The extraction process consists in separating the oil from the rest of the olive components.
We use two different extraction processes:
1. The Traditional Method: The Olive Press
Pressing is the oldest method of the olive oil extraction processes.
The method for extraction is:
- First of all, apply pressure to stacked filter mats
- Secondly, smeared with paste
- Finally, central spike allows the expressed oil and water (olive juice) to exit.
The machinery is cumbersome, the process requires more labor than other extraction methods, the cycle is not continuous, and the filter mats can easily become contaminated.
Oil from presses falls into both extremes; producing the best olive oil when properly operated because they tend to have greater flavour and higher polyphenol content.
2. The Modern Method: Decanter continuous Centrifugation
The modern method of olive oil extraction process uses an industrial decanter to separate all the phases by centrifugation.
- Olives are crushed to a fine paste.
- Paste is pumped in to an industrial decanter where the phases will be separated.
- Water is added to facilitate the extraction process with the paste.
- The decanter is a large capacity horizontal centrifuge rotating approximately 3000 rpm, the high centrifugal force created allows the phases to be readily separated according to their different densities (solids > vegetation water > oil).
- Inside the decanter’s rotating conical drum there is a coil that rotates a few rpm slower, pushing the solid materials out of the system.
- The separated oil and vegetation water are then rerun through a vertical centrifuge, working around 6000 rpm that will separate the small quantity of vegetation water still contained in oil and vice versa.
Different kinds of centrifugation:
- Three phases (oil, water and solids). With the three phases oil decanter, a portion of the oil polyphenols is washed out due to the higher quantity of added water , producing a larger quantity of vegetation water that needs to be processed.
- Alpechín is the water contained in the olives and the added water.
- Orujo is the solid part, the remains of flesh, skin and stone.
- The two phases: This type of decanter, instead of having three exits (oil, water and solids), has only two.
- The water is expelled by the decanter coil together with the “orujo”, resulting in a wetter pomace “alperujo” that is much harder to process industrially. Many pomace oil extraction facilities refuse to work with these materials because the energy costs of drying the pomace for the hexane oil extraction often make the extraction process sub-economical.
- The two and a half phase oil decanter is a compromise between the two previous types of decanters. It separates the olive paste into the standard three phases, but has a smaller need for added water and also a smaller vegetation water output. Therefore the water content of the obtained pomace comes very close to that of the standard three phase’s decanter, and the vegetation water output is relatively small.
The olive paste that remains is still rich in oil and is squeezed again three more times.
The first oil pressure is the most valued, and it is obtained by squeezing oil of different qualities. 5 kilos of olives are needed to extract 1 litre of olive oil from first press. These oils are known by the generic name of virgin olive oil. The waste is known by the name of “orujo”.
Olive pomace (orujo) oil extraction
- The remaining paste (pomace) still contains a small quantity (about 5-10%) of oil that cannot be extracted by further pressing, but only with chemical solvents, mostly hexane. This is done in specialised chemical plants, not in the oil mills. The resulting oil is not “virgin” but “pomace oil”. The term “first press”, sometimes found on bottle labels, is technically meaningless, as there is no “second” press.
Refined olive oil
- This oil is obtained by refining virgin olive oils (not olive-pomace oils) that have a high acidity level and/or organoleptic defects which are eliminated after refining.
Stages of oil refining:
- Degumming: which eliminates the lecithin and gums.
- Fading: Color removal by activated carbon or by land absorbents such as bentonite.
- Neutralization: Elimination of the acid by treatment with alkali hydroxide, an operation called saponification, soaps of these fatty acids, obtained by addition of soda, are easily removable to be insoluble in the oil.
- Deodorization: with water treatments at temperatures between 160 and 180 ° C at high vacuum, which removes certain aldehydes.
This process is done in specific refineries.
Read also Olive Milling.