What does egg actually do in a recipe?

An egg is not one ingredient. The white is mostly water and protein and does structure, aeration and binding; the yolk is fat, protein and emulsifiers and does richness, emulsification and colour. Recipes that call for a whole egg are using both, usually for different reasons, and that is why 'can I leave the egg out' has no general answer.

Step by step

1. Structure and setting.Egg proteins coagulate on heating into a solid mesh that holds everything else in place. This is what makes a quiche a quiche and a custard set. If the egg is doing structure, removing it leaves you with a liquid, and no substitute short of another setting agent will help.
2. Aeration.Whisked whites hold air in a protein film, and whole eggs beaten with sugar do the same less powerfully. This is the lift in a whisked sponge, a soufflé, a mousse and a meringue. Here the egg is a physical scaffold for bubbles, and the substitute has to be able to foam — aquafaba can, mashed banana cannot.
3. Emulsification.The yolk's lecithin keeps fat and water dispersed in one another. It is why a creamed cake batter stays smooth rather than curdling, and it is the whole mechanism of mayonnaise and hollandaise. If a batter splits when you add the eggs, this is the job that failed.
4. Binding.The setting protein glues loose ingredients together — burgers, fishcakes, meatballs, stuffings, breadcrumb coatings. This is the job that is easiest to substitute, because you only need something sticky that firms with heat. Flax, chia, mashed potato and breadcrumbs all work.
5. Moisture, fat and richness.An egg is about three-quarters water, and the yolk brings fat. In a cake the egg is a significant part of the liquid and much of the tenderness. Take it out and you have to put both back — this is why fruit purée substitutions work in some cakes and produce bricks in others.
6. Glaze, colour and browning.Egg wash on pastry gives shine and colour, because the proteins and sugars brown. Yolk alone gives the deepest colour, white alone gives shine without much colour, and a whole egg with a splash of milk sits between them.
7. Identify the jobs in your own recipe before changing anything.One egg in a batch of fishcakes is binding. Four eggs in a quiche filling are structure. Three separated eggs in a sponge are aeration and structure. Two yolks in a batter are emulsification and richness. The count and the treatment tell you which.

Tips

• Whole egg, yolk only and white only are three different ingredients. A recipe specifying one of them is telling you which job it wants.
• More eggs is not a general improvement. Extra egg in a cake makes it drier and more rubbery, because you are adding more setting protein than the crumb wants.
• In enriched doughs — brioche, challah, hot cross buns — eggs do richness, colour and softness, and the tenderising effect of the yolk fat is a large part of why they stay soft.
• If a recipe splits when the eggs go in, add them one at a time and slightly warm. Cold eggs into creamed butter is the classic cause of a curdled batter.
• A single egg in a large quantity is nearly always binding, and binding is the job with the most usable substitutes.

Questions people ask