Olive oil extraction is the process of extracting the olive oil present in olive drupes. Olive oil is produced in the mesocarp cells, and stored in a particular type of vacuole called a lipo vacuole, i.e., every cell contains a tiny olive oil droplet. Olive oil extraction is the process of separating the oil from the other fruit contents (vegetative extract liquid and solid material). It is possible to attain this separation by physical means alone, i.e., oil and water do not mix, so they are relatively easy to separate. This contrasts with other oils that are extracted with chemical solvents, generally hexane.F. D. Gunstone, Vegetable Oils in Food Technology: Composition, Properties, and Uses, 2002, CRC Press, 351 pages The first operation when extracting olive oil is washing the olives, to reduce the presence of contaminants, especially soil which can create a particular flavor effect called "soil taste".
Olive presses
Roman era olive presses survive to the present time, with a notable collection present at Volubilis in Morocco but many others across the Mediterranean and Europe (e.g. Italy).C. Michael Hogan, Volubilis, The Megaoithic Portal, ed. Andy Burnham (2007) 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.
Olive presses were traditionally built within walled structures.Columella, De Re Rustica (Book 1, chapter 6:18): "The press-rooms especially and the store-rooms for oil should be warm, because every liquid is thinned with heat and thickened by great cold; and if oil freezes, which seldom happens, it becomes rancid. But as it is natural heat that is wanted, arising from the climate and the exposure, there is no need of fire or flame, as the taste of oil is spoiled by smoke and soot. For this reason the pressing-room should be lighted from the southern side, so that we may not find it necessary to employ fires and lamps when the olives are being pressed." Traditional olive-presses consisted of a crushing basin which is a large, cylindrical millstone mounted by an upper milling-stone (crushing stone) used to grind the olives and their pits into a pulp.Mishnah (Baba Bathra 4:5), which reads: "He that sells an olive-press has ipso facto sold therewith the lower millstone [attached to the ground], and the upper milling-stone [used for grinding], and the [upright] posts, but he has not sold the pressing boards [used to hold the stone weight], nor the lever wheel (winch), nor the [lever] beam.""The lower millstone, etc." Translation here follows the interpretation of the Hebrew words = yam, and = memal, as explained by Mishnaic exegetes Hai Gaon (939–1038) on Mishnah (Taharot 10:8); Maimonides on Mishnah Baba Bathra 4:5 and in his Mishne Torah (Hil. Mekhirah 25:7) as pointed out by Rabbi Vidal of Tolosa; and by Nathan ben Jehiel (1035–1106) in his Sefer ha-Arukh, s.v. ; Obadiah Bartenura in his commentary on Mishnah (Maaserot 1:7), as well as by Moses Margolies (1715–1781) in his commentary P'nei Moshe on the Jerusalem Talmud (Baba Bathra 4:5), and by Nissim of Gerona on Rav Alfasi's commentary on Avodah Zarah 75a, among others. See also , s.v. Formerly, the upper milling-stone was rotated by a beast of burden harnessed to a wooden beam, or horizontal shaft, and which turned the crushing stone. The draft animal which rotated the horizontal shaft was always blindfolded to prevent it from getting dizzy. After which, the pulp was collected and kneaded. It was then placed within frails (being no more than flexible, woven baskets made of thick fibrous material, usually of rushes, palm fronds, hemp or willow splints), stacked one on top of the other, to which was applied a stone weight to release the oil from the pulp.The stone weight, which could be as much as 400 kg, was lifted by means of a lever beam resting on a pivot (fulcrum) between two vertical posts. The extracted liquid which is obtained consists of oil and vegetable water (amurca = watery lees) mixed together, and runs off into a pit. After settling, the oil rises to the surface and is removed by way of decantation. Filtering the oil produces a clearer batch of oil. The olive residue that remained was used for lighting fires.
About 50–70 kilograms (110–155 pounds) of olives were spread within the [crushing] basin and crushed for 30-50 minutes. Sometimes water would be added to facilitate the movement of the crushing stone. From time to time a worker would push the olives into the path of the moving stone with a spade-like tool.
The following basic method is still widely used today, and remains 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 at a corporate oil mill. The olive paste generally stays under the stones for 30–40 minutes. This has three objectives: Ensure that olives are well ground Allow enough time for the olive drops to join to form the largest droplets of oil Allow the fruit enzymes to produce some of the oil aromas and taste
In modern-day mills, after grinding, the olive paste is spread on fibre discs, which are stacked on top of each other, then placed into the mechanical press. In modern times, these discs are made of synthetic fibres which are easier to clean and maintain.
