Adding cheese to a sauce that is too hot is one of the most common mistakes in Italian cooking. When finely grated parmesan or pecorino hits liquid at a rolling boil, the proteins seize almost instantly and the sauce becomes grainy, stringy or broken rather than smooth and glossy. For cacio e pepe, the heat must be reduced to a gentle simmer before the cheese is added, stirring continuously as it melts slowly into the starchy cooking liquid. Marcus Wareing warned specifically that if the sauce continues to boil vigorously after the cheese is added, it will curdle back in the pan. Low and slow is the rule: patience at this stage makes the difference between a silky, restaurant-quality sauce and a grainy disappointment.