They can be picked by hand, but for large quantities a mechanical shaker is often used. The harvested olives are then sorted, washed in cold water and left to dry.
The olives are crushed with their stones.
Traditionally the olives are crushed between 2 stone grindstones until an oil paste is obtained.
This paste is a semi-fluid mass made up of a solid fraction (fragments of stones, skins and flesh), and a liquid fraction (emulsion of water and oil).
It is a little known fact that the juices are not released due to the crushing action, but because of the rubbing of the sharp edges of the olive stone fragments on the flesh of the olives. The role of the wheels is therefore to crush the olive stones and mix the olive paste.
In olden times the grindstone consisted of a single wheel driven by an arm harnessed to a donkey or horse. Nowadays the grindstone is driven by a motor with a slow rotation speed, of twelve to fifteen revolutions per minute and lasts twenty to forty minutes. The quantity of olives processed in one cycle is 2.5 to 3 quintals.
In modern continuous cycle installations, the preferred mechanisms are entirely metallic hammer mills, blade mills and disc mills because they are perfectly adapted to deal with automation requirements for very large quantities.
The paste obtained is then pressed using weights.
This produces a juice consisting of water and oil which is mixed in a steel tank where rotating helical blades impart a slow mixing movement to the paste. The mixing action breaks down the emulsion and then improves the yield of the oil's must. This phase is very important in order to achieve the right compromise between oil quality and quantity.
Heating increases the efficiency of mixing since it maximises the quantity of oil produce, but it has a detrimental effect on oil quality above a certain temperature.
For high quality oils, mixing takes place at cold temperatures.
This consists in separating the oil must from the husks, a solid fraction made up of the pieces of olive stone, skins and pieces of flesh.
Extraction is performed by various systems:
Extraction by pressure: this is the traditional method in which oil must is separated from the husks by pressurised filtering. The pressure is produced using an open hydraulic press. The oil paste is laid in thin layers sandwiched between fibre disks, called "scourtins" in a mobile tower.
Extraction by centrifugation: The oil paste is centrifuged in a conical drum rotating on a horizontal axis (settling device). The centrifuging is performed at a speed of roughly 3,400 revolutions per minute.
Since the density of olive oil (0.92) is less than that of water, it naturally rises to the surface and can be collected. This is the traditional method. However nowadays centrifuging methods are usually used.
The oil must is centrifuged at 6,000-7,000 revolutions per minute.
Due to the difference in density, oil and water separate into two separate flows.
When the oil flows out of the centrifuge outlet, it is ready for consumption.
It is then stored in steel tanks to prevent it oxidizing. It will be filtered before being bottled, to eliminate any suspended solids that would make it cloudy.