On one hand, an "extra" category extra virgin olive oil can easily be produced at the oil mill, provided good quality, freshly harvested, raw material (olives), at a good stage of ripeness, is available. Yet, on the other, it is not always easy to obtain a product possessing the desired organoleptic properties unless all the production stages, from the farming techniques to the technology involved, are monitored and followed rationally and the aim kept clearly in sight.

We describe below the proper procedures to be followed at the oil mill in order to obtain high quality oil possessing the desired organoleptic properties.

Olive storage at the oil mill

Olive storage, which takes place after the harvest and prior to mechanical oil extraction, must not exceed 24-48 hours, using perforated boxes (25-30 Kg capacity), or perforated plastic bins (250-350 Kg capacity) because they are good for preventing the layers of olives from being too thick, since that can result in the fruit getting squashed, with an unavoidably negative effect on oil quality. Using plastic bins is rational and useful because they allow the olives to breathe, especially when leaves are present, and because they can be stacked to reduce the amount of space needed for storage. Finally, bins are useful because they can be moved by mechanical equipment, such as forklift trucks, thus cutting down and rationalizing labour times. Furthermore, olives should not be stored in great heaps in direct contact with the ground, while jute or plastic bags should be completely banned.

In any case, olives must be stored in roofed, cool, aired premises that are sheltered from the rain, excessive cold and, especially, from frost, which makes oil extraction difficult during the subsequent stages of processing. Olives should also be stored away from the odours (whether pleasant or unpleasant) of other activities and from high temperatures (which should never exceed 20 °C) and dampness, which could result in mould forming on the drupes.

Deleafing and cleaning the olives

Olives destined to produce high quality olive oil must be deleafed and washed before entering the processing cycle. This is usually done by a machine that carries out both operations sequentially. This stage is important not only for health reasons but also for technical-mechanical and oil quality purposes. Since the oil is a food product, it is not only proper but mandatory, in compliance with the most basic hygiene-health regulations, for the raw materials to be clean of any foreign objects, whether vegetable or mineral.

If the oil mill uses a metallic crusher to prepare olive paste and a centrifugal decanter to extract the oil, it is especially advisable to remove the vegetable matter, dirt or sand that often come with the olives. Leaves can affect the oil's organoleptic properties, while siliceous mineral particles can have an abrasive effect on the metallic parts of high speed rotating machines.

Naturally, olives are washed with drinking water, which is changed often enough to ensure a hygienic process and avoid bad smells that could affect the oil. Washers are equipped with a nozzle sprinkler for the final washing, using tap water to eliminate any risk of pollution and remove any potential organic residues used in plant protection.

Olive milling and crushing

Extracting oil from the olives using mechanical equipment is possible thanks to a series of technological operations intended to free the drops of oil from the vegetable tissues that contain them, and to increase their dimension until the oil is separated from the paste. The oil is contained in the cells of the olive pulp: most of it in the vacuoles; some in the colloidal system of the cytoplasm, and a minimal percentage in the olive epicarp and endosperm.

Milling or crushing the olives is the main (and also the most important) operation in oil extraction. This process determines the deeper, or not so deep, breaking up of the pulp cells that contain the oil and consists of using granite millstones or high speed rotating metallic crushers.

Millstones (between 2 and 6) have a low processing capacity, since olives are milled for a relatively long time, between 15 and 25 minutes, with a slow and non-violent action that guarantees a good paste and oils with harmonious and balanced organoleptic properties and milder notes of bitterness and spiciness. These properties are essentially a result of the smaller quantities of phenolic substances present in the oil when compared to oils extracted from pastes processed by stronger crushing methods.

Metallic crushers, on the other hand, can process a higher amount per hour, since olives are crushed very rapidly through a motion that can be more or less violent, depending on the type of crusher involved. Fixed metallic hammer and disc crushers are to be considered the most violent, while blade, movable hammer, conical and roller crushers act with less violence.

Oils obtained from pastes obtained by a metallic crusher are more bitter and spicy when compared to those obtained from pastes obtained with a millstone, since they have a higher overall phenolic content, which, aside from having antioxidant properties, gives the oil a bitter and spicy taste.

However, between the two types of crusher, there are no marked differences in the parameters that define the quality of virgin olive oils, such as free acidity, peroxides value and UV spectrophotometric absorptions, since these properties depend on the quality and health of the olives, rather than the processing techniques used in milling them.

Thus, different milling methods result in oils with different organoleptic properties and this can mean opportunities for producers who want to obtain a standard kind of oil as well as for those who want to obtain oils with different kinds of properties in order to satisfy customer demand.

Although oils with a medium-high total phenolic content, even bitter and spicy ones, are considered of good quality and of higher nutritional value, one must bear in mind that some consumers prefer oils with less intense organoleptic properties, with a sweeter flavour and ripe fruitiness, which blend better with their regular diet. Therefore, the flavour of virgin oils obtained from a single batch of olives can be altered (within certain limits) through crushing or milling, or by combining both methods, for an uniform or differentiated production.

Olive paste kneading

The olive paste, which has to be sent into the centrifugal decanter or a press or a percolator for oil extraction, is prepared by kneading it with a slow and continuous movement generated by the rotation of helical bands and blades soldered onto a horizontal axis that spins between 20-30 times per minute. Kneading machines must be hermetically sealed, and are usually shaped like a semi-cylinder, built in stainless steel and have an external casing where hot water flows in order to heat the paste to the appropriate temperature, relative to the extraction system in use.

