How to Fold Egg Whites

To fold egg whites into a batter, first stir in about a quarter of the whites to loosen the batter, then add the rest and gently cut down through the middle with a spatula, sweep along the bottom and lift up the side, turning the bowl as you go. Stop as soon as only a few streaks remain. Folding mixes the two without popping the air bubbles that make soufflés, sponge cakes and mousses light.

Why folding protects the air

Whipped egg whites are a foam: billions of tiny air bubbles, each wrapped in a thin film of unfolded egg proteins. The film is strong enough to hold its shape but easily torn. Stirring, beating or pressing the foam against the bowl pops those films and lets the air escape.

Folding moves batter and foam past each other in big, slow strokes. Instead of shearing every bubble, it layers the foam into the batter and lets the two blend with as little friction as possible. Every bubble that survives expands in the oven and lifts the cake.

Whip the whites correctly first

Folding goes much better when the whites are right. Aim for glossy peaks that stand up but curl slightly at the tip.

Overwhipped whites look dry, grainy and cottony. Their proteins have bonded too tightly, so they break into clumps that won't blend into the batter. You end up folding much longer, and that's what deflates it. A pinch of cream of tartar (about 1/8 teaspoon per white) or a little sugar makes the foam more stable and harder to overwhip.

Fat is the other enemy. Even a trace of yolk or grease in the bowl weakens the foam, so use a clean glass or metal bowl.

Lighter into heavier

Add the whites to the batter, not the batter to the whites. A dense base such as chocolate, egg yolk and sugar, or a thick cheese mixture, would crush the foam if poured on top.

The first quarter of the whites is sacrificed on purpose. You stir it in briskly to loosen the base so it's closer in texture to the foam. After that, the rest folds in quickly with far less damage.

For very thin batters, such as a crêpe-like soufflé base, you can reverse it and pour the batter down the side of the bowl into the whites.

Steps

1. Whip the egg whites to soft-to-firm glossy peaks. Do this just before folding; whipped whites start to weep and lose volume within about 5 minutes.
2. Stir about a quarter of the whites into the batter to lighten it. Mix briskly with a spatula or whisk until fully combined. Don't worry about deflating this portion.
3. Add half of the remaining whites on top. Scrape them onto the batter in a single mound.
4. Cut the spatula straight down through the centre to the bottom. Hold it like a knife with the edge leading.
5. Sweep along the bottom and lift up the side of the bowl. Turn your wrist as you rise so the spatula flips the batter over the top of the whites.
6. Give the bowl a quarter turn and repeat. Expect about 10 to 15 strokes per addition. Work quickly but gently.
7. Add the last of the whites and fold until only a few streaks remain. A few small white streaks are fine; large clumps need another stroke or two.
8. Pour into the pan and bake right away. Bubbles start to rise and escape the moment you stop.

Common mistakes and fixes

• Batter is flat and runny. Too much folding, or the whites were stirred in. Use fewer, larger strokes and stop at faint streaks.
• Lumps of white in the baked cake. The whites were overwhipped and clumped, or the folding stopped too early. Whip only to glossy peaks.
• Heavy, dense base at the bottom. The spatula didn't reach the bottom of the bowl. Scrape all the way down on every stroke.
• Whites won't whip at all. Yolk, grease or a plastic bowl. Start again with a clean glass or metal bowl.
• Mousse turned grainy. Warm chocolate seized when cold whites hit it. Let the chocolate base cool to about 35 °C / 95 °F before folding.

Folding other foams

The same method works for whipped cream in mousses, aquafaba meringue in vegan bakes, and flour folded into a whipped-egg sponge. For flour, sift it over the foam in three additions so it doesn't land in heavy clumps.