Cold pressing - less oil is extracted but the oil is purer and of a higher quality. Hot pressing - more oil is extracted but the oil is mixed with other compounds that are made soluble by the higher temperatures, and the oil may be chemically changed, all of which makes for an oil of inferior quality. Seeds or fruits are crushed, then agitated in an organic solvent. The solvent is driven off the extract by distillation and reused. This removes any mucilaginous materials.
Triglycerides are usually odorless and tasteless.
Taste and color of vegetable oils comes from other compounds extracted with the oil. Bleaching - removal of pigmented substances by filtering through diatomaceous earth, to which color particles adhere. Deodorization - removal of odors by being heated with steam under a vacuum. Hydrogenation - hydrogen is bubbled through the oil, usually with a catalyst such as nickel. Hydrogen binds to previous double bonds, resulting in increased saturation.