Olive Oil extraction is the process of separating the oil from the fruit’s pit and pulp. It is possible to attain this separation by physical means alone (since oil and water don’t mix), so they are relatively easy to separate. This contrasts with other oils that are extracted with chemical solvents. The first stage for extracting Olive Oil is washing the olives, to reduce the presence of contaminants, especially soil which can create a particular flavour defect called "soil taste".
There are 3 basic methods (and one rare method) of Olive Oil extraction - summarized below:
The Traditional Method: The Olive Press
This method has been used for thousands of years, such as when Greeks first began pressing olives over 5000 years ago. An olive press works by applying pressure to olive paste to separate the liquid oil and vegetation water from the solid material. The oil and vegetation water are then separated by standard decantation. This basic method is still widely used today, and it’s still a valid way of producing high quality Olive Oil if adequate precautions are taken.
First the olives are ground into an olive paste using large millstones. The olive paste generally stays under the stones for 30 to 40 minutes. This has three objectives:
- to guarantee that the olives are well ground
- to allow enough time for the olive drops to join together, to form the largest droplets of oil
- to allow the fruit enzymes to produce some of the oil aromas and taste
After grinding, the olive paste is spread on fibre disks, which are stacked on top of each other, then placed into the press. Traditionally the disks were made of fibres from hemp or coconut, but today they are made of synthetic fibres for easier maintenance.
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).
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 and produce inconsistencies of flavours (called defects) that will contaminate the Olive Oil. A similar problem can affect the grindstones, so these also require cleaning after each usage to ensure perfect quality.
The advantage of the Traditional Olive Press is a better grinding of the olives, thereby reducing the release of oil oxidation enzymes.
The Modern Method: Decanter Centrifugation
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 to a fine paste. This can be done by a hammer crusher, disc crusher, de-pitting machine or knife crusher. This paste is then mixed or stirred for 30 to 40 minutes to allow the small olive droplets to combine into larger droplets. The aromas are created in these two steps through the action of fruit enzymes.
Afterwards the paste is pumped into 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 at approximately 3000 rpm. The high centrifugal force created allows the oil and the water to be easily separated because of their different densities (we all know oil and water don’t mix well). 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 at nearly 6000 rpm that will separate any other small quantities of oil from the water.
The advantage of Decanter Centrifugation is that it results in the highest percent of oil extraction.
The Sinolea Method: Cold Dripping
The Sinolea method is a cold pressing milling system, which employs no pressure or added water and was first introduced in 1972. It consists of a stainless steel comb or set of blades that dip into the olive paste. At every dip the comb (or blades) is covered in a thin layer of oil, which then gets wiped off into a bowl. It’s based on the different surface tension of the vegetation water (water that is naturally within the olive – not added) and the oil. The concept is borrowed from a very simple law of physics: liquids with different molecular cohesion have different surface tensions.
This can be understood with a simple home experiment. Place water in a cup and then put some oil in it. Dip a stainless steel fork into it and notice the oil will cling onto the metal but not the water. Patiently, let the oil drip off the fork into another empty container and you will have a separate container of only Olive Oil. It takes a very long time, but the argument is that the oil is more superior in quality due to less stress imposed on it during the process. This is also referred to as “cold dripping”.
In addition to saving the oil paste from too much mechanical handling, the Sinolea method acts as a mild bouquet and flavour enhancer in extra virgin Olive Oils by allowing them to retain the highest percentage of natural antioxidants and all of the various components that contribute to an Olive Oil's quality.
The advantages of the Sinolea Method are:
- it produces an oil of higher polyphenol content,
- it results in lower acidity levels in the oil
- the Olive Oil paste is not subjected to any temperature increase
- a higher olive oil quality
Olive Oils extracted with the Sinolea Method tend to be more costly because of the time involved, the small quantity of high quality oil produced, and the larger amounts of olives required compared to the other methods.
The Affiorato Method: Rising to the Surface
There exists another ancient method for olive oil extraction that is very simple, and extremely rare in today's Olive Oil world. This method kown as “affiorato”, meaning “rising to the surface”, is very labour intensive and produces a fantastic quality Olive Oil. For this process, olives are hand harvested, lightly crushed, and then placed in special containers. Then the artisans will wait for the oil to rise or “surface”. The oil, being lighter than water, will separate itself from the vegetal water present in the olive fruit. Once the oil has surfaced or risen, the “floating” oil is manually collected and skimmed off the surface using special jugs. The result is a particularly fresh and delicate olive oil. Today, there are only a few producers in Italy that still create the labour-intensive and traditional “affiorato” olive oil. Due to the artisan method used, it is common for sediment to appear in "affiorato"-style oils. These oils are very sought after by connoisseurs for their superb flavour, and usually demand higher prices.
The advantages of the Affiorato Method are:
- it produces an oil of extremely high quality,
- the Olives are not subjected to any temperature increase
- the Olives are subjected to minimal physical pressure