Olive Oil Extraction

Mediterranean diet Health Effects

Olive oil extraction is the process of extracting the oil present in the olive drupes for food use. The oil is produced in the mesocarp cells, and stored in a particular type of vacuole called a lipo vacuole, i.e. every cell contains a tiny olive oil droplet. Olive oil extraction is the process of separating the oil from the other fruit contents (vegetative extract liquid and solid material). It is possible to attain this separation by physical means alone, i.e. oil and water do not mix, so they are relatively easy to separate.

This contrasts with other oils that are extracted with chemical solvents (generally hexane).[1] The first operation when extracting olive oil is washing the olives, to reduce the presence of contaminants, especially soil which can create a particular flavour defect called "soil taste".

Traditional method: olive press

Grinder with two millstones and in the foreground several fiber disks
Press with stack of fiber disks

People have used olive presses since Greeks first began pressing olives over 5000 years ago.[citation needed] Extant Roman era olive presses survive to the present time, with a notable collection present at Volubilis in Morocco. An olive press works by applying pressure to olive paste to separate the liquid oil and vegetation water from the solid material. The oil and vegetation water are then separated by standard decantation.

This basic method is still widely used today, and it’s still a valid way of producing high quality olive oil if adequate precautions are taken.

First the olives are ground into an olive paste using large millstones. The olive paste generally stays under the stones for 30 to 40 minutes. This has three objectives:

to guarantee that the olives are well ground
to allow enough time for the olive drops to join to form the largest droplets of oil
to allow the fruit enzymes to produce some of the oil aromas and taste

Modern method: decanter centrifugation

The modern method of olive oil extraction uses an industrial decanter to separate all the phases by centrifugation. In this method the olives are crushed to a fine paste. This can be done by a hammer crusher, disc crusher, depitting machine or knife crusher. This paste is then malaxed for 30 to 60 minutes in order to allow the small olive droplets to agglomerate. The aromas are created in these two steps through the action of fruit enzymes.

Afterwards the paste is pumped in to an industrial decanter where the phases will be separated. Water is added to facilitate the extraction process with the paste.

The decanter is a large capacity horizontal centrifuge rotating approximately 3000 rpm, the high centrifugal force created allows the phases to be readily separated according to their different densities (solids > vegetation water > oil). Inside the decanter's rotating conical drum there is a coil that rotates a few rpm slower, pushing the solid materials out of the system.
Diagram of a working decanter. The three phases are separated according to their densities

The separated oil and vegetation water are then rerun through a vertical centrifuge, working around 6000 rpm that will separate the small quantity of vegetation water still contained in oil and vice versa.
Three, two, and two and a half phases decanters

With the three phases oil decanter, a portion of the oil polyphenols is washed out due to the higher quantity of added water (when compared to the traditional method), producing a larger quantity of vegetation water that needs to be processed.

The two phases oil decanter was created as an attempt to solve these problems. Sacrificing part of its extraction capability, it uses less added water thus reducing the phenol washing. The olive paste is separated into two phases: oil and wet pomace. This type of decanter, instead of having three exits (oil, water and solids), has only two. The water is expelled by the decanter coil together with the pomace, resulting in a wetter pomace that is much harder to process industrially. Many pomace oil extraction facilities refuse to work with these materials because the energy costs of drying the pomace for the hexane oil extraction often make the extraction process sub-economical. In practice, then, the two phases decanter solves the phenol washing problem but increases the residue management problem. This residue management problem has been reduced by the collection of this wetter pomace and being transported to specialized facilities called extractors which heat the pomace between 45ºc and 50ºc and can extract up to a further 2 litres per 100 kilos of pomace using adapted two phase decanters.

The two and a half phases oil decanter is a compromise between the two previous types of decanters. It separates the olive paste into the standard three phases, but has a smaller need for added water and also a smaller vegetation water output. Therefore the water content of the obtained pomace comes very close to that of the standard three phases decanter, and the vegetation water output is relatively small, minimizing the residue management issues.

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