As dry heat cooks a roast, some of the internal meat juices flow to the surface and evaporate, leaving behind an exposed coating of natural sugars, amino acids, and other solid substances. The Maillard (also called browning) reaction combines the sugar and amino acids on the meat's surface into new compounds that enhance the flavor, texture, and color.
Most of the new compounds created by the Maillard reaction remain on or just below the meat's surface. Some fall off the meat and coagulate on the pan's bottom, providing the flavor foundation of the esteemed pan drippings. You can promote browning by basting the surface of the meat with butter. Not only does butter contribute extra sugar and amino acids for the Maillard reaction, its fat content helps prevent the meat's surface from drying out while its surface molecules dutifully brown. Basting with the pan juices has the same effect.
** Asian Recipes