Cooking is a good way to learn about chemistry: During food preparation, obvious changes take place. Milk turns into cheese, a fish fillet starts out translucent then becomes opaque, eggs change from liquid to omelets, and egg whites morph into stiff meringue. All of these are examples of the denaturation of protein.
Denaturation
There are many different kinds of protein, and each serves a different purpose in the body. Proteins are made of squiggly shapes, or alpha helixes, held together by hydrogen bonds. But these bonds are easily disrupted. When something causes the bonds to break and the alpha helixes to unfurl, the protein loses its function and becomes denatured.
Denaturation and Eggs
The protein in eggs is most often denatured by heating or beating. Heat vibrates the protein molecules, making them uncurl and flatten out, resulting in greater surface area. You can see this happen as a runny egg white hardens and turns white in your pan. The vibrations break the original hydrogen bonds, and new bonds with other protein molecules form. When you beat egg whites, you also break bonds and add air into the equation. Some of the amino acids in the egg protein are attracted to water; that is, they're hydrophilic. Others are repelled by water, or hydrophobic. As your beaten eggs are exposed to air, the hydrophilic parts sink into the watery part of the egg, and the hydrophobic parts reach for the air. As the molecules form new bonds, air bubbles keep the beaten egg whites in place.
Salt and Denaturation
Fluctuations in salt concentration can also cause denaturation of proteins. But this depends on the salt. Ammonium sulfate usually stabilizes proteins, resulting in an increase in melting temperature. Some salts, like calcium chloride, have the opposite effect: The melting temperature decreases. So adding different types of salt to eggs also affects their protein molecules.
Permanence vs. Impermanence
You might wonder if these changes caused by denaturation are reversible. In the case of egg whites, it depends how long you beat them. If beaten until stiff, egg whites are fully denatured and lose all their elasticity, so they cannot return to their original state. However, if you only partially beat your egg whites so that they rise in soft peaks, they retain some elasticity; they’re only partially denatured. If you heat these partially denatured egg whites, they solidify around tiny air bubbles. This creates light, fluffy meringues and soufflés.
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