gas slowly over hours, as yeast bread does. The result is a dense, tender loaf made in under an hour.

Bakers used chemical leaveners before sodium bicarbonate became widely available in the nineteenth century. One was pearl ash, a refined form of potassium carbonate made from wood ashes. It reacted with acids such as sour milk to release carbon dioxide, and American Cookery, published in 1796 and often called the first American cookbook, includes recipes using it. Pearl ash was harsh and could leave a bitter taste, which is why cooks welcomed bicarbonate of soda as a milder replacement. Baking powder followed, and the English chemist Alfred Bird is often credited with an early version in the 1840s.

Another old leavener, hartshorn, is still in use. Originally made from ground deer antler, it is now sold as baker's ammonia, a form of ammonium carbonate. When heated it breaks down completely into gases: carbon dioxide, ammonia and water vapour, leaving no salty residue. That gives an unusually crisp, light texture, but ammonia smells strongly while baking. It only works well in thin, dry biscuits, where the gas can escape completely before the biscuits cool. Some traditional Scandinavian and German biscuits are still made with it.

A little arithmetic shows why recipes need so little baking soda. A level teaspoon weighs roughly 5 grams. Sodium bicarbonate has a molar mass of 84 grams per mole, so a teaspoon is about 0.06 moles. When fully reacted with acid, each mole releases one mole of carbon dioxide, and at room temperature a mole of gas fills about 24 litres. One teaspoon can therefore produce around 1.4 litres of gas, and the oven's heat expands those bubbles further. That is enough to lift a whole tin of batter, which is why an extra half teaspoon can make a cake rise dramatically and then sink.

The same chemistry explains common substitutions. Baking powder contains only part baking soda, with the rest made up of acid and starch, so you need roughly three times as much powder to replace a given amount of soda, and the result will taste tangier because the recipe's acidic ingredients are no longer neutralised. Going the other way, you can make a simple single-acting baking powder at home by mixing one part baking soda with two parts cream of tartar, an acidic powder that is a by-product of winemaking. Because it starts reacting as soon as it gets wet, batter made with it should be baked straight away.

Baking soda's effect on pH can show up as colour as well as browning. Blueberries, red cabbage and many other red, purple and blue foods contain pigments called anthocyanins, which change colour with acidity. They look red in acid, purple near neutral and blue or greenish in alkaline conditions. A blueberry muffin made with too much soda, or with soda that has not fully met its acid, can develop odd green or blue-grey streaks around the berries. It is harmless, but it is a visible sign that the batter ended up more alkaline than the recipe intended.

Cocoa shows the same chemistry from the other side. Natural cocoa powder is mildly acidic, so it can react with baking soda and help a cake rise. Dutch processed cocoa has been treated with an alkali during manufacture, which darkens its colour, mellows its flavour and removes most of its acidity. That is why recipes built around Dutch processed cocoa usually rely on baking powder, which brings its own acid. Swapping one cocoa for the other without adjusting the leavening can leave a cake flat, or soapy and unusually dark. The chemical difference between the two powders matters as much as the difference in taste.

Baking soda produces carbon dioxide when it meets an acid, giving fast chemical rise, and it also raises pH so baked goods brown more, while baking powder is simply baking soda with its own acid built in.