Ingredient

Yoghurt

Yoghurt does two jobs that pull against each other: it adds acidity and richness, and it splits the moment you heat it carelessly. Knowing why turns it from an unpredictable ingredient into a reliable one.

Why it splits

Yoghurt is milk whose protein has been partly coagulated by acid produced by bacteria. The proteins are already at the edge of coming out of suspension — that is what makes yoghurt thick — and two things push them over: heat, which makes them clump together, and more acid, which does the same.

So yoghurt tipped into a boiling curry curdles into grains in a watery liquid. It has not gone off and it is perfectly edible; it just looks and feels wrong. Once it has happened, whisking will not fully undo it.

How to stop it

• Use full-fat. Fat physically gets between the proteins and slows clumping. Low-fat and fat-free yoghurt split far more readily.
• Temper it. Stir a few spoonfuls of the hot liquid into the yoghurt first, then return the loosened yoghurt to the pan. Never the other way round.
• Take the pan off the heat, or turn it right down, before the yoghurt goes in. It should never boil afterwards.
• Stabilise it with starch. A teaspoon of cornflour whisked into the yoghurt before it goes in is what many traditional recipes do, and it works reliably — this is standard practice in Turkish and Greek cooking.
• Or use Greek or strained yoghurt, which has less whey and more solids and is markedly more stable.
• Stir gently and in one direction once it is in, rather than beating it.

The kinds

Natural or plain

Unstrained, pourable, tangy. General purpose, and the most likely to split.

Greek or Greek-style

Strained to remove whey, so thicker and richer. "Greek-style" is often thickened rather than strained — check the ingredients.

Skyr and quark

Very high in protein, very thick, low in fat. Excellent cold, poor when heated.

Labneh

Yoghurt strained until it is spreadable. A cheese in all but name.

Coconut and other non-dairy

Behave quite differently. Most contain added starches and gums and are actually more heat-stable than dairy yoghurt.

Using it well

• As a marinade for meat, where the acidity and the calcium both help tenderise, and the coating protects the surface from fierce heat. This is why tandoori works.
• As a cooling counterweight to chilli and spice — raita, tzatziki, a spoonful on a curry. The fat carries capsaicin away.
• Stirred in off the heat at the end of a curry, soup or stew, for richness and a sour lift.
• In baking, where its acidity reacts with bicarbonate of soda for lift and its water content keeps cakes moist.
• Strained overnight in a cloth to make labneh, which is a very cheap luxury.
• Do not freeze it and expect it to survive. It separates on thawing and is only useful in cooking afterwards.

Common questions