If covered, the meat will be cooked, in part, by the moist heat generated by the steam trapped inside the covered pan. The meat's surface will tend to be mushy and pale brown. If cooked uncovered, the meat will be cooked by dry heat. An important chemical change called the Maillard reaction will occur. The Maillard reaction gives a meat's surface an appetizingly intense flavor, crusty texture, and dark brown pigmentation.
** Asian Recipes