How Do Egg Whites Turn Into Meringue?

Egg whites turn into meringue because whisking unfolds their proteins, which then link up into a stretchy film around millions of tiny air bubbles. Sugar dissolves into that film and makes it thick and glossy, so the foam holds its shape. Even a speck of fat gets in the way of that film forming, which is why a trace of yolk can stop whites whipping at all.

What happens when you whip an egg white?

The proteins doing the work

An egg white is about 90% water and 10% protein, with almost no fat. That little bit of protein is a team of different molecules, and several have a job in a foam:

• Ovalbumin, more than half the protein, gives the foam strength once it's baked.
• Ovotransferrin and the globulins are good at rushing to the surface of bubbles and forming a film quickly.
• Ovomucin, a long, stringy protein, makes the white thick and helps stop bubbles draining and collapsing.
• Lysozyme sticks to other proteins and helps the network hold.

Water alone can't hold air for more than a second. Its surface tension pulls bubbles apart. Proteins can, because they have both water-loving and water-avoiding parts. That makes them natural foam builders.

What the whisk does

Each stroke of the whisk does two things: it drags air into the liquid and it stretches and batters the proteins.

Proteins in a raw white are folded into compact shapes, with their water-avoiding parts tucked inside. When a protein lands on the surface of an air bubble, it partly unfolds so those water-avoiding parts can point into the air and the water-loving parts can stay in the liquid. This is surface protein denaturation, caused by the air-water boundary rather than by heat.

Once unfolded, neighbouring proteins grab onto each other, forming a flexible, elastic skin around every bubble. Keep whisking and the bubbles get smaller and more numerous, and the skins get stronger. You can watch it happen in stages:

Why does a speck of yolk ruin egg whites?

Fats and oils are even better than proteins at sitting on a bubble's surface. A drop of yolk, which is about a third fat, or a film of grease on a plastic bowl, races to the bubbles and takes up the spots the proteins need. Proteins can no longer link up into a continuous skin, so the bubbles pop and the foam won't build.

It doesn't take much. A tiny streak of yolk can cut the volume noticeably. Practical fixes:

• Separate eggs one at a time into a small cup before adding each white to the main bowl, so one broken yolk doesn't spoil the batch.
• Use glass, metal or ceramic bowls. Plastic is slightly porous and holds onto grease, even after washing.
• Wipe the bowl and whisk with a paper towel dampened with a little vinegar or lemon juice to cut any leftover fat.

Separate eggs while cold, since the yolk is firmer and less likely to break, then let the whites warm up a bit. Whites at room temperature whip faster because their proteins move and unfold more easily, though cold whites will still whip with a bit more patience.

Chef Fizz says: egg whites are great team players, but they will not share a stage with fat. Not even a little bit.

When and why do you add sugar?

Sugar is the reason meringue is meringue and not just foam. It does two big things:

1. It stabilises. Sugar dissolves into the water in the bubble walls, making them thick and syrupy. Thicker walls drain more slowly, so bubbles last longer, and the foam turns shiny and smooth.
2. It protects against overbeating. Sugar slows down how far the proteins bond, so it's harder to whip a sugared meringue until it turns grainy.

The catch: sugar also slows the foam forming in the first place. Add it all at the start and you'll whisk for ages. That's why most recipes wait until soft peaks, then add the sugar gradually, about one spoonful every 15 to 20 seconds, so each addition dissolves.

A firm meringue uses about twice the white's weight in sugar. A large egg white weighs about 30 to 33 g, so that's roughly 50 to 60 g (around 1/4 cup) of sugar per white. Superfine (caster) sugar dissolves faster than granulated. Rub a little meringue between your fingers; if it feels gritty, keep whisking.

This is one of many jobs sugar does in the kitchen; what sugar does in baking covers the rest.

What do cream of tartar and copper bowls do?

Both stop the proteins from bonding too tightly, which is what overbeating really is.

When you overwhip, proteins form too many bonds, especially sulfur-sulfur bonds between protein chains. The network tightens, squeezes out water and breaks into dry clumps, much like an overcooked custard curdles.

