Oil production by pressing
Nowadays, these presses are only used to obtain fats and oils that are manufactured in small amounts. Typical examples of these are shea butter and cocoa butter. The presses consist of a perforated cylinder known as the sieve (sieve press).
The sieve is stabilised with rods, and the oil escapes through a slit which is kept open by the sieve rods. A piston is used to raise the pressure inside the sieve, pushing a plunger down onto the contents that are to be pressed. During pressing, fat is fed away via trays inside the sieve into special ducts.
This mode of pressing is used mainly to obtain natural or cold-pressed vegetable oils. Typical of this category are cold-pressed sesame, sunflower and safflower oils, also, increasingly, cold-pressed rape oil. The devices used for this purpose are referred to as 'worm presses', in which the shaft is in the shape of a continuous worm drive and fits horizontally inside the press. In order to increase the pressure whilst material is being passed through the press and to compensate for any pressure loss, the diameter of the worm housing tapers gradually in the direction of travel. The worm normally consists of several segments, with varying pitches and shapes, allowing the press to adapt to different seeds.
Nowadays, open pressing is only used in olive oil production and is a discontinuous process. Olive oil pulp is spread out on frames fitted with filter cloths; these frames are then stacked (frame press). Processing, which takes place inside presses capable of handling somewhere in the region of 500 kg of pulp, takes up to two hours. In the middle of the frame there is a perforated bung projecting upwards, through which the olive oil can run out. The necessary pressure is generated by means of a hydraulic piston. Previously, olive oil was obtained by grinding olives in olive oil mills.