A method for heating a medium that includes an object having a variable weight is provided. The method comprises (A) placing medium including an object having variable weight into a variable enclosure having adjustable dimensions; (B) initiating a heating process by subjecting the medium including the object to a variable AC electrical field; (C) substantially continuously measuring an impedance of medium including the object during the heating process; (D) adjusting dimensions of said variable enclosure to optimize the heating process; (E) adjusting parameters of the variable AC electrical field to optimize the heating process; and (F) controlling the heating process.