• Cream of tartar (potassium bitartrate) is a mild acid. Lowering the pH makes those sulfur bonds slower to form, giving you a more stable, more forgiving foam. Use about 1/8 teaspoon per white, or 1/2 teaspoon of lemon juice or white vinegar as a stand-in. Add it once the whites are foamy.
• Copper bowls release a tiny amount of copper ions into the whites. Copper binds to ovotransferrin and makes it more resistant to unfolding too far, which stabilises the foam in a similar way. The result is a slightly golden, very stable foam. A copper bowl is lovely but not necessary; an acid does a similar job.

French, Swiss or Italian meringue: what's the difference?

They're three ways of combining whites and sugar, and each gives a different texture.

French meringue

Raw whites whipped, then sugar added gradually. It's the lightest and simplest, used for crisp baked meringues and folded into soufflés and cakes. Since the whites are raw, it's best baked or cooked.

Swiss meringue

Whites and sugar warmed together over a pan of simmering water, whisking, until the sugar dissolves and the mixture reaches about 71°C (160°F), then whipped until thick and cool. It's dense, silky and very stable, and the heat dissolves the sugar completely, so it rarely beads. Great for buttercream.

Italian meringue

Hot sugar syrup cooked to about 115 to 121°C (240 to 250°F) streamed into whites that are already whipping. The syrup partly cooks the proteins, so the foam is the sturdiest of the three. Used for lemon meringue pie toppings, macarons and mousses. Hot sugar syrup can burn badly, so pour it carefully down the side of the bowl, away from the whisk.

Why does baked meringue weep, bead or go sticky?

A few classic faults, each with a different cause:

• Beading (clear amber droplets on top): usually undissolved sugar or overbaking. Dissolve sugar fully and don't overcook.
• Weeping (a watery layer under a pie topping): the bottom of the meringue didn't cook enough and its proteins released water. Spread the meringue onto a still-hot filling so it cooks from below too, and seal it to the crust edges.
• Stickiness or softening: sugar is hygroscopic, meaning it pulls water from the air. On humid days, crisp meringues soak up moisture and go chewy. Store them airtight as soon as they cool.
• Cracking: the outside dried and set before the inside stopped expanding. Bake low, around 90 to 120°C (195 to 250°F), and let meringues cool slowly in the switched-off oven.

Want to practise reading peaks without wasting a dozen eggs? The Cooking Scientist app has a short lesson on stable meringue, with a mini game for spotting soft, firm and stiff peaks.

Try it at home

Here's a simple fat-versus-no-fat test using 2 egg whites, 2 clean glass bowls and a hand mixer or whisk.

1. Separate 2 eggs, putting one white in each bowl. Make sure no yolk gets into either.
2. Add a single drop of oil (or a tiny dot of yolk on the tip of a spoon) to one bowl.
3. Whip each white on the same speed for exactly 2 minutes, cleaning the beaters thoroughly between bowls.
4. Compare the bowls, then tip each one gently.

What you'll see: the clean white rises into a thick, snowy foam that holds soft peaks or better. The oily white stays loose, bubbly and much lower in the bowl, and may never reach peaks.

Why: the oil reached the bubble surfaces first and blocked the proteins from linking into a continuous film. Without that film, bubbles burst as fast as you make them.

Common mistakes

• Letting a little yolk slip in. Yolk fat blocks the protein film. Separate each egg over a small cup first, and start again with that white if any yolk gets in.
• Using a plastic bowl. Plastic holds a thin film of grease. Use glass or metal, wiped with a little vinegar.
• Dumping in all the sugar at once. It slows the foam and may not dissolve. Add sugar gradually once soft peaks form.
• Whipping past stiff peaks. Overbeaten proteins clump and squeeze out water, leaving grainy, dry foam. Stop as soon as peaks stand up straight and glossy, and use cream of tartar for insurance.
• Spreading pie meringue onto a cold filling. The bottom stays underbaked and weeps. Top a hot filling and seal to the crust.
• Baking meringues on a humid day and leaving them out. Sugar pulls moisture from the air and they go sticky. Store airtight once cool.