When the ingredients arrive at the margarine manufacturing facility, they must first undergo a series of preparatory measures. The oil—safflower, corn, or soybean, among other types—is treated with a caustic soda solution to remove unnecessary components known as free fatty acids. The oil is then washed by mixing it with hot water, separating it, and leaving it to dry under a vacuum. Next, the oil is sometimes bleached with a mixture of bleaching earth and charcoal in another vacuum chamber. The bleaching earth and charcoal absorb any unwanted colorants, and are then filtered out from the oil. Whatever liquid is used in the manufacturing process—milk, water, or a soy-based substance—it too must undergo preparatory measures. It also undergoes pasteurization to remove impurities, and if dry milk powder is used, it must be checked for bacteria and other contaminants.