Egg yolk
Fat, emulsifiers and colour — the part that enriches and tenderises, and the part that cannot set a structure on its own.
Also called yolks (INTERNATIONAL); egg yolks (INTERNATIONAL)
The yolk carries the egg’s fat, its emulsifiers — principally lecithin — and most of its flavour and colour. It contains protein too, and that protein sets on heating, but the fat around it keeps the set soft rather than firm.
In baking it does three distinguishable things. It emulsifies, holding fat and water together so a batter stays smooth rather than splitting. It tenderises, because the fat interrupts protein and gluten networks. And it enriches — the richness of a brioche, a custard or an enriched dough is largely yolk.
Adding an extra yolk to a cookie or a cake is one of the few genuinely reliable adjustments in domestic baking: more fat, more emulsifier, a softer and moister result, and no extra water to rebalance.
What changes if it is left out
Losing the yolk loses fat, emulsification and colour together. Batters split more readily, the crumb is drier and firmer, and the bake is paler.
Follows from the composition. The emulsification consequence is the one most often overlooked.
RecordedFollows from evidence recorded elsewhere on BakeHQ
Related
What it does
Emulsification
Lecithin holds fat and water together, which is what stops a creamed batter splitting.
Gluten inhibition
Yolk fat interrupts protein and gluten networks, which is the tenderising half of what an egg does.