Ingredient

Egg yolk

The yolk is where the fat, the emulsifying power and most of the flavour of an egg are. It thickens without flour, holds oil and water together, and enriches anything it goes into — three quite different jobs from the same ingredient.

Three jobs, one ingredient

A yolk is roughly half water, a third fat and a sixth protein, plus a significant quantity of lecithin — a molecule with a water-loving end and a fat-loving end.

The protein is what thickens. Heated gently, yolk proteins partially unfold and link, holding the surrounding liquid and turning it from thin to creamy. That is a custard, a crème anglaise, and the sauce in a carbonara.

The lecithin is what emulsifies. It sits at the boundary between oil droplets and water and stops them merging, which is the whole basis of mayonnaise, hollandaise and aioli.

The fat is what enriches — it carries flavour, coats the tongue, and gives pastry, ice cream and enriched dough their character.

Where a yolk is doing what

Custard and crème anglaise

Thickening. Gentle heat, constant stirring, and it curdles a few degrees past where it thickens.

Pastry cream

Thickening, stabilised with cornflour so it can be boiled without curdling.

Mayonnaise and aioli

Emulsifying, cold. One yolk will hold a surprising volume of oil if added slowly enough.

Hollandaise and béarnaise

Emulsifying and thickening at once, over gentle heat — which is why they are the most fragile sauces.

Ice cream

Thickening and enriching. A custard base gives a smoother, less icy result than a cream-only one.

Enriched doughs — brioche, challah

Fat and colour. The yolks are why brioche is yellow and tender.

Carbonara

Thickening, with pasta water and cheese, off the heat. It curdles the instant the pan is too hot.

The habits that stop it curdling

• Temper. Pour a little hot liquid into the beaten yolks while whisking, then return the mixture to the pan. Never the other way round.
• Whisk yolks with sugar first. Sugar physically gets between the proteins and raises the temperature at which they set — real insurance, not a flourish.
• Low heat and constant stirring, with a spatula that reaches the corners of the pan.
• Take it off before it looks thick enough and pour it straight into a cold bowl.
• A teaspoon of cornflour makes a custard far more forgiving, at a small cost in silkiness.
• Strain it anyway. It takes ten seconds and catches anything that started to set.

Using up spare yolks

The recurring problem of anyone who makes meringue.

• Custard, crème brûlée, crème anglaise, ice cream base.
• Mayonnaise, aioli, hollandaise, Caesar dressing.
• Carbonara and other pasta sauces thickened off the heat.
• Lemon curd, pastry cream, sabayon.
• An egg wash for pastry — a yolk with a splash of milk gives the deepest colour.
• Enriching mashed potato, risotto or a soup, stirred in off the heat.
• They keep a couple of days in the fridge covered with a little water or cling film pressed onto the surface, and they freeze with a pinch of salt or sugar to stop them going gluey.

What the yolk actually does

• Emulsifies. Lecithin in the yolk is what holds mayonnaise and hollandaise together. This is its most useful property.
• Thickens. Its proteins set gently at a low temperature, which is what makes custard, crème anglaise and pastry cream possible.
• Enriches. Fat and colour, in a cake, a pasta dough, a brioche or an ice cream base.
• Tenderises. Extra yolks in a biscuit or a cake give a softer, richer, denser crumb — which is why swapping a whole egg for two yolks is such a useful adjustment.
• Glazes. An egg wash of yolk with a little water or milk gives the deepest colour and shine on pastry.
• Keeping them. Yolks do not freeze well on their own; a pinch of salt or sugar whisked in first stops them going gluey.

Common questions