Olive oil processing
The extraction of olive oil is an agro-food transformation process for getting oil from the drupes of the olive tree. Since ancient times, all the peoples who lived on the Mediterranean Sea knew the unique and extraordinary value of olive oil (Given a crucial condiment), taking the populations above all for the extreme simplicity with which it was extracted. Today, thanks to technology, techniques that have been handed down for centuries have been improved. This transformation, carried out in a structure called olive oil mill (Oleificio or frantoio), takes place in two fundamental phases: pulp milling and the subsequent separation of the oily fraction from the other components (Water-Olive pomace).
OLIVE OIL PRODUCTION PHASES
The modern techniques used for the extraction of olive oil consist of different applications in the individual processing phases. Below, we will see the most common ones, also used for the production of our extra virgin olive oil:
The olive harvest is the first phase of the extra virgin olive oil production process. It is done by carefully evaluating the state of ripeness of the fruit. The techniques used can be different, both manual and mechanized. We prefer manual harvesting (Bacchiatura), as it is considered of great importance to preserve both the quality of the fruit and the health of the tree. This technique consists in using a comb which, passed over the branches, causes to fall olives onto a large net specially placed under the trees. Subsequently, most of the foreign matters (Branches, leaves, etc.) are manually removed, placing the olives in special boxes ready for transfer to the olive oil mill.
It is the phase the olives are separated from the rest of foreign matter like a piece of branches and leaves. In ancient times, there was a tendency to leave a few leaf in order to give the oil a fruity taste.
The weighing is done in the olive oil mill before putting the olives in the processing line, in order to calculate the oil yield. The quality of the final product depends on the state of conservation of the olives (Well ventilated environment), in fact, after this phase we proceed immediately to the milling, carried out within 24 hours of harvesting, avoiding fermentation processes that could compromise the quality of the oil.
Once the olives have been placed inside the hopper (The first step begins in this phase, as the olives are introduced into the processing line), they are subjected to further checks for the possible presence of other foreign matters and transferred with belt conveyor to the immersion tank (Where a forced movement of the water is maintained to improve the cleaning process). Subsequently, the olives are dried (With jets of compressed air), ready for the next phase: the milling.
Milling is the real extraction phase. The olives are subjected to mechanical actions that cause the breaking of the cell wall and membranes with the leakage of juices and oil, obtaining the so-called “oil paste”, composed of a solid part (Seeds, skins and pulp) and a liquid part (Water-oil). The systems used are basically of two types:
TRADITIONAL MILLING: Derived from the ancient millstones (Molazza), it’s composed of a tank with a granite or steel bottom and raised edges, with a system of 2-6 stone wheels (Granite) in an horizontal axis, arranged in pairs. The wheels are raised by a few millimeters respect to the bottom of the tank and it’s adjustable in order to obtain stone fragments of a suitable size. The operation takes place through the mechanical action by the rotation of the wheels on the mass being processed, causing the leakage of juices and oil, caused by the rubbing of the pieces of seeds on the pulp of the olives. The wheel function (In ancient times, moved by donkeys or horses, with considerable space; Currently, it is operated with a slow rotation electric motor: 12-15 rpm for 20/40 minutes) therefore, is to crush the seeds into suitable sizes and remix the mass being processed.
HAMMER MILL: It’s the most popular tool in modern times as it integrates perfectly with automation needs. Loading is done mechanically from above, with belt elevators, while unloading takes place from below, by pouring the oil paste into the kneading machines. It is made up of a series of rotating disks with sharp edges, which break the pulp and part of the seeds.
The kneading process has the purpose of breaking the emulsion between water and oil and making the oil micelles flow into larger drops which tend to separate spontaneously from the water. It is carried out with kneading machines, substantially composed of a steel tank in which helical blades rotate (20-30 rpm), slowly mixing the oil paste for about an hour, improving the oil yield in the following phase.
