Tempering egg yolks

Why

Egg yolk proteins start to thicken at roughly 65 °C and set at around 70 °C; diluted with liquid and protected by starch or sugar they can take more – a thick custard with flour or starch can even take a brief boil – but in a soup, boiling is almost always too much for the yolk. Pour a yolk straight into boiling soup and its edges cook before it can disperse, leaving the pot full of flecks of scrambled egg. To enrich a soup with yolk, you therefore first mix the yolk with cream or lukewarm liquid and add the hot soup to it a spoonful at a time – the yolk is diluted gradually and warms evenly. Only then does this mixture go into the pot, which is no longer boiling, to be heated through while stirring constantly; boiling it again would tighten the proteins and make the soup or sauce grainy.

How to do it

• Mix the yolks in a bowl with cream or a little lukewarm liquid.
• Take the pot off the heat and let the soup or sauce stop bubbling.
• Stirring constantly, add the hot liquid to the yolks a spoonful at a time, at least one ladleful in total.
• Only then pour the mixture into the pot and stir straight away.
• From now on just heat it through over low heat and don't let it boil.
• When reheating soup enriched with yolk, never bring it back to the boil.

If it goes wrong

Strain out fine flecks through a sieve or blitz them with a stick blender. A badly curdled soup won't become smooth again, but it is perfectly edible – next time temper the yolk and don't boil the pot once it is in.