Why Egg Whites Turn Into Clouds When You Whisk Them

Crack an egg, tip the white into a clean bowl, and start whisking. For the first minute nothing much happens except a lot of noise. Then the clear, slippery liquid turns cloudy, then white, then into something you could pile on a spoon and hold upside down. A few minutes earlier it poured like water. Now it stands in peaks like a tiny snowy mountain range, and it is still, mostly, water.

About nine tenths of an egg white is water. The remaining tenth is protein, dozens of kinds of it, dissolved and folded up into tight little balls. The most common, ovalbumin, makes up just over half of that protein. In the fresh white these molecules drift about minding their own business, which is why the white pours.

The whisk changes that. Every stroke drags the liquid through itself and pushes air into it, and the pulling force begins to unfold the proteins. An unfolded protein has parts that like water and parts that would rather avoid it, so it heads for the nearest boundary between water and air, which is the skin of a bubble. There it links up with its neighbours, and together they make a thin, flexible wall around every bubble. Millions of bubbles, each wrapped in protein, are what you are looking at when you see foam.

Cooks describe the foam by how it behaves when you lift the whisk out. At soft peaks the tip flops over. At firm peaks it bends a little. At stiff peaks it stands straight up and stays there. Beat past that point and the protein walls knot together too tightly, squeeze out their water, and the foam turns grainy and begins to weep. Fat is the other enemy: a single streak of yolk, or a trace of oil on the bowl, gets in among the proteins at the bubble walls and stops them joining up, and the whites may refuse to rise at all.

That is why so many old kitchens kept a copper bowl just for egg whites. Cooks in France were using them in the 18th century, long before anyone could say why they worked. A pinch of cream of tartar, which is a mild acid, does a similar job and is far cheaper. Sugar goes in slowly, a spoonful at a time, because it pulls water towards itself, thickens the liquid in the bubble walls and makes the foam glossier and steadier. Tip it in all at once and the foam can collapse. Meringue, which is really sweetened, baked egg-white foam, has been made this way for a long time: the earliest recipes recognisable as meringues sit in two English manuscript books from the 17th century.

So what is the copper actually doing? When whites are whisked in a copper bowl, a tiny amount of copper passes from the metal into the egg. There it binds to one of the white's proteins, conalbumin, and the copper and protein together are steadier than the protein on its own. The pair copes better with heat and keeps the foam from being overbeaten, so the peaks hold without turning grainy. Other metals, such as iron and zinc, bind to conalbumin as well, but they do not bring the same benefit, which is why a stainless steel bowl behaves like any other bowl. A silver-plated bowl, oddly enough, works about as well as copper.

The acid works by a different route. By lowering the pH of the white, it brings the proteins closer to the point where they unfold and bind to each other easily, so they build their bubble walls sooner and hold them better. Every meringue, souffle and fluffy mousse rests on the same small miracle: a liquid that is almost all water, persuaded by a whisk to hold its breath.