Food for thought: Why do egg-whites foam when you whisk them?

THE METAMORPHOSIS of globular egg-white to fluffy peaks of foam is one of the many culinary facts we take for granted. But how does it work?

As liquid egg-white is whisked, the mechanical action causes its proteins to unfold and form a network, trapping air in tiny pockets. As the whisking continues, the air pockets become smaller. The change in colour (from translucent to brilliant white) is due to a trick of the light with the bubbles, rather than the egg's pigment. Essentially, the foam is composed of small gas bubbles dispersed through the egg-white. If the foam is left to stand, it will eventually collapse back into a liquid. But the physical change caused by whisking denatures the protein, depriving it of elasticity - it cannot be re-whisked successfully. If the foam is baked, protein coagulates and moisture is driven off, forming a solid foam commonly known as meringue. Roy Ballam, British Nutrition Foundation