Butter should be kept in a cool place, and well covered, otherwise it is liable to become rancid; this is due to the albuminous constituents of the milk, acting as a ferment, setting free the fatty acids. First-quality butter should be used; this does not include pat butter or fancy grades. Poor butter has not been as thoroughly worked during manufacture, consequently more casein remains; therefore it is more apt to become rancid. Fresh butter spoils quickly; salt acts as a preservative. Butter which has become rancid by too long keeping may be greatly improved by melting, heating, and quickly chilling with ice-water. The butter will rise to the top, and may be easily removed.