There are two systems that can be followed to extract olive oil from olives: the so-called traditional or discontinuous method, and the modern or continuous one. Read more on our article How is olive oil made?
Traditional extraction, or discontinuous method
In the traditional method, olives are crushed by big millstones (most commonly made of granite) at a moderate-low speed – this naturally helps to protect the paste from a relevant increase of temperature throughout the process; when the temperature is constantly kept under 27°C, the product can be defined as “cold-pressed” (n.b. “cold pressed” is not a synonym for “cold extracted”, more on the difference between the two terms here). The millstones rotate for 30-40 mins, during which the olive paste is formed; here begins the malaxing stage, which lasts for an additional 10-30 mins maximum once the millstones stop working. The olive paste obtained as a result is then spread over “fiscoli”, circular fiber disks piled up on top of each other (like in a giant sandwich) from which the oil can be extracted by the work of hydraulic presses: the pressure applied to the disks makes the solid phase compacting and the liquid phases (oil and vegetation water) percolating. After this phase, the olive oil produced is centrifuged, filtered (in most cases, at least) and bottled.
The traditional system is called discontinuous because the manufacturer has to stop the process to clean all the tools and machineries used at every new olive grinding: it is extremely important to sanitize everything accurately to produce quality EVO oil without defects. The use of the discontinuous system has many disadvantages that led many manufacturers to switch in time to the continuous system instead. It is important to keep in mind that oxygen, heat, and light are dangerous enemies of olive oil.
Disadvantages of the traditional systems for olive oil
The main disadvantages of the traditional systems are:
- Usually, different farmers bring their olives to the same oil miller. Given the considerable length of time of the extraction phase and consequential cleaning of the traditional mill, the olives are kept in their bags or containers for a prolonged period of time, a procedure which allows the start of fermentative degenerations.
- Millstones and fiscoli disks must be cleaned at every usage.
- The prolonged exposure of olive paste to oxygen: traditional grinding machines do not operate in a closed system; as a result, the oil produced starts to oxidate immediately, thus reducing the olive oil shelf life.
- The fibers from which the fiscoli are made absorb all the components of the product during the process, thus creating a favorable environment for microorganisms growth and fermentative events, even when the fiscoli are thoroughly cleaned after each use; this augment in the potential presence of defects may then be transferred to every future production process in which the fiscoli are involved.
Modern extraction, or continuous method
In the modern (or continuous) mill, all the extraction phases are conducted in a “closed” system, so that oxidation levels are kept at lower degrees than those typical of the traditional system.
First the olives are mashed by stainless steel crushers made of a body and toothed rollers or hammers. In modern systems it is possible to choose the rotation and speed of the machine, which will result in a difference in the characteristics of each olive oil produced (e.g. the use of a low frequency usually results in the production of less bitter olive oils than the ones manufactured at a high rotation frequency).
The malaxing lasts longer than in the traditional method, about 45 mins, as oil drops are smaller and need more time to aggregate. New malaxers operate in a controlled atmosphere (nitrogen) to slow down the oxidation process. In the continuous system, olive oil extraction is performed by centrifugation using either three-phase or two-phase decanters (to separate, respectively, oil, water and solids, or oil from wet olive pomace). Two-phase extraction is preferable as it does not require the extra supplement of water, which is instead needed when the three-phase system is used; water dilutes and eliminates polyphenolics substances, and creates additional toxic waste (vegetable waters).
The use of the continuous system implies a better control in sanitation, temperature and oxidation processes, and an increased work capacity than in the discontinuous method. The disadvantages are in the need to constantly control the temperature reached during the grinding phase, as due to friction the temperature tends to naturally increase. When the temperature of the product is controlled and maintained under 27°C throughout all phases of the process, the olive oil produced is defined as “cold extracted”.
Percolation, or Sinolea method
Percolation or Sinolea method is a third olive oil extraction method, much less common than the other two already described. In respect to the other two methods, the Sinolea system applies a different technology to separate the olive oil drops from water and solids; the process is based on the principle of different surface tension of the vegetation water and the oil: olive oil adhere to steel plaques, thus separating itself from water and solids. This process is not completely efficient as it still leaves a large quantity of oil in the olive paste.