Why won't egg whites get fluffy if some yolk gets in?

Have you ever listened to a whisk going tap, tap, tap against a bowl of egg whites? The clear, slippery egg white turns into a fluffy white cloud, soft as a pillow. But if one little drop of yellow yolk falls in, the cloud never comes.

So why won't egg whites get fluffy if some yolk gets in? Let's peek inside an egg white to find out.

Egg white is mostly water, with something special mixed in called protein, which means tiny strings curled up into balls. These strings are far too small to see. They are what let the egg white turn into a cloud.

When a grown-up whisks the egg white, the wires push air into it as little bubbles. Those bubbles are what will make the cloud light and puffy.

Every beat of the whisk also tugs on the curled protein strings. They unroll and stretch out long, so they can reach around the bubbles.

The stretched protein strings grab onto each other around every bubble, like friends holding hands in a ring. Together they make a thin wall that keeps the air inside. What do you think a drop of yolk might do to all those holding hands?

Egg yolk has fat in it, the same slippery, greasy stuff that's in butter. The fat slips in right where the protein strings try to hold hands. Now the strings can't hold on to each other, so they never build a strong wall.

Without strong walls, the bubbles pop and the air escapes. No matter how long the grown-up whisks, the egg white stays runny instead of fluffy.

The word to remember is protein. When a grown-up whisks egg whites, protein strings hold hands around the air bubbles, but the fat in yolk slips between them, so the bubbles pop and no cloud can grow.

Sleep tight, and may your dreams be as soft and fluffy as a big bowl of whipped egg whites.