There are 3 olive oil extraction techniques that are generally adopted to achieve a complete processing process: by pressure (classic, discontinuous method),by centrifugation (modern, continuous method), and finally, percolation by selective filtration (modern, continuous method).
Olive oil extraction: pressure method
When considering the olive oil extraction techniques, it is necessary to outline that the more traditional system is the pressure method: it is classic and ancient, however, it has the disadvantage of being discontinuous.
The olives arriving at the mill must first be cleaned of leaves, earth and anything else that could damage the organoleptic characteristics of the oil and the plant itself. The drupes will then undergo one or more passages in suction machinery and washing tanks. Moreover, for what concerns the the production of particularly valuable oils, hand sorting can be carried out, removing the olives that do not meet quality standards.
In addition, immediately after the olives have been cleaned, they are pressed or crushed by mechanical means. The drupes are placed inside the mullers. It is a special metal vats equipped with 2, 3 or 4 very heavy wheels made of granite. In fact, these wheels rotate on themselves thus causing the olive to be crushed.
Crushing has the purpose of damaging the pulp cells. This last aspect is very important because, as the kernel is equipped with a woody shell, when it breaks, it produces splinters.
In more modern mills, the millstones are replaced by metal hammer, cylinder, which allow production times to be cut down.
CENTRIFUGATION METHOD, which is continuous, modern, faster, but removes much of the phenolic component.
The initial processes, cleaning, crushing and kneading, are the same as for the classic pressure method. After crushing, the olive paste is mixed with 30 per cent water (it is diluted, made more liquid) and enters a centrifugal extractor, which can be either a three-way (from which the pomace, oil and vegetation water come out) or two-way (from which only the oil and vegetation water come out).
The centrifugal extractor, called a decanter, consists of an endless screw that rotates. By doing so, it carries the olive paste forward by compressing it. The crushed pomace comes out from one side, the liquid goes to the bottom and, as it goes down, there is also the separation of the water (which comes out from the bottom) from the oil (which comes out from the top, because it is lighter).
The separation, however, is not clean and both are immediately subjected to centrifugation, to recover the small percentage of oil present in the water on one side, and to remove the small amount of vegetation water present in the oil on the other.
SELECTIVE FILTRATION: a new generation method; fast and continuous, it allows the phenolic component to be preserved.
In this case too, the initial stages of cleaning, milling and gramolatura are the same as for the previous methods. Gramolatura is followed by extraction of the oil by percolation.
The olive paste is placed on filtering systems consisting of a steel or nickel grid. Since these lamellae are made of metal, the olive oil adheres to them much better than water; when they retract, the retained oil drips out. This technique exploits the different interfacial tension existing between metal and oil and between metal and water. The same phenomenon can be appreciated by immersing a knife or fork in water and oil.
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