How does flour protect a custard?

Thanks for the question reader Alan. As you may recall from other posts on custard, custards are gels that are created when egg proteins are heated. Gentle heat coaxes these long, bunched-up molecules into releasing their chemical bonds and opening up. As they unwind they get tangled up with and/or bond with their neighbors. That has the effect of restricting the flow of the water and fat molecules that are are among them, and the mixture firms.

The problem comes when too much heat is applied. At that point the proteins begin to re-bunch themselves (coagulate). The water between the proteins gets squeezed out, the custard breaks and the result is a bunch of curds floating in water. Starch protects against this process by getting in among the proteins and stopping the re-bunching process from happening. Of course it only works up to a point. If the intense heat continues for too long you’ll get curdling no matter what.