Olive oil extraction
Olive Oil extraction is the process of extracting the oil present in the olive drupes for food use. The oil is produced in the mesocarp cells and then stored in a particular type of vacuole called lipovacuole, i.e. every cell contain a tiny olive oil droplet. Olive oil extraction is the process of separating the oil from the other fruit contents (vegetation water and solid material). This separation is attained only by physical means, i.e. oil and water don’t mix so they're relatively easy to separate; as opposed to standard food oils that are extracted through the use of a chemical solvents (generally hexane).
The first operation when extracting olive oil is washing the olives, in order to reduce the presence of contaminants, especially soil which can create a particular flavour defect called “soil taste”.
Traditional Method – Olive Press
The Olive Press has been in use since Greeks first began pressing olives over 5000 years ago, it works by applying pressure to olive paste thus separating the liquids (oil and vegetation water) from the solid material. The two liquids are then separated by standard decantation.
Although this method is still widely used today, and if adequate precautions are taken it’s still a valid way of producing high quality olive oil, most Olive Oil today is produced using more modern methods.
First the olives are ground into an olive paste, using large grindstones. The olive paste generally stays under the stones for 30 – 40 min, this has three objectives:
- to guarantee that the olives are well ground
- to allow enough time for the olive drops to join to form the largest droplets of oil
- allow the fruit enzymes to produce some of the oil aromas and taste
In some olive oil mills a modern crushing method is used in conjunction with a traditional press, but this is a rare occurrence.
The olive paste is then spread on fiber disks, which are stacked on top of each other, then placed into the press. Traditionally the disks were made of hemp or coconut fiber, but nowadays they’re made of synthetic fibers which are easier to clean and maintain.
These disks are then put on a hydraulic piston, forming a pile. Pressure is applied on the disks, thus compacting the solid phase of the olive paste and percolating the liquid phases (oil and vegetation water). The applied hydraulic pressure can go to 400 atm.
To facilitate separation of the liquid phases, water is run down the sides of the disks to increase the speed of percolation.
The liquids are then separated either by a standard process of decantation or by the means of a faster vertical centrifuge.
The traditional method is a valid form of producing high quality olive oil, if after each extraction the disks are properly cleaned from the remains of paste; if not the leftover paste will begin to ferment thereby producing inconsistencies of flavours (called defects) that will contaminate the subsequently produced olive oil. A similar problem can affect the grindstones, that in order to assure perfect quality, also require cleaning after each usage.
Advantages and Disadvantages
Proper cleaning will produce higher quality oil. The grindstones, while ancient in design, are the more adequate way to grind the olives because it breaks up the drupes pulps while only slightly touching the nut and the skin. This reduces the release of the oil oxidation enzymes present in these organs. In addition, in this extraction method when compared to the modern one, the introduction of water is minimal, thus reducing the washing off of the polyphenols. The exhausted paste, called Pomace, has a low content of water making it an easier residue to manage.
Advantages
- Better grinding of the olives, reducing the release of oil oxidation enzymes
- Reduced added water, minimizing the washing of polyphenols
- Pomace with a low content of water easier to manage
Disadvantages
- Difficult cleaning
- Uncontinuous process introducing waiting periods thus exposing the olive paste to the action of oxygen and light.
- Requires a more hand labour
- Longer time period from harvest to pressing
Modern Method
The modern method of olive oil extraction uses an industrial decanter to separate all the phases by centrifugation.
In this method the olives are crushed by the use of a rotating mechanical mill that grinds the olives in to a fine paste. This paste is then malaxed for 30 to 40 min in order allow the small olive droplets to agglomerate. The aromas are created in these two steps through the action of fruit enzymes.
Afterwards the paste is pumped in to an industrial decanter where the phases will be separated. To facilitate the extraction process with the paste there is a certain quantity of water added.
The decanter is a large capacity horizontal centrifuge rotating around 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 decanters 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 trough a vertical centrifuge, working around 6000 rpm that will separate the small quantity of vegetation water still contained in oil and vice versa.
Three, Two and a half, and Two phases decanter
Over the years there have been introduced a few technological variations to the industrial decanter. The standard three phases oil decanter presents several problems, part of the oil polyphenols are washed due to the higher quantity of added water (when compared to the traditional method), which on the other end produces a larger quantity of vegetation water that needs to be processed.
