Olives are picked at La Mouissone from November to February and often much beyond that, depending on the weather. They can be picked by hand, but for large quantities a mechanical shaker (peigne vibreur) is often used. The harvested olives are 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 little known 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 earlier 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 at a slow rotation speed, of twelve to fifteen revolutions per minute and the process lasts twenty to forty minutes. The quantity of olives processed in one cycle is 2.5 to 3 quintals (hundredweight).
In modern continuous cycle installations, the mechanisms are entirely metallic hammer, blade and disc mills as these are adapted to deal with the automation requirements of very large quantities.
The paste obtained is pressed using weights.
This produces a juice consisting of water and oil which is mixed in a steel tank where rotating blades deliver a slow mixing movement to the paste. The mixing action breaks down the emulsion and improves the yield of the oil's must. This phase is very important in order to achieve the right level 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. This is the reason that for high quality oils, mixing takes place at cold temperatures.
his involves 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 undertaken by various systems:
Extraction by pressure: the traditional method in which oil must is separated from the husks by pressurised filtering. The oil paste is filled into plaited bags of plant material or coco fibre, called "scourtins", laid in layers sandwiched into a stack. The pressure is produced using an open hydraulic press on to the stack of scourtins, and the oil leaches out.
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 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, the oil and water each separate into separate flows.
When the oil flows out of the centrifuge outlet, it is ready for consumption. It is in stored in steel tanks to prevent it oxidizing and is usually left to clear for six to eight weeks when it will be filtered before being bottled, to eliminate any suspended solids that would make it cloudy.