The pre-grated, packaged Parmesan cheese at the grocery store contains additives to prevent the grated cheese from clumping. These are often called “non-caking or non-clumping agents.” Because of this, the cheese might not fully melt, resulting in a grainy sauce. Invest the few minutes it takes to grate the cheese yourself. It will keep the additives out of your homemade food, and make a better sauce, too.