Production Units and Factories

Olive oil production systems

The process of olive oil production can be subdivided in two main phases:

  1. preparation of a homogeneous paste and
  2. oil extraction and purification.

In the 1st phase, the olives are processed by means of grinding and mixing pulp and olive stone, followed by a heating process to further break down olive cells and to create large oil droplets. In the 2nd phase, oil is extracted by a press or a decanter. Water and solids are thus separated from the oil and further centrifuged in order to recover residual oil. Oil is purified through clarification by sedimentation or filtration by vibrating screens. OMWW streams are also clarified before disposal. Residual solids from the purification step are mixed with those coming from the extraction step. In modern olive-mills, extraction from the olive paste is based on the principles of:

  • pressing (traditional or classical system);
  • centrifugal (continuous system)
    • three-phase
    • two-phase
  • stone-removing process
  • percolation (selective filtering)
  • chemical separation
  • electrophoresis.

The last four methods are hardly used. In the pressing system and the three-phase continuous system the waste is a liquid waste product (OMWW) and a solid waste product; in the olive-mills of two-phases, the waste is a slurry waste (2POMW). A critical aspect of olive oil production is represented by high energy requirement of the milling process. According to the technology utilized in olive processing, namely pressing or continuous centrifugation systems, the amount of energy consumption, with reference to one ton of treated olives, is 40,000–50,000 and 48,000–65,000 KJ, respectivel (Niaounakis, M., and Ch. Halvadakis, Olive processing-Waste Management, Literature review and patent survey. Elsevier, 2006).

Olive Processing

1. Olive Harvest

  • While olives are on the tree the acidity level is <0.5%. Changes occur after harvest.
  • Harvest methods all vary on how much damage they do to the fruit. Hand harvest is the best, but very expensive. Mechanical harvest can be almost as good if done properly with the right equipment. The key is not to break the fruit skin in any way.
  • Harvest date is a maze of choices between type of oil desired, long-term stability of the oil, color, and linoleic acid content. For example, with Arbequina variety (Spanish), a one month delay in harvest can cause a four month loss in soil stability due to the drop in polyphenol content.
  • Olives should be stored in shallow bins for transport (to prevent smashing bottom fruit) and the bins must have ventilation holes (to reduce fermentation).
  • Ideally, olives should be processed within a few hours of harvest.

2. Handling fruit before milling

  • Fruit is classified and separated by quality. Fruit with defects is processed separately because a very small portion of bad fruit can ruin the whole batch. One liter of defective oil can ruin 1,000 liters of good oil
  • Olives that do not need washing are processed “as is” because extra moisture can cause problems. Only olives that have been harvested from the soil or require removal of copper, sprays, etc. are washed.
  1. Washed fruit has more moisture; water is really on the exterior of fruit
  2. Extractability is lower (6-7%) on washed olives if crushed in a hammer mill because an emulsion forms between the oil and water.
  3. Polyphenol content is lower in washed olives; there can be as high as a 49% loss in oil stability by washing olives
  4. Wash water is usually dirty and has a good chance of passing flavors into the oil
  5. Sensory rating for a defect is usually affected negatively with washed olives compared to non-washed olives. Oil from washed olives has lower bitterness rating, lower “piquant” rating and less fruity flavor.
  6. It is important that no fruit remains stuck in the bins, hoppers, etc., at the processing plant as it can ferment, and ruin the oil.
  7. Olives are stored for as short period as possible and at temperature between 40o and 45o F. Temperatures above 50oF can cause problems. Wet fruit is also much more likely to ferment than dry fruit.

3. Milling

  • Stone mills, because of their inefficiency, have been replaced by hammer mills in many plants and enterprises. The stones are also more difficult to be cleaned, and the long milling time can increase oxygen exposure and paste fermentation.
  • The hammer mill is easier to be cleaned and much faster, allowing for the deployment of a continuous flow system . Oil produced from a hammer-mill is generally greener, since the skins are broken up more efficiently. The disadvantages include the creation of an emulsion between the oil and the water, heat that is generated during the process, and metal flavors that can be introduced into the oil. The emulsification problem can be overcame by malaxation for a slightly longer period and the metal flavor problem has been generally overcome with new metal alloys that are much stronger. New stainless steel mills reduce oxidation and do not impart a metallic flavor into the oil.
  • The size of the hammer mill mesh is changed as the season progresses and the fruit becomes riper and softer. A smaller mesh screen is required to produce a finer paste from firm olives. This breaks the cells containing the oil more efficiently. As the fruit ripens, the cells break much more easily, a larger mesh screen can be used, and a more coarse paste can be worked out.

4. Mixing of the olive paste (malaxation)

  • The paste is slowly mixed, bringing small droplets of oil in contact with each other to form larger droplets. This improves the extractability of the oil. Optimally, the malaxator is designed to assure thorough mixing, leaving no portion unmixed.
  • Malaxation usually requires 45 min to 1.5 hours. More than 1.5 hours has shown no benefit in oil extractability. Moreover, the prolonged contact of oil and of the water content of the olives, reduces the final polyphenol content of the oil.
  • The paste temperature during malaxation procedure is very important. It should be between 80o and 86o F to improve the viscosity of the oil and to improve extractability.
  • D. Temperature values above 86oF may cause problems such as loss of fruit flavor, increase in bitterness and in astringency.
  • Sometimes the low extractability is owed to the moisture content of the olives. The solution is to let olives for a few days in a well ventilated area, raise the temperature of the paste, or add talc to adsorb the excess moisture. A paste moisture of <45% is easily processed but when moisture content is >50% then is more difficult to succeed satisfactory extraction.

