Most of us get away with making meringues and egg white foams in clean, stainless steel bowls, yet chefs insist copper is better, and have done so for hundreds of years. Is there a scientific reason behind using copper bowls to whip egg whites?
The Science of Whipped Egg Whites
As you beat those whites, the whipping action of your whisk forces the tangled and tightly clumped proteins in the egg whites to unfold and loosen up from each other. As air is incorporated into the mixture, the proteins rearrange to form a network around the air bubbles, trapping air within the egg white foam. The types of bonds holding the network of proteins together can be ionic (like between a positive charge and a negative charge) or much stronger (like disulfide bonds that form between two sulfur groups on the egg white proteins). But there's a catch: The more this network forms, the more likely the proteins will bind even more tightly. As the proteins tighten up, water can get pushed out; the proteins set up into gritty, grainy, dry networks of egg whites separated from the liquid that is usually trapped within. The whipped egg whites essentially break down.
Copper to the Rescue
The neat thing about whipping egg whites in a copper bowl is that as the whisk hits the bowl, tiny bits of copper break off from the surface of the bowl and mix in with your egg whites. Since copper can bind sulfur groups, those tiny bits of copper bind to the egg white proteins and therefore, those sulfur groups are unable to form the strong disulfide bonds that can lead to gritty, dry whipped egg whites. Therefore, egg whites whipped in a copper bowl stay glossy and firm without as much risk of becoming overwhipped or grainy.
Adding an Acid Will Also Do the Trick
For those of us who might not have the budget for a copper bowl, there's another option: adding an acid, like cream of tartar. Cream of tartar is actually the potassium acid salt of tartaric acid and, added to egg whites, will lower the pH of the egg whites and reduce the likelihood of those disulfide bonds. Again, forming overwhipped, gritty egg whites is less likely.
Do you have any tips or tricks for achieving perfectly whipped egg whites?