Plant oil sources are typically the seeds or seed coats of plants. Plant breeding and genetic engineering have made available many plant oils with fatty acid compositions quite different from the typical values cited in the accompanying table.

Oils are extracted from plants by using pressure or solvents, usually the petroleum fraction hexane. Olive oil, for example, is a typical seed coat oil and is extracted by multiple pressings of the fruit pulp. The oil from the first pressing has the best quality and is termed virgin oil. Oilseeds may be extracted with pressure in a mechanical expeller but usually are cracked and pressed into flakes for extraction with hexane. The hexane is removed from the extracted crude oil by distillation.

Crude oils contain small amounts of undesirable pigments, phospholipids, and free fatty acids (i.e., fatty acids not chemically linked to glycerol) that make the...