Whipping egg whites

Why

As you whip egg whites, their proteins unfold at the surface of the air bubbles and link up into a flexible network that holds the foam together. Fat gets there first – even a trace of yolk or a greasy film on the bowl – and stops that network from forming; plastic bowls are hard to get completely grease-free, so glass or metal is the safer choice. Sugar makes the foam firmer and glossier, but added right at the start it slows the whipping down, while a little acid (lemon juice, cream of tartar) keeps the proteins from bonding too tightly. Overwhip and the network contracts so much that it squeezes out water: the foam turns grainy, dull and starts to weep. A pinch of salt does not stiffen the whites, whatever the old advice says; if anything, it weakens them slightly.

How to do it

• Use a glass or metal bowl and whisk, both clean and dry; plastic tends to hold on to grease.
• Crack each egg over a cup first, then tip the white into the bowl, so one broken yolk can't ruin the whole batch.
• Let the whites come to room temperature; thinner whites from older eggs whip faster, fresher ones give a more stable foam.
• Add the sugar a spoonful at a time, and only once the whites have formed a soft foam.
• For a more stable foam, add a few drops of lemon juice or a pinch of cream of tartar.
• Stop as soon as the whites are glossy and hold a peak; dull and grainy means overwhipped.

If it goes wrong

If yolk gets into the whites, scoop it out before you start with a clean spoon or a piece of shell; if they still refuse to foam, turn them into an omelette and start again. Overwhipped plain whites, with nothing else added yet, can usually be revived with one more fresh white: add it and whip briefly until the mixture comes back together.