Buttermilk's acidity is essential in soda bread — it reacts with bicarbonate of soda to produce carbon dioxide, giving the bread its lift without any yeast. If you don't have buttermilk to hand, you can make a reliable substitute in five minutes: stir one tablespoon of lemon juice or white wine vinegar into 280ml of whole milk and leave it to stand. The milk will curdle slightly and thicken, mimicking buttermilk's tangy acidity and creamy body. Skimmed milk produces a thinner result and a less reliable rise, so whole milk is strongly preferred. Use the substitute immediately once it has rested — the acid reaction with the bicarbonate of soda begins as soon as the two meet, so speed matters once mixing starts.