This work collects a global vision of the technology necessary for the extraction of virgin olive oil, comparing the different extraction processes, that is, the old three-phase centrifugation and pressure systems with the current two-phase centrifugation system.
The extraction of virgin olive oil is done today by two main methods:
- Pressure (traditional system) and
- Centrifugation (continuous system).
However, previously a device can be inserted that extracts a significant part of the loose oil obtained in the malaxer: Selective filtration (partial extraction).
1.- PARTIAL EXTRACTION
The physical principle behind selective filtration is the different surface tension (surface tension is the force that holds together the molecules of a liquid) of the oil and the vegetation water. Thus, the surface tension of the oil is less than that of the water, which means that when putting both liquids in contact with the pores of a filtering surface, the oil passes through them but not the water, since the force with the that the molecules of the latter bind.
There are currently two devices on the market to carry out this extraction on an industrial scale. The objective of both is not to obtain all the oil from the dough, but only part of it, the rest will be extracted by other systems. The factors that influence the amount of oil extracted through this system, whatever the industrial device used, can be summarized in three:
- Amount of loose oil present in the dough.
- Duration of the process, establishing an optimal time of 30 minutes.
- Other factors: characteristics of the machine, the filter surface, the revolutions of the extractors, the oil / water and oil / solids ratios of the mass, etc.
The oil obtained leaves with a humidity of 1% and must be centrifuged quickly.
In a market characterized by substitute products at a lower price, olive oil will only survive by enhancing its differential characteristics, be they chemical, organoleptic, nutritional, etc.
In the manufacturing process, we must avoid that these qualities disappear or are attenuated, so that we obtain a quality product that compensates with its higher price for the higher cost of raw material and the lower price obtained with lower quality oils.
In this commercial scheme, the introduction of a device that separates an important part of loose oil, at low temperature, without having been subjected to pressure, centrifugation, water additions and with a period of contact with the rest of the phases of the mass very short, it appears as very interesting to obtain a product that is suitable for that for which the market is willing to offer a higher price.
The oil from the partial extractor maintains better organoleptic characteristics, lower levels of acidity and greater resistance to rancidity. In addition, the following operational advantages of this system can be pointed out:
- -Low cost of installation and maintenance.
- -Possibility of including it in a pre-existing extraction line, either traditional or continuous.
- -Low need for additional labor and low energy consumption.
- It favors the subsequent treatments of the paste facilitating the exhaustion of the marc.
The laborious extraction with this system is rewarded with the high quality of the 60% oil obtained on average, which can be considered an excellent oil. On the other hand, having been obtained simply by dripping, it preserves the natural composition and organoleptic characteristics of the fruit.
2.- PRESSURE EXTRACTION
Historically, pressure extraction is the oldest and most used procedure for obtaining olive oil. The instruments used are hydraulic presses. The prepared paste is placed in thin layers on filtering material discs called capachos. The capachos are arranged one on top of the other, in a wagon and are guided by a central needle. This set of wagon, needle and blankets with their load of paste, constitutes the charge, which is subjected to each pressing operation. It is therefore a discontinuous system with charge formation, pressing and stripping.
The hydraulic press is made up of a low bridge embedded in the ground and a high bridge, joined by steel columns. The piston, located inside a cylinder attached to the low bridge and embedded in the ground or on the high bridge, receives the hydraulic pressure generated in a pump and transmits it to the cargo through the truck, which is driven by the columns in order to maintain its verticality.
Pressurized water is introduced into the pump box into a small diameter body, which is transmitted to the pump piston according to Pascal's physical principle, with the same intensity. As the piston section is greater, the force generated in the pump is multiplied by the ratio of piston and pump sections, resulting in much higher. The pressures exerted on the pump box range from 300 to 400kg/ cm2.
Modernly, the pump boxes are equipped with automatic manometers that entail to program the desired pressures. All this allows choosing and knowing the most suitable pressures in each case (depending on the type, variety, maturity, etc., of the olive), comparing the exhaustion achieved in the marc with the selected mode of operation.
The oil flow produced during pressing is positively influenced by the presence of a degree of humidity and a high percentage of incompressible solid matter (bone) in the paste, conditions that facilitate drainage of the liquid phases through the cake pomace.
It is usual to do the so-called "knocking", that is, when the maximum pressure has been reached, the pressure is removed, and when the charge separates from the upper part, pressure is reapplied. In the interval, the mass and the layers have swollen, some liquid outlet channels have been reconstructed and it is possible to extract a new fraction of oily wort.
