On arrival at the mill, the olives are first of all weighed and, before proceeding to the oil extraction stage, are subjected to defoliation and washing.
The defoliator, using fans and rotating shafts, removes branches and most of the leaves, destined to be turned into fertiliser or fuel, while the washer removes soil residues or other impurities using drinking water.
At this point the olives are ready for the subsequent processing stages: crushing, malaxing, extraction, storage and filtration, which have followed an almost unchanged operational logic for millennia.
– [b]Crushing[/b] is the stage during which the olives are crushed to obtain a coarse paste containing skin (epicarp), pulp (mesocarp) and stones (endocarp), which have a draining function and facilitate the subsequent separation of the oil from the paste.
The two most widely used crushing methods today are the traditional one, which results in a discontinuous processing cycle, and the modern one, characteristic of continuous-cycle mills.
The traditional method, which is gradually disappearing, is based on the use of granite millstones that crush the olives with their weight; the modern method instead uses hammer or rotating-disc crushers, with the advantage of quickly crushing a large quantity of olives, obtaining a more uniform paste and minimising the harmful contact of the olive paste with oxygen.
– [b]Malaxing[/b] consists of slowly and continuously mixing the olive paste inside a machine called a malaxer. This is a fundamental and delicate step, as it allows the water-oil emulsions formed during crushing to break, and lets the droplets of oil-must merge into ever larger drops that will be easier to separate in the subsequent extraction.
Malaxing is now carried out in sealed stainless steel tanks, so as to control the amount of oxygen coming into contact with the paste, possibly also saturating it with inert gases such as nitrogen.
During the malaxing stage, which lasts about 30 minutes, the temperature of the paste must never exceed 29°C, in order to preserve the organoleptic characteristics and properties of the oil.
– [b]Extraction[/b] is the heart of the processing: from the paste obtained in the previous stages, the three components are separated: pomace, vegetation water and oily must, and it can be discontinuous or continuous.
Pressure extraction is a discontinuous system: the paste coming out of the mill is placed on circular filtering panels, perforated in the centre (“fiscoli” mats). These are stacked, alternated with steel discs to even out the pressure, and piled on trolleys placed under the press. Increasing pressure for about 1 hour allows the oily must (oil and vegetation water) to separate from the solid part (pomace) that remains on the mats.
The continuous methods, most widely used today, achieve separation of the oil from the solid part not through pressure but by using other physical principles. The most common extraction system, centrifugation, exploits the different specific weight of the individual components. The paste from the malaxer is fed into a large centrifuge or decanter, which, through high speed, separates the pomace from the oily must, which is in turn separated into oil and vegetation water by a dedicated centrifugal separator.
At the end of these operations, an oil is always obtained that is perfectly edible, but cloudy, because it has mucilage, air bubbles, pulp fragments and water residues in suspension. A period of settling is therefore necessary, so that all foreign substances settle at the bottom (sediment), or the process can be shortened by resorting to filtration through filtering systems of various types and composition.
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