The two phases oil decanter was created as an attempt to solve these problems. Sacrificing part of its extraction capability, it uses less added water thus reducing the phenol washing. The olive paste is separated in to two phases, oil and wet pomace. 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 pomace, resulting in a wetter pomace 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 aren’t compensated by the price of the extracted oil. So in practice the two phases decanter solves the phenol washing problem but increases the residue management problem.
The two and a half oil decanter is a compromise between the two previous types of decanters. It separates the olive paste in to 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 phases decanter, and the vegetation water output is relatively small, minimizing the residue management issues.
Sinolea
This is the most recent method to extract oil from the olives, rows of metal discs or plates are dipped into the paste; the oil preferentially wets and sticks to the metal and is removed with scrapers in a continuous process. It’s based on the different surface tension of the vegetation water and the oil, these different physical behaviors allow the olive oil to adhere to a steel plaque while the other two phases stay behind.
Sinolea works by continuously introducing several hundreds of steel plaques in to the paste thus extracting the olive oil. This process is not completely efficient leaving a large quantity of oil still in the paste, so the remaining paste has to be processed by the standard modern method (Industrial Decanter).
Advantages and Disadvantages
Advantages
- Higher polyphenol content of oil
- Low temperature method
- Automated
- Low labor
- Oil/water separation step is not needed
- Low energy requirement
Disadvantages
- Often must be combined with one of the above methods to maximize oil extraction which requires more space, labor, etc.
- Large surface areas can lead to rapid oxidation
- Sale of future machines currently outlawed in European Union due to difficulty with cleaning such large surface areas.
First Cold Pressed – Cold Extraction
Many oils are marketed as first cold pressed or cold extraction, this is a denomination describing the temperature at which the oil was obtained.
In the EU these designations are regulated by article 5 of Regulation 1019 of 2002. This article states that in order to use these designations the olive oil bottler must prove that the temperature of Malaxation and Extraction was under 27ºC (80ºF).
For olive oil bottled outside EU countries this regulation does not apply, and therefore the consumer has no assurance that these statements are true.
The temperature of malaxation and extraction is crucial due to its effect on olive oil quality. When high temperatures are applied the more volatile aromas are lost and the rate of oil oxidation is increased, producing therefore lower quality oils. In addition, the chemical content of the polyphenols, antioxidants, and vitamins present in the oil is reduced by higher temperatures. The temperature is adjusted basically by controlling the temperature of the water added during these two steps. High temperatures are used to increase the yield of olive oil obtained from the paste.
Consumer point of view
High quality olive oil can be obtained by all the methods if proper measures are taken. Olive oil quality is equally dependent on the quality of the olives themselves and on the time they have to wait from harvesting to extraction, in addition to the extraction method itself.
The 2 worst enemies of olive oil are: Oxygen and light. Once an olive is harvested, it should be pressed within 24 hours. Oxidation begins immediately upon harvesting. In the period between harvest and grinding, the olives are still a live organ and therefore being separated from the tree go under stress. During this period the fruits enzymes are very active, and the endogenous oil degradation increases, and therefore the obtained oil is of lower quality presenting a higher the acidity (oleic acid percentage).
In addition, if, during the extraction process, additional oxygen is allowed to interact with the olive paste, the acidity level will increase as well. Sealed extraction methods are best to prevent the continued introduction of oxygen, as well as light to the oil.
Lastly, after extraction of the oil is complete, the oil must be stored in cool stainless steel silo's that are pumped free of oxygen. This will ensure the quality of the oil and the integrity and stability of the chemical makeup of the oil.
Future Prospects
The future of olive extraction points to reducing the negative aspects of the present methods, decreasing the degradation oil produced by the extraction process in itself.
- Reducing the oxidation by performing part of the process of malaxation and the extraction under a controlled nitrogen atmosphere
- Extracting the nut of the olive before grinding, this will reduce the release of negative oxidative enzymes present in this organ, and the pomace doesn’t contain any wood residues making it possible to be used in animal feeding
- Reducing the addition of water to minimize the washing of polyphenols