Increasing the viscosity can be done by adding flour or cornstarch and this can also protect against curdling. Curdling occurs when the sauce is cooked too quickly over directly and actually brought to a boil, causing the egg proteins to denature and rearrange or coagulate into curds by bonding to one another. Starches, such as flour and cornstarch, protect against curdling when the starch granules absorb water and being to leak long starch molecules into the liquid. These long starch molecules prevent curdling in two ways. First, they absorb heat and prevent some egg proteins from denaturing. Second, the long dissolved starch molecule get in the way of the egg protein molecules and impede bonding.