How does protein react when heat is applied?

When heat is applied to protein, it undergoes a process known as coagulation. This is due to the denaturation of proteins, where the heat causes the protein's structure to unfold and lose its original configuration. Once denatured, the proteins can then form new bonds with each other, which leads to coagulation or solidification. This is a critical process in cooking, as it contributes to the texture and structure of a variety of foods, such as eggs and meats, resulting in firming up when cooked. The coagulation of proteins is essential in many culinary applications, influencing the final outcome of dishes.

The other options, while related to different processes, do not accurately describe the reaction of proteins to heat. For example, proteins do not dissolve in heat since this typically pertains to solutes like sugar or salt in solutions. Melting refers to the phase change of solids to liquids, applicable to fats rather than proteins. Evaporation is the transition from liquid to gas, which is also not a relevant change that occurs in proteins when subjected to heat. Thus, coagulation is the most appropriate reaction of proteins when exposed to heat.