What Does Baking Soda Do in Baking?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Baking soda produces carbon dioxide gas, which makes cakes and cookies rise without yeast. But it only works when it meets an acid, and that condition explains most of the confusion between baking soda and baking powder.
What it does chemically
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.
Why it needs an acid
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 soda versus baking powder
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.
Its other jobs
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.
Getting the amount right
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.
How to test if yours is still good
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.
The takeaway
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.