Olives are harvested by hand or by shaking the tree or by machinery. After harvest, the fruit is transferred as soon as possible to the olive press (or otherwise an olive oil mill, or oil mill, "lubricant" or factory) to prevent the fermentation and development of mold that degrades the quality of the olive oil.
In the olive mills the treatment begins with weighing, separating and washing the olives. Olives, placed in crates or bags, are transported by lifts to a large basin on an elevation of the mill. From there they fall through crustaceans into crushers or millstones.
Then slip in the olive pulp, stirring for half an hour in apparatus called "softeners". There, the olive oil can also be heated to improve oil yield. For good quality olive oil the kneading is "cold", ie at room temperature.
After grinding, the traditional presses (which are now scarce in Greece), with the help of powerful hydraulic presses, extract the olive oils, which are a mixture of olive oil with aqueous components of the fruit.
In modern mills, liquids are separated from solids by centrifugation on devices called "decanter". The residual solid by-product is called the "core".
The liquids obtained from the decanter (a basic mixture of water and oil) are directed to one or more centrifugal separators where the oil is separated from the aqueous portion due to a difference in density. The aqueous components that are eliminated are commonly called "kale" and are dark brown in color and characteristic odor.
In modern times these wastes are considered as environmental pollution and various measures are taken for their treatment or storage in septic tanks. In some cases, by centrifuge liquid waste, industrial olive oil is produced for either refining or industrial use.
In any case, the management of liquid wastes in mills is difficult because of their high organic load and odor.
Solid waste is commonly called "core". It consists mainly of cellulosic substances from pomace, flesh and olive bark. The core contains a significant amount of olive oil which is obtained by physicochemical methods in the pomace oils. There, the material is essentially dried and then extracted with an organic solvent. The olive oil produced there, raw pomace oil, is unsuitable for editing due to its high acidity and unpleasant odor and taste. In order to be edible there is refining or "refining" in special plants called "refineries". Refining is an intense chemical process that destroys much of the taste, aroma and some useful nutrients of olive oil. The olive oil produced is no longer called "virgin olive oil" and is only available for consumption as a blend of virgin. It is the lowest of the categories of edible olive oil available on the market.
From the processing of the kernel is left the dry substance or "kernel" which is usually used as fuel, often returning to the mills that need sufficient energy to heat the water. The kernel can also be used as a fertilizer or feed.
Depending on the stage of ripening of the olives, some virgin oils are called virgin oils. The average yield of 100 kg of oils, depending on the quality, year and treatment system, is about 10-25 kg of oil, 35-50 kg of olive kernel and 35-50 kg of wet residue.