Olive oil extraction
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HOW OLIVE OIL IS OBTAINED
The olive oil production process takes place in special facilities called OIL MILLS.
The difference between extracting oil from various types of seeds (sunflower, soybean, sesame, peanut, castor, flax, etc.) and extracting oil from olives is that in the latter case, the oil is obtained by squeezing a fruit (drupe) and not a seed.
Olive oil is found inside the so-called lipovacuoles of the olive pulp cells. With the extraction process there is therefore the separation of the liquid part (water contained in the cells), the oily part (proper oil) and the solid part.
After the olive harvest has taken place (see the olive harvest page) the following steps are:
Sorting is the operation that frees the olives from any earth, leaves, mud, branches, etc. This operation can be carried out in the field or at the oil mill with special rotary machines equipped with a perforated rotating drum that allow the olives to fall free from impurities.
The weighing is done at the time of delivery to the oil mill to know the quantity of olives delivered.
The olives, in the case of small batches, are stored in order to reach the sufficient quantity to carry out a processing cycle. Once the storage took place in jute bags. Today, not too large perforated boxes are used to allow good ventilation between the olives and avoid the start of fermentation processes. The storage times are generally short, again to avoid the onset of fermentation of the olives.
The olives through special conveyor belts arrive at the washing station where they are immersed in water in special tanks or in special washing machines to have a clean final product. This phase is very important to finally have a quality oil and therefore the water used must be changed often. At the end of the washing the olives are simply drained from the water.
5. MILLING OR CRUSHING OF OLIVES
This is the actual start phase of the oil extraction. In fact, there is the crushing of the olive with the breaking of the pulp and the stone with the consequent leakage of cellular juices and oil. The product obtained by this operation is called oil paste and is composed of:
- a solid part made up of fragments of pulp, peel and stone;
- a liquid part formed by water and oil.
The systems used to obtain the oil paste can be of two types:
to) classic crusher system with the muller
The muller is formed by a basin, once made of granite today in stainless steel, where the olives are poured and where there are 2 or 4 granite stones (slaughterhouses), placed perpendicularly to the basin which, rotating, crush the olives. In this phase the action of the so-called is important servants they are nothing more than simple stirrers that bring the dough back under the stones at each turn. The rotation is very slow, carrying out from 12 to 15 revolutions per minute for about 20-40 minutes. Generally 2-3 ql of olives are processed in one cycle. This is in fact the quantity sufficient to put the olive paste in the extraction system.
Three stone muller
However, the release of the juices is not caused by the crushing of the olives but by the rubbing of the pieces of the stone, sharp, which break the cells making the juice escape. The task of the stone is therefore to crush the stones, not the pulp. In fact, the stones are not placed on the bottom of the tank but are 2-3 mm apart (however adjustable according to the type of olive) precisely to prevent excessive crushing of the stone, leaving it of the appropriate size to perform its oil extraction work.
The muller is equipped with blades that remove the dough that adheres to the wheels and improve the mixing of the dough itself.
Once the muller was moved by donkeys or horses. Today the system is motorized.
- this type of pressing involves a low degree of oil emulsification with high qualitative and quantitative yields;
- the oil has a less spicy and bitter taste than the crusher method (see below) because the amount of polyphenols is lower.
- oil paste has a higher exposure to air due to the longer extraction times and this can be harmful. However, the quality of the final product is still high.
b) crushing system with the crusher
The crusher is the system used in the most modern continuous cycle plants. The olives are poured from above by means of belt elevators that collect the olives that come out of the washing machines.
The crusher consists of a series of rotating discs with sharp edges (hammers) that rotate at a speed of 1200-3000 rpm. In this way the olive breaks thanks to the collisions with the rotating devices at high speed and only in part by the mechanical action caused by the fragments of the core.
The processing is very fast and is in a continuous cycle with automatic loading and unloading.
- the oil obtained is richer in chlorophyll and more stable during storage;
- the system has a considerable hourly working capacity;
- there is a perfect integration of the different processing phases.
- the fact that the pressing is more rapid and violent causes a very high degree of emulsification between water and oil, therefore the yields are lower;
- there is a much more intense extraction of polyphenols and this results in an oil with a spicier taste and a bitter aftertaste. However, if this taste is associated with olives that already have a spicy and bitter taste in themselves, it goes without saying that the final quality of the oil will not be good with a consequent depreciation.
6.KNEADING OR KNEADING
The malaxing is carried out with machines called kneaders or kneaders. This is the process that follows the pressing and has the purpose of breaking the emulsion between water and oil so that the oil particles spontaneously separate from the water.
The malaxers are steel tanks inside which helical blades rotate at a speed of 20-30 revolutions per minute. The oil paste is mixed inside this tank: in this way the emulsion between oil and water is broken.
There are different types of malaxers with blades arranged in series or in parallel. In any case, the machine is equipped with a heating system consisting of the passage of hot water in a cavity to heat the oil paste and facilitate the separation between water and oil. The temperature ranging from 22-24 ° C to 27 ° C and not higher than 30 ° C is fundamental. Obviously the higher the temperature, the better the yield but the worse the quality of the oil. In fact, the oil in this case will acquire a metallic taste. The loss of the fruity taste is due to the loss of volatile substances and the loss of polyphenols, vitamin A and tocopherols (vitamin E).
