Cocoa, baking soda and why your cake was flat

Baking soda is a base and needs an acid to react and produce carbon dioxide. Natural cocoa, at pH 5.3–5.8, supplies that acid. Dutch-process cocoa has been alkalised to pH 6.8–8.1 and supplies none.

Baking powder contains its own acid and works with either. That single fact resolves most cocoa substitution questions.

Using Dutch-process where the recipe wanted natural plus baking soda: replace the soda with roughly three times as much baking powder, or add an acid — buttermilk, yoghurt, a spoonful of vinegar — to give the soda something to react with.

Using natural where the recipe wanted Dutch-process plus baking powder: it will generally work, with a slightly sharper flavour and a lighter colour. This is the more forgiving direction.

The two also differ in flavour and colour. Natural cocoa is fruitier and sharper with a reddish tone; Dutch-process is smoother and darker. Recipes that describe a deep dark crumb usually assume Dutch-process, and using natural will give a lighter, redder result even if the leavening works.

Devil's food cake is the classic case where the reddish tone of natural cocoa reacting with soda is part of the intended appearance.

Labels are inconsistent. Look for alkalised, processed with alkali, or Dutch-process on the ingredients. If nothing is stated, US-market cocoa is more often natural and European-market cocoa is more often alkalised.

The fallback test: a spoonful in a little water with a pinch of baking soda. Natural cocoa fizzes; Dutch-process does not.