Final answer:
A custard mixture made of whole eggs and milk coagulates at approximately 160°F, as this heat causes the proteins in the eggs to denature and form a solid structure.
Step-by-step explanation:
A custard mixture made of whole eggs plus milk coagulates at about 160°F when heat is applied. Proteins such as albumin found in eggs begin to denature and form a solid structure when heated to this temperature, which is sufficient to form hydrogen bonds and create a semi-solid consistency making the custard set. This is similar to the process that occurs when eggs are boiled or when other protein-rich mixtures, such as yogurt, are heated during the cooking process, despite the fact that each type of protein might have specific coagulation points.