Vacuum kettles speed up cooking and prevent discoloration and flavor loss by causing evaporation to occur at lower temperatures than normal. Pressure cookers and vacuum kettles are mainly used for jams, jellies, and other preserves. Jacketed kettles can use steam, electric or gas for heating. Some kettles have different kinds of internal agitation or no agitation at all. Internal agitation can include scrape surface which passes a brass or metal scraper along the inside of the kettle to remove the product from the kettle walls.