Slow kneading of the olive paste helps gather smaller drops of oil together, which then form into larger drops that tend to separate into a continuous liquid phase. This increases the amount of free oil and therefore helps to increase the oil yield through the various extraction means used.

Technological kneading parameters (time and temperature) are important, since they can have an impact on oil yield and some of its properties. Proper and rational processing of the olives involves about 20-30 minutes of kneading if the paste is then pressed, and 45-60 minutes if a 2 or 3 phases centrifugal system is used. It is best to avoid kneading for an excessively long time, since this could favour the action of the enzymes, which exist naturally in olive paste, as they could have a negative impact on oil quality, especially as far as phenolic substances are concerned.

The kneading temperature must be controlled through the device's own thermostat, so as to properly prepare the olive paste and to avoid negatively affecting the oil produced. Regardless of what system the oil mill uses, kneading temperatures should never exceed 28-32 °C, since those values ensure the best extraction and oil quality.

However, if the producer wants to market oil with "cold extraction" or "first cold pressing" labels, temperatures throughout all of the production stages must never exceed 27 °C, as per the provisions of art. 5 of (EC) Regulation no. 1019/2002 of 13-6-2002.

Separation of oil from the other phases of the olive paste

Olive paste, prepared using the technological operations previously described, is ready to be separated from the oil, which is done by a mechanical system that separates the liquid phase from the solid one. Currently, the industrial methods used to mechanically extract oil from the olives are: pressure, percolation and centrifugal systems, which respectively use the principles of applied pressure (high force), surface tension (weak force) and centrifugal force (high force).

- Pressure system

This is the oldest system used to extract oil from olive paste, and in the past decades it has been perfected mechanically in order to allow a high efficiency extraction level and has been improved by adding a robot for preparing the tower to be pressed.

According to the quality and the state of hygiene and ripeness of the olives, the pressure system allows good quality oils to be obtained, which are usually harmonious and balanced from the organoleptic point of view, especially thanks to the use of granite millstones during the milling stage. Nevertheless, this system calls for a lot of care in cleaning the facilities and machinery, especially the mats (plastic fibre filtering diaphragms), so as to avoid polluting the oil with newly formed volatile substances originating from alterations in the organic matter caused by microorganisms found in the olive paste and in the environment.

- Percolation system (or selective filtration)

Oil extraction by percolation is done by dipping a sheet of steel into the olive paste. The oil adheres more to the metallic surface because it has a lower surface tension in relation to steel than that of vegetable water. This method, usually combined with centrifugation, although not very common, makes it possible to obtain the best quality oils in relation to the quality of the olives used, since the machine is inert and only works through the motion of the blades, without needing any other means like water or filters to draw out the oil.

This system also requires very careful cleaning of the basin where the blades and combs are, so as to avoid polluting the oil with volatile substances produced by the fermentation of vegetable residues that remain in the machine.

- Centrifugation system

This is the most widely used processing system in the field of oil making, and continues to increase in popularity. Continuous centrifugal processing with high speed (approximately 3500 revolutions/minute) horizontal axis decanters requires the addition of water in order to fluidise the olive paste (3 phases decanter). The centrifugal system first appeared in the 60s, and solved the problem of continuously processing olives, which is impossible with non-continuous systems like presses and percolation. This allowed increasing the productive capacity of oil mills while at the same time cutting down on costs.

From a quantity standpoint, the oil yield of a three-phase centrifugal system depends on the decanter's supply flow and the amount of fluidization water added. To obtain a satisfactory amount of oil, good oil mill practice recommends sending into the decanter an amount of olive paste that does not exceed 80% of the maximum set by the equipment's manufacturer.

From a quality standpoint, oil extracted with a three-phase centrifugal decanter is usually free of pollution from residues of previous olive batches and its organoleptic properties vary in relation to the olives used, the crushing method and the water added, since the latter removes part of the hydrosoluble phenolic substances of oil.

A few drawbacks of the three-phase centrifugal decanter (high volumes of vegetable water and oils with a lower phenolic content) have been overcome by two-phase (which only produce oil and olive pomace residue) or two-and-a-half phase (which produce oil, olive pomace residue and a little vegetable water) decanters. These new decanters can separate the oil from the olive paste using no, or very little, dilution water, and therefore the production of vegetable water can be avoided or only a limited amount produced (10-30 1/100 kg of olives), thus obtaining oils with a high total phenols content.

Bulk storage of virgin olive oil

After its production at the oil mill, virgin olive oil is still more or less cloudy even after separating the liquid phase using a vertical centrifugal separator. This is because there are small amounts of vegetable water and even vegetable microfragments that tend to settle after some time. Cloudy oil, while awaiting bottling, is stored in homogeneous batches in separate aerial tanks made from stainless steel, in storage facilities at temperatures between 13-18 °C, avoiding both low and high temperatures.

During bulk storage impurities settle on the bottom of the containers and should be removed. If the tank has a conical bottom, this can be done directly; otherwise, the oil should be decanted into another container.

So as to decrease the risk of oxidation, tanks should be filled with oil and the air volume should not exceed 2-3% of tank total volume. Under these conditions, the oxidation process is slow and the oil retains its original quality properties for several months. If a large portion of tank volume is taken up by air, oil oxidation is unavoidable; to avoid this, the container must be conditioned using an inert gas, usually nitrogen.