The air created by beating an egg white denatures or makes some of its proteins unwind. The denatured proteins bump into each other and form a lining around each air bubble. The idea is to create an ideal egg white foam in which the denatured proteins link together loosly and stay moist and elastic. If you overbeat it, the links tighten and squeeze out the moisture.
For years, chefs believed that beating egg whites in copper bowls improved egg white foam. While copper bowls don’t produce a greater volume of foam than a glass or steel bowl, there is a major difference in the volume or “loft” of the cooked dish. It turns out that the protein in egg whites that links around the bubbles is conalbumin, which loves to combine with copper. If the copper is left in contact with the egg white for more than two minutes, it forms copper-conalbumin around the air bubbles. The copper-conalbumin is more stable, dries out less easily, and requires a higher temperature for coagulation.
Food science writer Harold McGee attempted to test whether whipped egg whites lost more moisture when beaten in a copper bowl. He found that after 10 minutes, egg whites whipped in glass bowls were quite dry, while those whipped in copper bowls stood moist and stiff even after 20 minutes. He suggests that the conalbumin-copper combination acts as a reinforcing cement in the bubble walls of the foam.
Egg whites beaten in copper bowls remain elastic and moist. When heated in an oven, the air bubbles expand and create lofty baked goods.