From Meathead at amazingribs.com: Grease and oils get rancid, particularly in hot weather, and rancid grease on the grates can make your food taste bad. Rancid

From Meathead at amazingribs.com: Grease and oils get rancid, particularly in hot weather, and rancid grease on the grates can make your food taste bad. Rancid grease on the bottom of the grates can vaporize and flavor the food, too. The black crust on the grates is mostly carbon. It tastes bad. In addition carbon insulates the grates and can stick to food. So it is vital that your food go on clean grates.