How is olive oil made?
Modern olive mills are partially or fully automated and have replaced granite crushers with metal crushers. They consist of a stainless steel body and a stainless steel crusher that rotates at high speed. The olives are typically thrown against a hammer-shaped metal grating, thus the name hammermill. Alternatives include toothed disc, cylinder, and roller mills. Modern milling is very gentle in order to avoid overheating of the paste. Cold pressed (extra virgin) oils must not exceed 27˚C at any step in the processing of the oil.
Modern malaxers are horizontal troughs with spiral mixing blades. Typically, two or three cylindrical vats are used in tandem, mixing the paste at slow speeds (15–20 rpm) for anywhere between 20 minutes and 75 minutes. The vats are jacketed so that the paste can be heated or water added during this process to increase the yield, although that generally results in a lowering of oil quality. New malaxers have an atmosphere controlled by an inert gas (i.e., nitrogen or carbon dioxide) to reduce oxidation and produce higher-quality oils.
Following modern milling processes, the paste is pumped into an industrial decanter where the phases are separated using centrifugation. This step can involve a three-phase decanter or a two-phase decanter. Water can be added to facilitate the extraction process. The decanter is a large-capacity horizontal centrifuge rotating approximately 3,500 rpm. In three-phase decanting, inside the rotating conical drum is a coil that rotates more slowly than the drum. This pushes the solids out of one end of the system and the water and oil out the other end. Three-phase decanting results in loss of a portion of the oil polyphenolics due to the higher quantity of water used. It also produces larger quantities of vegetative water that then need to be processed and have negative environmental effects. Two-phase decanters were created to solve these problems. In this process, the olive paste is separated into two phases, oil and wet pomace. The decanter has two exits instead of three, and the water is expelled with the pomace, resulting in a wetter pomace. The two-phase decanter thus solves the phenol washing issue and uses less water, but increases the waste produced. Regardless of the process used for oil extraction, a final centrifugation process is needed to separate the oil from the vegetation water. Vertical centrifuges, operating at lower speeds of 6,000 rpm under controlled temperature conditions, are used for this purpose.