The higher the heat, the more the Maillard reaction
takes place. There are other variables that also determine the quality of a roast's crust. One variable is cooking time: the longer the roast cooks, the crustier the meat is. Another is the use of a lid or foil: an uncovered meat develops a crispier, richer-colored, and more delectable crust than a covered one. Yet another variable is the fat content of the meat or basting liquid. Fat helps to keep the meat's surface from scorching and drying out, thereby retarding crust formation.
** Asian Recipes