When making a meringue, you must not get even a tiny speck of yolk in the egg whites or they won’t beat up properly
April 20, 2012
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There’s a kernel of truth in this one. When you beat egg whites until they are stiff, you are actually creating a foam in which the egg white proteins form bubbles with the air you are beating in. Fats tend to collapse foams and an egg yolk contains a lot of fat, hence the origin of this “rule.” It may have had some validity when people made meringues by hand, but with today’s power mixers you’ll be able to make a perfectly good, stiff meringue even if a bit of yolk gets mixed in.