Olive Oil Extraction

Olive oil production is divided into three fields:

Oil mills, which process the olives into oil and oil cake
Refineries, where the non-consumable oil is refined,
Plants where the oil cake is processed and residual oil is extracted from the waste resulting from olive oil extraction.

To get the highest possible yield of valuable virgin oil, the olives have to be processed at low temperature (< 40 C). For olive oil extraction three processes are applied:

Pressing, traditional system.

Continuous process with three-phase decanters.

Continuous process with two-phase decanters. It generates alpeorujo, the waste treated in the project IMPROLIVE.

Smaller plants work in batch operation. Here the washed olives are grinded together with the stones, the mixed pulp is then pressed several times by means of hydraulic open cage presses. The off-running oil is collected and clarified by settling in containers. The preliminary separation of the stones from the pulp can be advantageous. With this production variant the stones are processed separately into oil. In larger plants, however, high-mechanised methods are largely used, which allow a quasi-continuous material flow. After grinding the pulp is led into malaxeurs. With the help of this mixing and kneading device the coagulation process and, with this, the development of larger oil droplets is favoured.

Oil extraction can be divided into seven steps:

I) Delivery
The olives are delivered and stored in the yard in collection boxes. This offers the possibility to determine the quality and rate of yield, and serves as basis for the settlement of accounts between the oil mill and olive farmer. The production plant is fed in charges. The olives are filled in baskets, transported by donkey carts to the soil funnel, and from there by a belt conveyor to the first processing stage.

II) Washing of the olives
The olives are filled in charges into a soil funnel and transported by a belt conveyor into a sucking device, where leaves, wood particles and other disturbing solids are removed. Subsequently they are cleaned in a washing plant. In some plants the washing water is recycled into the process after sedimentation of the solids or filtration. In other plants the olives are directly processed without being washed. For washing of the olives 100 - 120 l of water per 1000 kg of olives are required. With the help of a perforated vibrating screen they are transported into a funnel. By the funnel tip they get into the loading opening of an eccentric worm pump which conveys them into a grinding device.

III) Grinding
The olives are ground up, together with their stones, and mixed into a homogeneous pulp. This is carried out in a stone-mill consisting of a horizontally lying granite block with a granite millstone resting perpendicular to it. Metal mills and hammer mills are also used for this purpose. If frozen or very dry olives are processed, a small quantity of water is added (100 - 150 l per 1000 kg of olives.

IV) Mixing/beating of the olive pulp
In downstream-arranged malaxeurs the pulp is mixed after adding of warm water. For further breakdown of the olive cells and to create large oil droplets, the pulp is beaten. For this purpose salt is often added which aids the osmotic breakdown of cells in the olives and so the separation of the oil and water from each other is eased. Beating of the olives is repeated several times. For oil extraction by centrifuges the pulp is heated to improve the separation process. After beating the pulp is further ground up. In a malaxeur up to 100 % water are added before conveying the pulp by an eccentric worm pump into the two-phase- or three-phase decanter.

V) Oil extraction
In small oil mills the olive oil is extracted in batch operation using the traditional press method. The oil extracted is collected in containers and clarified by sedimentation. About 200 kg of oil result on an average from one ton of processed olives. To improve the separation of oil and pulp, biological or chemical aids can be added that attack the cell walls. With traditional presses the energy demand for olive oil extraction processes is 40 - 63 kWh per ton of processed olives, and 90 - 117 kWh with three-phase decanters.

VI) Purification of the oil
By means of a vibrating screen fine solids that might still be contained are removed from the oil. To enable the separation of small waste water quantities and suspended solids that accompany the oil, 100 - 150 l of water per 1000 kg of centrifuged oil are normally added to the purification process, which is achieved with the help of centrifugation.

VII) Extraction of residual oil
The high-loaded waste water from the three-phase decanter is treated by means of a vibrating screen and a centrifuge, the residual oil is extracted. The oil centrifuged twice flows into a collecting tank and is pumped by eccentric worm pumps in surface or underground storage tanks. The solid waste from oil extraction by pressing still contains about 6 % oil, using the continuous three-phase decanter, still 4 % olive oil. The oil content in the solid-liquid mixture from the dual-phase decanting process is 2.5% to 3.5 %. In specialised plants the solvent extraction process is used for this purpose. First the waste is completely dried and then extracted using hexane as solvent. The dry residues can be used as concentrated fodder. In some extraction plants the stones are separated from the pulp after extraction and used as fuel for heating of the driers. The pulp is sold as fertiliser or fodder. In some oil mills the solid waste from the press is directly used as fuel for the heating of water.

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