How Does Egg White Turn Into Foam?

Step 1 of 5

Egg white is protein coils in water.

It is about ninety per cent water. The rest is proteins wound into tight balls that slide past each other, which is why it pours.

Step 2 of 5

Beating tears the coils open.

The whisk stretches and shears the liquid hard enough to unwind the balls into long strands. Heat does the same thing by a different route.

Step 3 of 5

The strands line up at the bubble surfaces.

One end of each strand likes water and one avoids it, so they arrange themselves neatly around every bubble of air the whisk folds in.

Step 4 of 5

Whisking too long breaks it.

Keep going and the strands bond to each other instead of to the bubbles. The foam turns grainy and weeps water — it has over-whipped.

Step 5 of 5

Fat is the enemy, acid is the friend.

A speck of yolk stops the strands from linking at all. A pinch of cream of tartar makes them a little less eager, so the foam is harder to ruin.

Copper bowls really do help — copper binds to one of the proteins and stops it over-bonding.