Egg white
Water and protein, no fat — which makes it the only part that whips to a stable foam and the part that toughens if overused.
Also called albumen (TRADE); egg whites (INTERNATIONAL)
The white is roughly nine parts water to one part protein, with essentially no fat. Everything it does follows from that.
It whips, because unfolded proteins can form a film around air bubbles and there is no fat present to disrupt that film. It sets firmly on heating, contributing structure — which is why a meringue holds its shape and why an egg-white-heavy formulation is drier and firmer than one with whole egg. And it dries: the water leaves during baking, leaving protein behind, which is what makes whites the route to crispness in things like a pavlova shell.
The cost of using whites alone is tenderness. Without yolk fat and emulsifiers to interrupt the protein network, the result goes rubbery — which is the usual outcome of substituting two whites for a whole egg to reduce fat.
Composition and specification
Composition, against yolk
The white is predominantly water and protein with effectively no fat; the yolk carries the fat, the emulsifiers and most of the flavour. They are not fractions of a common substance and do not substitute for one another.
RecordedHarold McGee, Scribner
What changes if it is left out
A formulation using yolks alone loses structure and moisture and gains richness — custards and some enriched doughs are built on exactly that trade.
Follows from the composition rather than from a separate measurement.
RecordedFollows from evidence recorded elsewhere on BakeHQ
Why a trace of yolk stops whites whipping
Fat interferes with the protein film that forms around each air bubble. A speck of yolk, or a bowl with a greasy residue, is enough to make a white foam poorly or refuse to foam at all.
The symptom looks like a technique problem and is not, which is why more whisking does not fix it. Clean equipment and a clean separation do.
Related
What it does
Structure building
Protein that sets firmly on heating with no fat to soften it, which is why white-heavy formulations are drier and firmer.
Aeration
The only part of the egg that whips to a stable foam, because there is no fat present to disrupt the protein film.
Hydration
The white is the wetter half of the egg — it is where most of the egg’s water is, with the protein that sets it dissolved in that water rather than displacing much of it.
Used in
The foam, and the only structure.
The cake is a set egg-white foam.