Definition: a slightly acidic cultured milk that tenderises crumb and reacts with bicarbonate of soda to help a bake rise.
Buttermilk earns its place two ways: its acidity tenderises the crumb for a softer, moister bake, and it activates bicarbonate of soda for lift. It’s classic in red velvet, soda bread and buttermilk scones. No buttermilk in the fridge? Make a stand-in: stir a tablespoon of lemon juice or white vinegar into 240ml of milk and leave it for five minutes to thicken and curdle slightly — it does the same job.
In practice: the quick lemon-milk swap works well; give it a few minutes to clabber before using.
Related:bicarbonate of soda,crumb.