A small amount of cornstarch (or flour) added to the recipe helps with this. The starch molecules slow protein coagulation, making the egg proteins more resistant to overcooking and curdling (clumping).
In a custard or pudding, this coagulative property is responsible for the custard texture and mouthfeel. As one source says, “Eggs are the main thickener in most custard and the yolks make them smooth and rich.”3 Starch is often added to custard to slow the process of coagulation to help prevent overcooking the mixture.
Egg proteins denature and coagulate over a wide temperature range. Natural protein consists of complex, folded and coiled individual molecules. Loose bonds across the folds and coils hold each protein molecule in a tight, separate unit. These bonds can be disrupted when exposed to heat or acid, or by physical means such as whipping, causing the protein to denature.
Coagulation or gelation in eggs can be achieved by several different means, including heat (protein denaturation), mechanical (beating or chipping), sugar (raises the temperature for coagulation), acids (decrease temperature where coagulation is achieved), alkali (high alkali can induce gelling of egg white).4, 7
When two unfolded protein molecules with their oppositely charged ends approach each other, the molecules unite. Essentially, millions of protein molecules join in a three-dimensional network, or simply, they coagulate, causing the egg product to change from a liquid to a semisolid or solid.7
Coagulation influences egg products’ ability to foam, seal, thicken and more. This thickening capacity impacts viscosity in products such as pie fillings (i.e., pumpkin5) and desserts, such as cheesecake, where a lack of eggs or substitutions can negatively impact final product height, appearance, firmness and mouthfeel.6
There are more than 40 different proteins in a whole egg, some only located in the white and others predominantly in the yolk. These proteins influence the rate of denaturation and coagulation. Egg white protein coagulates between 144° F and 149° F (62.2° C and 65° C); egg yolk protein coagulates between 149° F and 158° F (65° C and 70° C); and whole egg protein coagulates between 144° F and 158° F (62.2° C and 70° C). However, a number of variables influence the rate of coagulation, as well as the ability of the proteins to remain in the three-dimensional network.