In the case of obtaining oils for direct consumption 30 ° C must not be exceeded while if rectified oil is produced (oil subjected to both physical and chemical operations), then we work with temperatures higher than 30 ° C to have higher yields.
The malaxing takes 20-40 minutes. Longer times damage the final oil because the pasta would remain exposed to the air for too long, causing oxidation. One realizes that the kneading is over when touching the dough with the hands it is greasy to the touch and does not stain even if today, the most modern systems have sensors for the control of the working process.
However, a good malaxing also depends on how the milling was done. The milling done with the classic method (with the muller) has a low degree of emulsification of the oil paste therefore a cold kneading is sufficient to have a good oil yield. If, on the other hand, the milling is done with the pressing then there is a more stable emulsion of the oil paste that needs to be heated up to 28-29 ° C to have acceptable yields.
7.EXTRACTION OF OIL
This phase consists in the separation of the solid part called pomace (consisting of stone fragments, peels and pulp fragments) from the liquid one.
The extraction methods can be of three types:
1. Extraction by pressure with presses
This is the oldest and most traditional method of olive oil extraction. The extraction takes place by mechanical squeezing as the olive paste is placed on top of vegetable or synthetic fiber discs (called friscoli), interspersed with stainless steel discs (to make the pressure more uniform). Pressure is exerted on this pile thanks to a hydraulic press. The pressure that is exerted is increasing and it takes about an hour to have all the water come out oily must (vegetable oil and water) while the wet pomace (solid part) remains adherent to the friscoli.
Extraction by pressure with presses
This type of extraction involves a fair use of manpower compared to other systems but the oil obtained is of excellent quality provided that the pressing discs are washed often (at least every 2-3 days). Otherwise, enzymatic processes and anomalous fermentation begin which will give the oil unpleasant flavors.
2. Extraction by centrifugation (decanter)
Extraction by centrifuge is obtained with a single procedure that allows to obtain at the same time: pomace, virgin olive oil is vegetation water.
We use machines called decanters which can have two or three phases. The operating principle is identical: it is in fact a centrifuge which, turning at 3600 rpm, allows to obtain:
- two-phase decanter: very wet pomace and oily must (with the addition of a little water to emulsify the olive paste);
- three-phase decanter: very wet pomace, oily must and water (it requires a quantity of water added to the olive paste equal to 20-30% of the paste itself).
In both cases, given the discreet use of water to make the paste fluid, the pomace will be very moist (higher in the three-phase decanter) and therefore difficult to process. The oil obtained is also richer in polyphenols and therefore more bitter. Furthermore, a fair amount of vegetation water is obtained, the cost of disposal of which is not indifferent.
3. Sinolea extraction or percolation
It is a cold extraction method without the addition of water that involves the use of machines equipped with a series of very dense combs, formed by many stainless steel strips, which are immersed in the olive paste while moving slowly into the dough. With each pass, these blades collect a small amount of oil. In practice, the water and the oil separate because the oil, due to surface tension, adheres to the steel blades and is then dripped into a special container until the two liquids are completely separated.
- since with this method there is no mechanical operation on the olive paste and therefore on the oil, the oil obtained has very high qualities both in terms of fragrance and organoleptic characteristics.
- with this method it is not possible to extract all the oil contained in the paste, consequently it is necessary to pass the residue of the extraction in a decanter;
- it is almost impossible to wash the machine except by stopping the system with all the consequences that this entails. Therefore the residues of days and days of processing affect the quality of the oil;
- it has little processing capacity, no more than 25 q / h
For all the disadvantages listed above, this method is not actually used.
8.SEPARATION OF OIL FROM WATER
During the previous steps, an oil was obtained which contains about 20% of water which must be separated from the oil. This can be done in several ways:
1. By decanting
This is the traditional method with which in the past the oil was separated from the water. In fact, they exploited the properties that water and oil have not to mix with each other. The mixture was left to rest and then the oil that emerged on the surface was removed. Decanting today is an abandoned method as it is not very suitable for obtaining quality products.
2. For vertical centrifuge
Today the vertical centrifuge is present in almost all plants (with the exception of oil extracted with the sinolea). It is a machine made up of a cylindrical tank that contains a rotating drum made up of a series of overlapping conical perforated discs in stainless steel. The oil must is poured from the top and is subjected to centrifugation at 6000-6500 revolutions per minute. Due to the different oil / water density, the two parts separate and take two different paths. During the rotation there is also the separation of small solid residues (sludge) which are then expelled.
The types of oils obtained from this type of extraction are:
- extra virgin olive oil, the most valuable and obtained exclusively from the mechanical pressing of the olives;
- virgin olive oil always obtained with the mechanical pressing of the olives but in the second pass;
- olive oil, the least valuable, because it consists of a mixture of virgin olive oil and rectified oil, that is oil extracted by chemical means from pomace (the waste from pressing).
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