By Harold McGee
There’s no truth to the common saying that acidity and salt “toughen” egg proteins. Acids and salt do pretty much the same thing to egg proteins. They get the proteins together sooner, but they don’t let them get as close together. That is, acids and salt make eggs thicken and coagulate at a lower cooking temperature, but actually produce a more tender texture.
The key to this seeming paradox is the negative electrical charge that most of the egg proteins carry, and that tends to keep them at a distance from each other. Acids— cream of tartar, lemon juice, or the juice of any fruit or vegetable—lower the pH of the egg, and thus diminish the proteins’ mutually repelling negative charge. Similarly, salt dissolves into positively and negatively charged ions that cluster around the charged portions of the proteins and effectively neutralize them. In both cases, the proteins no longer repel each other as strongly, and therefore approach each other and bond together earlier in the cooking and unfolding process, when they’re still mostly balled up and can’t intertwine and bond with each other as tightly. In addition, coagulation of the yolk proteins and of some albumen proteins depends on sulfur chemistry that is suppressed in acidic conditions (see the discussion of egg foams). So eggs end up more tender when salted, and especially when acidified.