Final answer:
Older egg whites whip to a more stable foam due to changes in the protein structure over time, specifically the weakening of hydrogen bonds, which allows the proteins to denature more easily, creating a stable foam when air is incorporated during whipping.
Step-by-step explanation:
Older egg whites whip to a more stable foam because as eggs age, changes occur to the proteins within the egg whites. The albumin, which is a protein that makes up a significant part of egg whites, is initially held in a compact, ordered structure by numerous hydrogen bonds. Over time, the bonds weaken, allowing the proteins to denature more easily upon whipping. This denaturation creates space for water and air to become trapped between the proteins, resulting in a more stable foam. Furthermore, older egg whites have a more liquid consistency, making it easier for the proteins to unfold and for air to be incorporated.
When egg whites are whipped, agitation physically disrupts the hydrogen bonds and other interactions among the proteins. The agitation introduces air into the egg whites, and, at the same time, beating the eggs causes the proteins to unfold, thereby helping to stabilize the incorporated air. As the proteins unfold, they expose their hydrophobic regions, which tend to avoid water and trap air, contributing to the foam's stability. Older egg whites, having less tightly bound proteins due to the weakened hydrogen bonds, form a more stable, less likely to collapse structure when whipped.