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Milling consists in the very first phase of extraction. Here olives undergo mechanical actions leading to the break-up of their cell wall and membranes resulting in the spilling of oil and cell fluids. This action happens through the rubbing of pit fragments on pulps or as a result of the impact caused by mechanical devices rotating at high speeds on pulp masses. The product derived from this step in most plants is oil dough, a semi-fluid mass composed of a solid (pit fragments, peels and pulp) and a liquid (oil and water emulsion) part.
The systems used are basically of two types: Pressing, Separator.
Pressing
The hammer mill is the most appreciated tool in cutting-edge continuous cycle plants since it perfectly fits the automation needs of the plant.
The load is executed mechanically and top-down using belt highlifters that collect the olives coming out of the washing machine; olives are then mechanically unloaded from the bottom with the oil dough being poured into scutchers. A hammer mill is composed of a series of rotating disks equipped with sharp edges (hammers) upon a rotating speed of 1200-3000 rpm set into action by engines equal to 10-40 kW.
Under this system, the break-up of pulps is caused by the impacts on rotating devices. The working takes place in a very short timespan and it suits a continuous cycle operation with automated loading and unloading.
Separator
It consists in the separation of crude oil from olive pomace, the solid part made of pit and pulp fragments and hulls.
Extraction is executed with alternative systems that exploit mechanical principles being conceptually different.
In view of these differences, crude oil and olive pomace hold different properties according to the chosen method of extraction but remarkable differences regarding the plant, product quality, work organization and the management itself are also worth highlighting. There are basically three types of extraction methods.