Whites and yolks set at different temperatures and that gap runs the sweet kitchen

Two materials, one shell

The white is roughly nine parts water to one part protein, with almost no fat and very little else. The yolk is a far more crowded substance: about half water, around a third fat, and a suspension of proteins and phospholipids in among it, including the lecithin that makes it the most useful emulsifier in the kitchen. Treating an egg as a single ingredient works well enough in a plain cake and stops working the moment a recipe asks for a set, a foam or an emulsion, because those three jobs belong to different parts of the same egg.

That difference shows up first as behaviour under heat. Proteins in the raw state are folded up and drifting past one another, which is why both parts are liquid. Heat unfolds them, exposes sticky regions that were tucked inside, and lets them bond into a continuous mesh that traps the water. What we call setting is that mesh forming. The temperature at which it happens is a property of the particular proteins involved, and the white and the yolk don’t share a set of them.

The gap itself, and why it isn’t a fixed number

Egg white begins to turn opaque a little above sixty degrees Celsius and is firm by about the middle sixties. Yolk starts to thicken around the mid sixties and sets near seventy. That narrow separation is what a poached egg exploits, and it is also why a curd and a custard can be cooked to a specific texture rather than simply cooked. But neither figure is a constant, because both move with what else is in the pan.

Dilution raises the setting temperature, since spreading the protein through more liquid means the chains meet each other less often and need more energy to link up. Sugar raises it further, by getting between the proteins and interfering with the same bonding. Acid and salt push the other way. This is why a baked custard heavy with cream and sugar has to go well past seventy before it holds, while a lean mixture curdles almost the moment you stop paying attention.

Whites build, yolks soften

Whipped white is the strongest structure a kitchen can make from an egg, because there is nothing in it to interrupt the protein film that forms around each bubble. That is why an angel food cake made only with whites rises so high and eats so dry. It has structure and no tenderiser whatever.

Yolk does the opposite. Its fat coats proteins and starch granules and physically prevents long continuous networks, so a batter rich in yolks bakes shorter and more tender. Its lecithin holds fat and water together in one stable phase, which is what allows a génoise, a pastry cream or an ice cream base to be smooth rather than greasy. Swapping in whites for yolks by weight looks reasonable on paper and gives you a bake that is drier, paler and springier than intended.

Curdling is the same reaction gone too far

A custard that splits has not done anything different from one that set. It has done the same thing more. Past the point where the protein mesh is continuous, the strands go on bonding to one another, tighten, and squeeze the water out from between them — the visible result being solid curds in a thin watery liquid. Once the bonds have formed they won’t unform, so no amount of further stirring puts it back.

A blender will sometimes rescue a lightly split custard by breaking the curds down small enough to pass as smooth, and a starch-thickened mixture is far more forgiving because the starch physically obstructs the proteins. Neither is a reversal. The genuine defence is a lower temperature and more patience, or an insurance policy of a teaspoon of cornflour, which sounds like a cheat and is standard practice in plenty of professional kitchens.

Age changes an egg in ways a recipe assumes

Eggs lose carbon dioxide through the shell as they sit, and the white becomes more alkaline as they do. A fresh white is tight and holds together in the pan; an older one has thinned and spreads. For whipping this is a genuine trade rather than a decline: an older, thinner white whips faster and to greater volume, and a fresher one produces a foam that is slower to build and more stable once built.

The other thing age does is admit air, which is why an old egg floats and a fresh one lies flat. That is a useful test and not much more. For baking, the practical points are that eggs separate more cleanly when cold because the yolk membrane is firmer, that whites whip better at room temperature because the liquid is less viscous, and that any trace of yolk in a white will stop a meringue from forming, since the fat interferes with the protein film before it can set.