The main drawbacks for the practical application of this system are the high labor costs, the discontinuity of the process and the costs inherent in using filter materials under optimal conditions.
3.- CONTINUOUS CENTRIFUGATION SYSTEM (3 PHASES)
After many years of research, a solution was found to the problem of oil extraction due to the effect of centrifugal force using machines that rotate at high speed. The first practical experiences of extraction by centrifugation of the pasta were carried out with the Perogio equipment and, later, with the Corteggiani system, consisting of a centrifuge with a large-diameter tub, with a capacity for 100 kilos of pasta, which rotated at 900 revolutions. per minute.
Due to the speed, and by adding water, the oil separated and was evacuated, along with the water, by the machine, while to empty the pomace it was necessary to stop it.
Figure 1.- Continuous system by centrifugation in three phases
A general scheme of the process can be seen in Figure 1. The horizontal centrifuge (decanter) that allows the solid / liquid separation consists of a cylinder-conical drum that can rotate from 3,000 to 4,000 revolutions per minute and has a body inside hollow, similarly, with a helical shoulder. Thanks to a small difference between the rotation speed of the drum and the worm gear (faster), the pomace comes out one end of the centrifuge and the oil and water the other way. The oily musts obtained (oil with little water and water with little oil) are definitively separated in the vertical automatic discharge centrifuges.
The separation of the solid phase from the liquids by centrifugation is carried out by adding water (more or less hot) to the olive paste. Given the possible incidence of the volume of added water on the oil yield, it is necessary to adjust the paste / water ratio to the type of apparatus but, above all, to the rheological characteristics of the olives. Excess or lack of water leads to decreased extraction performance. The optimal proportion is determined empirically by observing the characteristics of the oil and water at the decanter outlet. As a guideline we will say that it varies from 1: 0.6 to 1: 1.
Certain improvements aimed at reducing water consumption have been introduced, thus experimental tests for recycling alpechin in the decanter have demonstrated the feasibility of this technique and its positive effects on the extraction performance and phenolic content of the oil while the viscosity the alpechín is not so high that it makes the water circulation difficult.
The advantages of the continuous extraction system by centrifugation include:
- Limited volume of the machines.
- Semi-continuous cycle, with automated operations.
- Reduced labor needs.
- The acidity of the oil is generally lower.
- The oil yields will be compared in general to those obtained in traditional installations.
- The continuous system guarantees perfect hygiene of the process.
However, it also has negative aspects:
- Considerable investment costs.
- Doubts about the stability of the oils and their organoleptic characteristics.
The addition of hot tap water to pasta, as required by the three-phase centrifugal extraction system, results in the elimination of certain substances contained in the alpechines, among which are especially the natural antioxidants present in the oils. Recycling the alpechines, replacing the tap water, eliminates this drawback and also reduces water consumption and the volume of wastewater to be evacuated.
The results obtained thanks to recycling have promoted the introduction of the two-phase decanter.
Furthermore, this technique has been used for a long time for drying the sludge.
By means of this extraction system, it is no longer necessary to add tap water or it has been reduced to minimum quantities in the case of olives with low moisture content, with the additional advantage of eliminating the evacuation of alpechines, although still more moist marc are obtained.
The two-phase decanter works best with olives from the beginning of the season or freshly harvested, which is related to the humidity of the olive, which is higher at the beginning of the season and for fruit that has not been burnt for a short time. the authors suggest the convenience, when the humidity of the olive drops, to add water to the paste, either in the mixer, or by injecting it directly into the decanter, which does not have to be more than 10-15% of the weight of the fruit.
4.- EXTRACTION SYSTEM BY CENTRIFUGATION (2 PHASES)
Figure 2.- Continuous centrifugation system in two phases
An outline of the complete process can be seen in Figure 2, which allows us to compare it with the three-phase system.
Regarding the mass flow rate injected into the decanter, it occurs the same as in the 3-phase system: when the injection rate is increased, the industrial performance tends to decrease (Espínola and Moya, 1995).
However, while in the three-phase phase the greatest increase in losses occurs in the alpechin, in the two-phase phase, logically, it occurs in the marc. There is no difference between the oil cleaning process between two and three phases.
The water from the washing of the oils has a fat level so low that it is not worth re-centrifuging, eliminating a vertical centrifuge (cross centrifugation). In other aspects of the manufacturing process (milling and shaking) there are no notable differences either (Giovacchino, 1994).
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