On Food and Cooking

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.