5. Sinolea Separation

  • The Sinolea machine is a selective filtration process that extracts oil from the paste with stainless steel blades. It produces excellent oil that should be kept separate from oil extracted during other processes, to take advantage of its unique quality and value.
  • No contact with pads and no pressure (no pressing gives an advantage to quality).
  • The equipment is complicated and requires frequent cleaning, maintenance of the stainless steel blade mechanisms and a constant heat source to keep the paste at an even higher temperature. Extraction is topped when vegetable water appears in the oil.

6. Traditional press

  • The traditional press uses filtration mats and pressure to separate the liquid and solid portions of the olive paste. This method requires little energy but a great deal of hand labor. During pressing, the paste that is too fine can squirt all over the walls. The solution is to make coarse paste, keep the center tube clear, press at a lower pressure, and allow more time for the liquid to filter through the mats.
  • Cleanliness of the mats is extremely important. Each time the mats are used small particles of paste plug the filtration channels and can cause a loss of oil. The number one problem with the use of traditional presses is in getting fermentation defects into the oil from the mats. Mats can start to ferment if not used continuously or if not cleaned regularly. The solution is to wash the mats every day, or to use the presses continuously until harvest is finished.
  • Experiments in Spain indicated that oil from presses had an overall higher free acidity and a lower rating in sensory analysis. Pressed oils tend to have greater flavor and higher polyphenol content. Many defects such as fermentation of mats is higher in press systems compared to continuous flow decanters.

7. Decanters, 3-phase and 2-phase

  • Decanters are large horizontal centrifuges that separate the oil from the solids and water. It is the same process as in a decantation tank, just more rapid. The time saving increases the efficiency of the system, but also decreases the time of contact between oil and the fermenting vegetation water.
  • The decanters spin at approximately 3,000 rpm. Centrifugal force moves the heavier solid materials to the outside; a lighter water layer is formed with the lightest layer of oil on the inside. There is no exact line of separation between the three phases of solid, water and oil, so the solid phase usually contains some water, the water contains some oil and subsequently the oil contains water. In the latter case, and in order to extract the maximum oil quantity, an additional centrifugation is required to remove all of the vegetation water from the oil.
  • The 3-phase system decanter separates the paste into a relatively dry solid, vegetation water, and oil. Water is added to this system to get it to flow through the decanter. A minimum quantity of water is added to separate the solid material efficiently and to retain water-soluble polyphenols.
  • The system should be run at approximately 65 to 70% of the maximum capacity to get good separation of phases.
  • Samples are taken every hour and analyzed daily to determine the status of the separation. Preferably, the solid contents of no more than 6-7% oil and 50% moisture, while the vegetation water does not contain more than 0.3% oil and 8% solids.
  • The first 2-Phase system decanters were introduced in 1992. They are based on the same principle as the 3-Phase decanters, except that the solid and vegetation water exit together. There is no need for water addition.
  • Experience with the 2-Phase systems has shown that the system has some advantages, i.e. better retention of polyphenols because no water is added and less oil loss, if the system operates properly. One problem with the 2-Phase system is the greater potential to lose oil when the olives are chatracterized by low moisture content because there is a thinner interface between the two phases during centrifugation. Another difficulty can occur because there is less visual evidence of what is happening with waste characteristics because the solid and vegetation water phases are mixed.
  • When very pure oil (containing no water) is obtained from a 2-Phase system, it means that there is a loss of oil to the waste solids because of the limited separation area within the decanter. The solution is to extract oil with some water in it and immediately run it through a vertical centrifuge or two to clean the oil further.
  • Water can be added to the paste just before the entrance in the 2-Phase decanter if the moisture content of the olives is below 42%.
  • Talc (a water absorbing neutral compound) is added sometimes to the paste, early in the season, if the olives have an excessive moisture content.

8. Vertical Centrifuge

  • Vertical centrifuges spin at two times the velocity of a decanter and provides four times the separation force for the solid, water and oil phases. They provide either clean oil (no vegetation water) or clean water (no oil), but not both.
  • Ideally, the centrifuges are opened every hour to clean and remove solids that accumulate.
  • Fresh warm water is added to clean the oil, creating a larger interface area between the phases. Many processors use two centrifuges, one for the wet oil from the decanter and a second one to separate the oil from the wastewater of the first centrifuge. Added water is only 2-4oF warmer than the water/oil mixture to be separated.

Source of data

In this section it is possible to find information about production data and mills in the different countries involved in the project:

  • This table shows the production of olive oil in the Mediterranean basin expressed in tons.
  • This table lists the production areas of olive oil in Spain together with production data expressed in tons.
  • This table lists the production areas of olive oil in Italy together with production data expressed in tons.
  • This table shows the number of mills and their distribution in Spain.
  • This table shows the number of mills and their distribution in Italy.

IOOC publisizes upadated data on its web site. Visit the site of IOOC