Why egg stiffens when whisked?

1. Protein Denaturation: Egg white is primarily composed of protein molecules called albumins. When you whisk egg whites, you're introducing air and disrupting the protein structure. This disruption causes the proteins to unfold and stretch out, a process called denaturation.

2. Foam Formation: As you whisk, the air bubbles get trapped within the unfolded protein chains. These air bubbles create a network of protein strands that hold the foam together.

3. Surface Tension: Egg whites contain a natural emulsifier called lecithin, which helps to lower the surface tension of the air bubbles, preventing them from collapsing.

4. Stabilization: As the proteins continue to unfold, they interact with each other and form a network of cross-links. These cross-links strengthen the foam and make it more stable.

5. Acidification: Adding a small amount of acid, such as lemon juice or cream of tartar, can help stabilize the foam. Acids denature the proteins even further, which helps them form tighter bonds.

In summary, the stiffening of egg whites is a result of:

* Unfolding and stretching of proteins.

* Air bubble incorporation and entrapment.

* Formation of a protein network.

* Surface tension reduction.

* Increased stability through protein interactions and acidification.

The result is a light, airy foam that can be used in various recipes, such as meringues, soufflés, and cakes.