Ingredient

Buttermilk

Buttermilk is acidic, thin and tangy, and its acidity is the whole reason it exists in a recipe. It reacts with bicarbonate of soda to produce the lift in soda bread and scones, it tenderises, and it adds a sourness that plain milk cannot.

Two different things under one name

Traditional buttermilk is what is left in the churn after butter has been made. It is thin, low in fat, and slightly sour because the cream was usually left to ripen before churning. It is a by-product, and it is rare in shops now.

Cultured buttermilk is what you actually buy. It is low-fat milk deliberately soured with a bacterial culture, in the same way yoghurt is, until it thickens slightly and turns tangy. It is made on purpose rather than left over, and it is more consistently acidic than the traditional kind.

For cooking they are close enough to be interchangeable, and cultured buttermilk is generally the more reliable of the two because its acidity is controlled. If a recipe is old enough to assume the churn version, the modern one will work.

What the acid is doing

Two things, and both matter.

It reacts with bicarbonate of soda. Bicarbonate needs an acid to produce carbon dioxide, and buttermilk supplies it. That is the entire mechanism behind soda bread and behind buttermilk scones and pancakes: no yeast, no waiting, just an acid meeting an alkali and producing gas immediately. It is also why those doughs must go into the oven promptly — the reaction starts the moment the liquid meets the dry ingredients and does not wait.

It tenderises. Acid weakens gluten and helps break down protein, which is why buttermilk-marinated fried chicken is tender and why buttermilk scones are shorter and softer than milk ones.

It also contributes flavour — a rounded, lactic tang that reads as richness even though buttermilk is low in fat.

What to use it in

• Soda bread, where it is not optional. Without an acid there is nothing for the bicarbonate to react with.
• Scones and pancakes, for lift and for tenderness.
• Fried chicken marinade, where the acid tenderises and the residue helps the coating stick.
• Ranch dressing and buttermilk dressings, thinned with mayonnaise or soured cream.
• Cakes, particularly chocolate and red velvet, where the acid also helps the crumb stay soft and reacts with cocoa.
• Cornbread.
• Cold soups, in Eastern European and Scandinavian cooking, where its thinness is the point.
• Not boiled. It is low in fat and high in acid, so it curdles readily. Add it off the heat.

Substituting it

All of these supply acidity. None is quite the same, and it matters less than people fear.

Milk plus lemon juice or vinegar

A tablespoon of acid per cup of milk, left for ten minutes until it thickens slightly. The standard substitute and it works well for baking.

Yoghurt thinned with milk

Closer in body and flavour than the lemon method. Roughly three parts yoghurt to one of milk.

Kefir

Almost a direct swap — thin, acidic and cultured. Arguably the best substitute there is.

Soured cream thinned with milk

Richer, and fine where extra fat is not a problem.

Plant milk plus acid

Works for the bicarbonate reaction. Soya milk thickens most convincingly.

Plain milk alone

Does not work where bicarbonate of soda is the raising agent. There will be no lift.

Common questions