Buttermilk
An acidic cultured milk whose value in baking is its acid — which is why the common substitutions work and why they are not identical.
Also called cultured buttermilk (US); soured milk (UK)
Traditional buttermilk was the liquid left after churning butter. What is sold as buttermilk now is generally cultured low-fat milk, deliberately soured by bacteria — and the culturing is the point.
In baking it does three things. Its acid reacts with bicarbonate of soda to produce carbon dioxide, which is why so many quick breads, scones and soda breads specify it. That same acid tenderises, by interfering with gluten development and by acting on the proteins present. And it is thinner and tangier than whole milk, which changes both the batter consistency and the flavour.
The usual substitution — milk with lemon juice or vinegar stirred in — replaces the acid and therefore the leavening reaction. It does not replace the cultured flavour or the slightly thickened body, so it is a good substitution for the chemistry and an approximate one for the result.
What changes if it is left out
Where bicarbonate of soda is the leavener, removing buttermilk removes the acid that makes it work: the bake rises less and can taste soapy from unreacted bicarbonate. Where baking powder is used instead, the loss is mostly flavour and tenderness.
Follows from the acid-base leavening model. The conditional is the important part — the consequence depends entirely on which leavener the formula uses.
RecordedFollows from evidence recorded elsewhere on BakeHQ
Why the lemon-juice substitution works better than it should
Adding an acid to milk and leaving it briefly does two things: it lowers the pH, which is the part that matters for leavening, and it curdles the milk slightly, which crudely imitates buttermilk’s body.
It is a genuinely good substitution for the chemistry and a rough one for everything else. What it does not reproduce is the cultured flavour, which is the reason buttermilk is specified in things like soda bread where the taste is part of the point.
Related
What it does
pH modification
THE reason it is specified. Its acid reacts with bicarbonate of soda, which is why soda bread, scones and many quick breads call for it.
Gluten inhibition
The same acid tenderises, interfering with gluten development and acting on the proteins present.
Hydration
Cultured buttermilk is a low-fat milk and is the liquid of a soda bread or a scone rather than an addition to one. As with milk, the capability is certain and the sufficiency depends on how much is used.
Can stand in for
Only where the recipe wants acid. Substituting water for buttermilk removes the acid a bicarbonate leavener may depend on, which is a leavening change rather than a liquid one.
Can be replaced by
Natural yoghurt— for the acid that activates bicarbonate of soda, thinning it if the batter needs it
The closest available swap, because the acid is the part that does the chemical work and both carry it. Yoghurt is thicker, so a batter built for buttermilk may need loosening — and a strained yoghurt needs it more than an unstrained one.
Compared in
One side of the comparison, so the comparison is reachable from the record a reader is more likely to land on first.
Used in
The traditional acid, and the liquid phase.
The acid the bicarbonate reacts with, and the tenderiser.