It consists in the separation of the liquid part (Oil/water) from the solid part (Olive pomace with mainly pulp, stone, peel) with systems that have different mechanical principles and technologies. The most popular are the following:
- PRESSURE EXTRACTION: Traditional method, the liquid part is separated from the olive pomace through filtration by means of a hydraulic press (Approximately 400 ATM), arranging the oil paste on layers alternating with circular filter diaphragms, perforated in the center (Usually nylon), so to form a “vertical tower” subsequently subjected to pressure (Arranging every three layers a diaphragm without oil paste and a steel disk, so as to distribute the pressure uniformly). The device used consists of a circular steel plate with slightly raised and shaped sides, placed at the bottom, which favors the outflow of the liquid part. Once the hydraulic press is activated, the liquid part separates from the solid part, flowing from the drainage system both externally and along the central part of the “tower”, accumulating inside the plate. After the extraction, the “tower” is disassembled and the olive pomace removed.
- CENTRIFUGING (DECANTER): The oil paste is subjected to centrifugation (3000-3500 rpm) in a conical rotating drum with a horizontal axis (Decanter) which, due to the different specific weight, separates the mass, also through the addition of a quantity of water. This system produces a low quantity of vegetation water, significantly reducing the polluting charge.
- “SINOLEA” EXTRACTION METHOD: The physical principle on which “Sinolea” is based (Also known as percolation or selective filtration, conceived in 1911), is the difference between the surface tension of vegetable water and oil: as a result, the oil tends to adhere more easily to a metal surface than water, which is separated by percolation. Basically, it consists of a tank containing the oil paste, in which a series of steel blades (About 5000) are immersed which, with a continuous alternating motion (Rather slow, 7-9 rpm, and low yield), cycle through the following stages:
- IMMERSION: The steel blades immerse themselves in the oil paste, taking the liquid part;
- LIFTING: At each lifting cycle, the device causes to drain of vegetation water by gravity while the oil adheres to the metal surfaces;
- OIL SCRAPING: During the return cycle, the metal surfaces come into contact with a oil scraper device, which removing and making it flow into a collection system.
The oil extracted with this system is without water and does not require the subsequent Oil-Water separation phase.
The liquid mass obtained still contains a residual quantity of water, which will be separated in this phase due to the effect of the different density, by decantation or centrifugation:
- DECANTING: Traditional method is based on the non-miscibility of Oil/Water, making the oil “Rest” which, being lighter, emerges on the surface separating itself from the water, requiring, however, long time of standing in special tanks. Decanting is almost completely abandoned as it is not suitable for obtaining quality products.
- VERTICAL CENTRIFUGING: Currently, it is the most widespread system. It consists of a cylindrical tank containing a rotating drum made up of a series of perforated and overlapping conical discs which, by subjecting the Oil/Water liquid to centrifugation (6000-6500 rpm), due to the different density it is separated into two outflows. In this phase, there’s also an accumulation of solid residues (Sludge), promptly expelled through a special system.
At the exit from the separator, the oil appears turbid and can preferably be subjected to filtration before packaging. Otherwise, it can be stored in special steel containers in contact with a nitrogen atmosphere to prevent oxidation and the sedimentation of the remaining residues present.
The indications contained on the labels, regarding the methods of extraction of the olive oil, are information to make people understand the quality level of the product. Therefore, we can say that there are two different extraction methods, different in yield and quality (According to current legislation):
- COLD PRESSED: When during all the production phases, it has never been subjected to temperatures higher than 27°C (Ideal temperature would be 25°C). In this way, you have lower yields, preserving the organoleptic qualities, in order to obtain a product of the highest quality and rich in polyphenols;
- HOT PRESSED: Defined in this way because, in some phases of production described below, it is subjected to temperatures above 27°C (about 35°C-40°C), increasing its yield to the detriment of the organoleptic properties and chemical characteristics of the oil:
- KNEADING: Equipped with an external “blouse”, overheating up to 40°C favors the emulsion of the oil, with a significant increase in yield;
- DECANTER’S EXTRACTION: In this phase the decanter develops temperatures around 40°C and, in some cases, hot water is also used, with consequent washing of the polyphenol;
- WASHING AT THE END OF PRESSING: Hot water is used in this phase, negatively influencing the finished product.
TYPES OF OLIVE OIL
This graph helps to better understand the aspects of the subdivision of olive oil into categories, therefore, it is essential to read the label, for an informed purchase (In the case of olive pomace, after appropriate drying, it can be subjected to further extraction process with chemical treatments, with an oil yield of around 3%-6%):