Milk vs buttermilk
Both supply the liquid. Only one supplies the acid, and where the recipe uses bicarbonate of soda that is the whole difference.
The short answer
Swap them freely where the leavening is baking powder — you lose some tang and tenderness and nothing else. Where the leavening is bicarbonate of soda alone, they are NOT interchangeable: bicarbonate needs an acid to react with, and plain milk does not supply one. A bake made that way rises poorly and can taste soapy from unreacted bicarbonate. Adding a spoonful of lemon juice or vinegar to milk restores the chemistry, which is why that substitution works better than it has any right to; what it does not restore is the cultured flavour or the body.
Where they differ
| Dimension | First | Second |
|---|---|---|
| Acidity | Near neutral. | Distinctly acidic — this is the reason it is specified. |
| Reaction with bicarbonate of soda | None to speak of. Bicarbonate needs another acid source. | Reacts, generating the carbon dioxide the bake is relying on. |
| Effect on gluten | Little. | Acid interferes with gluten development, giving a more tender crumb. |
| Flavour | Mild and slightly sweet from lactose. | Tangy; part of the point in soda bread and scones. |
Differences that do not matter
These are real distinctions that are unlikely to change your decision. They are listed so you can stop thinking about them rather than left out so the comparison looks decisive.
- Liquid contribution. Roughly seven-eighths water; a normal liquid phase. A low-fat cultured milk; comparable liquid, slightly thicker.
Related
Compares
One side of the comparison, so the comparison is reachable from the record a reader is more likely to land on first.
One side of the comparison, so the comparison is reachable from the record a reader is more likely to land on first.