These discs 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). The applied hydraulic pressure can go to 400 atm. To facilitate separation of the liquid phases, water is run down the sides of the discs to increase the speed of percolation. The liquids are then separated either by a standard process of decantation or by means of a faster vertical centrifuge.
The traditional method is a valid form of producing high-quality olive oil, if after each extraction the discs are properly cleaned from the remains of paste; if not the leftover paste will begin to ferment, thereby producing inconsistencies of flavors (called defects) that will contaminate the subsequently produced olive oil. A similar problem can affect the grindstones that, in order to assure perfect quality, also require cleaning after each usage.
Grades of olive oil In the ancient Levant, three methods were used to produce different grades of olive oil. The finest oil was produced from fully developed and ripe olives harvested solely from the apex of the tree, and lightly pressed, "for what flows from light pressure is very sweet and very thin.", pp. 288–289 The remaining olives are pressed with a heavier weight, and vary in ripeness. Inferior oil is produced from unripe olives that are stored for extended periods of time until they grow soft or begin to shrivel to become more fit for grinding.Cf. Others are left for extended periods in pits in the ground to induce sweating and decay before they are ground., s.v. Mishnah Tohoroth 9:1 (Maimonides' commentary) According to the Geoponica, salt and a little nitre are added when oil is stored. Traditionally, freshly collected olives were laid up within a large tub () and sprinkled with salt before they were to be crushed in the mill, and which function served to heat-up the olives and to induce sweating, thereby bringing them to an advanced stage of ripeness, and make it easier to extract the oil once the olives are brought to the mill.Jerusalem Talmud (Ma'aserot 4:3, Commentary of Solomon Sirilio, s.v. )
Advantages and disadvantages Proper cleaning produces higher-quality oil. Grindstones, while ancient in design, are a suitable way to grind olives, because this method breaks up the drupe's pulp while only slightly touching the nut and the skin. This reduces the release of the oil oxidation enzymes present in these organs. In addition, in this extraction method, the introduction of water is minimal when compared to the modern one, thus reducing the washing-off of the polyphenols. The exhausted paste, called pomace, has a low content of water, making it an easier residue to manage.
Advantages Better grinding of the olives, reducing the release of oil oxidation enzymes Reduced added water, minimizing the washing of polyphenols Pomace with a low content of water easier to manage
Disadvantages Difficult to clean Non-continuous process with waiting periods thus exposes the olive paste to oxygen and light Requires more manual labor Longer production time from harvest to pressing
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, depitting machine or knife crusher. This paste is then malaxed for 30 to 60 minutes in order to allow the small olive droplets to agglomerate. 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 approximately 3,000 rpm, the high centrifugal force created allows the phases to be readily separated according to their different densities (solids > vegetation water > oil). Inside the decanter's rotating conical drum there is a coil that rotates more slowly, pushing the solid materials out of the system.
The separated oil and vegetation water are then rerun through a vertical centrifuge, working around 6,000 rpm that will separate the small quantity of vegetation water still contained in oil and vice versa.
Three, two, and two and a half phases decanters With the three phases oil decanter, a portion of the oil polyphenols is washed out due to the higher quantity of added water (when compared to the traditional method), producing a larger quantity of vegetation water that needs to be processed.
The two phases oil decanter was created as an attempt to solve these problems. Sacrificing part of its extraction capability, it uses less added water thus reducing the phenol washing. The olive paste is separated into two phases: oil and wet pomace. This type of decanter, instead of having three exits (oil, water, and solids), has only two. The water is expelled by the decanter coil together with the pomace, resulting in a wetter pomace that is much harder to process industrially. Many pomace oil extraction facilities refuse to work with these materials because the energy costs of drying the pomace for the hexane oil extraction often make the extraction process sub-economical. In practice, then, the two phases decanter solves the phenol washing problem but increases the residue management problem. This residue management problem has been reduced by the collection of this wetter pomace and being transported to specialized facilities called extractors which heat the pomace between 45 °C and 50 °C and can extract up to a further 2 litres per 100 kilos of pomace using adapted two-phase decanters.
The two-and-a-half-phase oil decanter is a compromise between the two previous types of decanters. It separates the olive paste into the standard three phases, but has a smaller need for added water and also a smaller vegetation water output. Therefore, the water content of the obtained pomace comes very close to that of the standard three-phase decanter, and the vegetation water output is relatively small, minimizing the residue management issues.