By Harold McGee
Long before anyone knew about egg proteins or their chemical bonds, cooks had come up with a way of controlling them. The French tradition has long specified the use of copper utensils for making egg foams. One early trace of this tradition is a 1771 illustration in the French Encyclopédie that shows a boy in a pastry kitchen working with a straw whisk and what the accompanying key identifies as “a copper bowl for beating egg whites.” It turns out that along with a very few other metals, copper has the useful tendency to form extremely tight bonds with reactive sulfur groups: so tight that the sulfur is essentially prevented from reacting with anything else. So the presence of copper in foaming egg whites essentially eliminates the strongest kind of protein bond that can form, and makes it harder for the proteins to embrace each other too tightly. Sure enough, if you whip egg whites in a copper bowl—or in a glass bowl to which you’ve added a pinch of a powdered copper supplement from a health food store— the foam stays glossy and never develops grains. A silver-plated bowl will do the same thing.