Sugar

Besides diluting protein molecules, sugar slows egg protein coagulation in custards and baked goods by preventing the proteins from unfolding. If the proteins are slow to unfold, they will be slow to coagulate, unless temperatures are raised. This means that sugar helps prevent curdling. It is one reason why quiches, which are essentially egg custard made without sugar, are more apt to curdle and weep than egg custard itself.

It is no surprise that sugars are considered tenderizers in baked goods; by slowing coagulation, sugars slow the formation of egg structure. (Sugars also slow the formation of gluten structure and starch structure.) If enough sugar is present, coagulation is stopped completely, and the baked good appears undercooked, even after extended baking.