Olive Oil Extraction

IV) Mixing/beating of the olive pulp

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.
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.
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.
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.
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.
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.
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.

From the Project IMPROLIVE web site… The aim of the project IMPROLIVE is to improve treatments and validate the liquid-solid waste from the two-phase olive oil extraction (“ alpeorujo “). The project is being developed during the period 1997-99 within the Programme FAIR (CT96-1420), European Commission, Directorate General XII E-2.

… The research team is composed of seven European Participants:

• Universidad Complutense de Madrid, Department of Chemical Engineering, Madrid (Spain).
• University of Athens, Department of Biology, Institute of General Botany , Athens, (Greece).
• Forschungsinstitut für Wasser und Abfallwirtschaft an der RWTH Aachen EV. Aachen (Germany).FiW
• Westfalia Separator AG, Oelde (Germany).
• Cognito Quam Electrotechnologies Ltd., Melissia Attikis (Greece).
• Wye College, University of London, Microbiology Lab., Dept. of Biological Sciences, Wye (England).
• Harokopio University, Dept. of Nutrition, Dietetics and Food Science, Athens (Greece).

  
  From “The Olive Oil Source”:

Cleaning the Olives
Grinding the Olives into a Paste
Malaxing (Mixing) the Paste
Separating the Oil from the Vegetable Water and Solids
Optional Steps (Depitting, Additives, Additional Processing)

The basic steps in making olive oil are always the same, no matter what kind of equipment is used, from The Olive Oil Source First Press to very large commercial mills built to process many tons of olives per hour. Before trying to understand the pros and cons of different machinery and techniques, it is important to understand these basic principles. Check out the Figueroa Farms slide show to see images of the oil extraction process.

CLEANING THE OLIVES
The first step in the oil extraction process is cleaning the olives and removing the stems, leaves, twigs, and other debris left with the olives. The olives should be washed with water to remove pesticides, dirt, etc. Rocks and sand will damage a hammermill and quickly wear out a centrifugal decanter or oil separator, reducing life span from 25 to as little as 5 years. It is amazing, and sometimes entertaining, to see what can be found in the bins with the olives. We have heard millers talk not only about rocks and branches, but broken glass, rings, bracelets, pieces of metal, knives, and even razor blades. Light contaminants are removed by a heavy air flow (blower) and heavy objects sink in the water bath.

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GRINDING THE OLIVES INTO A PASTE
The second step is crushing the olives into a paste. The purpose of crushing is to tear the flesh cells to facilitate the release of the oil from the vacuoles. This step can be done with stone mills, metal tooth grinders, or various kinds of hammermills.

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MALAXING THE PASTE
Malaxing (mixing) the paste for 20 to 45 minutes allows small oil droplets to combine into bigger ones. It is an indispensible step. The paste can be heated or water added during this process to increase the yield, although this generally results in lowering the quality of the oil. The most common mixer is a horizontal trough with spiral mixing blades. Longer mixing times increase oil yield but allows a longer oxidation period that decreases shelf life.

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SEPARATING THE OIL FROM THE VEGETABLE WATER AND SOLIDS

How is oil separated from vegetable water and solids?

The next step consists in separating the oil from the rest of the olive components. This used to be done with presses (hence the now somewhat obsolete terms first press and cold press), but is now done by centrifugation, except in old facilities. Some centrifuges are called three-phase because they separate the oil, the water, and the solids separately. The two-phase centrifuges separate the oil from a wet paste. In most cases, the oil coming out of the first centrifuge is further processed to eliminate any remaining water and solids by a second centrifuge that rotates faster. The oil is then left in tanks or barrels where a final separation, if needed, happens through gravity. This is called racking the oil. Finally the oil can be filtered, if desired.

Now watch the entire old fashioned pressing from the “Old World”:
http://www.gloria.tv/?media=381655