Vegetable oil is made by pressing ground plant seeds such as grape seeds, soybeans or sunflower seeds together using extreme pressure. Some manufacturers use volatile hydrocarbons in the process of extracting oil from seeds as well.
Most seed oils are extracted using a process that includes both pressing the ground seeds and then extracting the remaining oil using a volatile hydrocarbon. Seeds are cleaned, ground and run through a screw press which extracts some of the oil, leaving what is called an oil cake. This oil cake is then processed using solvent extraction to produce more oil. During this process a volatile hydrocarbon dissolves the remaining oil. The remaining oil is then collected after the solvent is distilled out. Most of the solvent remaining is the oil evaporates out. The remaining solvent is removed using a stripping column which heats the oil using steam which separates the solvent. When the solvent cools and condenses, it is collected. Once collected after the extraction the solvent can be reused. The most common volatile hydrocarbon used in this process is hexane.
Unlike other plant seeds soybeans are often not pressed before they undergo solvent extraction because they contain so little oil. After extraction, most vegetable oils undergo further refining such as heating, bleaching and deodorizing so that the final product is light, and relatively clear and odorless. Refinement ensures that the final product is stable in high heat and has a longer shelf life.