By Harold McGee
Flour or cornstarch can protect against curdling in custards and creams, even if they’re cooked quickly over direct heat and actually boil. (The same is true for egg-based sauces like hollandaise.) The key is the gelation of the solid starch granules in these materials. When heated to 175°F/77°C and above— right around the temperature at which the egg proteins are bonding to each other— the granules absorb water, swell up, and begin to leak their long starch molecules into the liquid. The swelling granules slow protein binding by absorbing heat energy themselves, and the dissolved starch molecules get in the proteins’ way and prevent them from bonding to each other too intimately. Because they contain starch, both chocolate and cocoa can also help stabilize custards and creams.