Olive oil extraction is the process of extracting the olive oil present in olive drupes. Olive 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. The first operation when extracting olive oil is washing the olives, to reduce the presence of contaminants, especially soil which can create a particular flavor effect called "soil taste".
Olive presses
People have used olive presses since Greeks first began pressing olives over 5,000 years ago. Roman 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.
Olive presses were traditionally built within walled structures. Traditional olive-presses consisted of a large, cylindrical millstone mounted by an upper milling-stone used to grind the olives and their pits into a pulp. Formerly, the upper milling-stone was turned by a beast of burden pulling a wooden beam attached to the stone. After which, the pulp was collected and kneaded. It was then placed within frails (being no more than flexible, woven baskets made of thick fibrous material, usually of rushes, palm fronds, hemp or willow splints), stacked one on top of the other, to which was applied a stone weight to release the oil from the pulp. The extracted liquid which is obtained consists of oil and vegetable water (amurca) mixed together, and runs off into a pit. After settling, the oil rises to the surface and is removed by way of decantation. Filtering the oil produces a clearer batch of oil. The olive residue that remained was used for lighting fires.
This basic method is still widely used today, and it is 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 at a corporate oil mill. The olive paste generally stays under the stones for 30–40 minutes. This has three objectives: Ensure that olives are well ground Allow enough time for the olive drops to join to form the largest droplets of oil Allow the fruit enzymes to produce some of the oil aromas and taste
Olive oil mills very rarely use a modern crushing method with a traditional press.
In modern-day mills, after grinding, the olive paste is spread on fibre discs, which are stacked on top of each other, then placed into the mechanical press. In modern times, these discs are made of synthetic fibres which are easier to clean and maintain.
These discs 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 discs 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 discs 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.