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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 lipovacuole, 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".

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People have used olive presses since Greeks first began pressing olives over 5000 years ago. Extant Roman era olive presses survive to the present time, with a notable collection present at Volubilis in Morocco.[2] 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

Rarely, olive oil mills use a modern crushing method with a traditional press.

After grinding, the olive paste is spread on fiber disks, which are stacked on top of each other, then placed into the press. Traditionally the disks were made of hemp or coconut fiber, but in modern times they’re made of synthetic fibers which are easier to clean and maintain.

These disks are then put on a hydraulic piston, forming a pile. Pressure is applied on the disks, thus compacting the solid phase of the olive paste and percolating the liquid phases (oil and vegetation water). The applied hydraulic pressure can go to 400 atm.

To facilitate separation of the liquid phases, water is run down the sides of the disks to increase the speed of percolation.

The liquids are then separated either by a standard process of decantation or by means of a faster vertical centrifuge.

The traditional method is a valid form of producing high quality olive oil, if after each extraction the disks are properly cleaned from the remains of paste; if not the leftover paste will begin to ferment, thereby producing inconsistencies of flavors (called defects) that will contaminate the subsequently produced olive oil. A similar problem can affect the grindstones that, in order to assure perfect quality, also require cleaning after each usage.