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Vegetable water

Olive oil

Figure 1. Scheme showing a typical pressing process. The crushing and milling can be done with (a) a traditional mill with cylindrical stones, or with the most modern apparatus, (b) steel hammer, (c) mixer, (d) oil diaphragms on trolley, or (e) hydraulic press.

• It does not produce emulsions.

• It does not raise the temperature.

• There is no contamination from metals.

• It crushes the pits to the proper size and achieves malaxation.

However, it also has certain disadvantages:

• Low efficiency

• Discontinuous processing

• Need for highly skilled workers

• Increased oxidation due to lengthy air exposure of the paste

More recently, metal olive-crushing equipment, either roller, cylindrical, disc, toothed, or hammer, has been used. These work at much higher speeds (3,000 rpm) within a chamber that moves in the opposite direction at about 80 rpm. The advantages of these mechanical systems are:

• Continuous operation

• Higher capacity

Some drawbacks include:

• The high operating speed may leave a paste that is not properly prepared because the crushing is carried out quickly and may not be complete.

• Emulsions are created that are difficult to break.

• Alteration of the organoleptic characteristics of the oil induces bitterness.

• Wear and tear of the metallic parts results in contamination of the product.

Mixing (Malaxation). The mixing or malaxation of the paste resulting from the crushing involves stirring the mash slowly (—20 rpm) and continuously for 15 to 60 min. This process takes place in hemicylindric or semispheric water-heated (<30°C) stainless-steel vats with double walls and rotating blades. The mixing time depends on the crushing technique. When the traditional stone cones are used, 15 min at room temperature are enough, but when hammers are used, the mixing needs to be extended up to 60 min and the temperature increased to ~30°C. This temperature acts as an antiemulsionant and facilitates the ac-

tivity of enzymes in the olives, which disrupt the membranes that surround small oil droplets and prevent self-aggregation. Modern techniques include the addition of inert coadjuvants (talc powder) during the mixing step to facilitate the emulsion separation and of enzymes to disrupt the membranes.

The purpose of this operation is to break the oil-water emulsion and promote the fusion of the small oil droplets into droplets of a diameter greater than 30 //m, the minimal dimension required for oil separation in continuous phase. Even after optimal crushing, only 40 to 45% of oil droplets have a diameter >30 jum. This percentage increases to 80 to 85% following good malaxation. Therefore, this operation is essential to increase the yield of the extraction regardless of the techniques utilized. Droplets of smaller size (<30 //m) remain as emulsion, and they stay in the subproducts.

Temperatures >30°C, lengthy mixing times, or both will have a negative effect on the aroma of the oil (43), as well as its antioxidant and vitamin content.

Extraction. The olive oil is separated from the olive paste using selective filtration (partial extraction), pressure (traditional system), or centrifugation (continuous system).

Selective Filtration. Selective filtration is based on the lower superficial tension of the oil compared to that of the water. Therefore, when both liquids are put in contact with the pores of the filtering surface, oil will pass through whereas water will be retained. The objective of selective filtration is not the total extraction of the oil in the paste, but the extraction of the oil naturally separated during the mixing step. Therefore, the factors that influence the amount of oil extracted by this system depend on:

• The amount of "free" oil in the paste

• Duration of the process, with an optimal time of about 30 min

• The characteristics of the equipment (filtering surface, rpm of the extractors, oil/water and oil/solids ratios)

The oil obtained by this procedure has a humidity content of —1% and has to be centrifuged immediately.

Up to 60% of the oil in the paste can be extracted by this method. The product obtained from selective filtration maintains excellent organoleptic characteristics, low levels of acidity, and greater resistance to rancidity. Moreover, it has some other operational advantages, including:

• Low installation cost and maintenance

• Possibility of inclusion in previously established production lines (traditional or continuous)

• Low requirements in terms of labor and energy

• Facilitation of subsequent processing of the paste, thus improving the final recoveries

To understand the high cost of this product, is important to consider that the production of 1 L of olive oil by this process requires 11 to 12 kg of highly select olives, whereas only 5 kg are required for the extraction of 1 L of olive oil by the pressure processes described next.

Pressure. Pressure extraction the oldest procedure used to obtain olive oil. Originally, oil separation was achieved with pressure applied by humans or animals. Today, traditional olive oil mills use hydraulic presses. The olive paste is placed in thin layers (2-3 cm) over disks of filtration material (oil diaphragms). These diaphragm-paste layers are piled on top of each other and placed over a cart or trolley fitted with a central shaft to provide even distribution and support when these turrets are subjected to hydraulic pressure. This combination of trolley, shaft, oil paste, and diaphragms is subjected to pressure (300-400 kg/cm2), the oil is extracted, and the apparatus is disassembled. Therefore this is a discontinuous process. The factors affecting the pressure process include:

• The characteristics of the oil diaphragms, which depend in part on the olive kernels in the paste and the degree of dispersion and concentration of the colloid constituents of the olive paste

• The humidity of the paste, the size and shape of the particles, and the physical characteristics of the olive (type, variety, ripeness, and temperature)

It is common to include a second step. Once the maximum pressure has been achieved, the pressure is partially released, allowing sponging of the paste and the diaphragms and reopening of channels that after pressure is reapplied allows the extraction of additional oil.

This system produces oils of excellent quality due to the low temperatures used during the process; however, it has elevated costs due to labor, the discontinuity of the process, and the use of optimal filtering materials.

Centrifugation. The use of centrifugation for oil extraction is relatively recent and is based on the different densities of the oil, the water, and the solids (pomace). Some of the first practical experiences with this technique were carried out with the Corteggiani system, a centrifuge with the capacity to process about 100 kg of paste spinning at 900 rpm. Water was added to facilitate the oil separation, and the process had to be stopped for removal of the solids.

Currently the process is carried out with a horizontal centrifuge (decanter). This system allows continuous solid-liquid separation and consists of a cylindrical-conical bowl that spins at speeds of 3,000 to 4,000 rpm. The interior consists of a hollow component of the same shape containing helical blades. There is a small difference between the speeds at which the bowl spins and the inner screw gyrates. The faster speed of the latter results in the movement of the pomace to one end of the decanter and the olive and water to the other end. The oily must is then fed to a vertical centrifuge revolving at 6,000 to 7,000 rpm for the final separation of the oil.

The separation of the solid and liquid phases by centrifugation requires the addition of water to the olive paste. The amount of water and the temperature influence the oil yield, and it is necessary to adjust both factors for each type of equipment.

Some of the advantages of the centrifugation process are:

• Small size of the equipment

• Automated process with semicontinuous cycles

• Reduced need for highly skilled labor

• Lower acidity of the oil

• Similar yields to those obtained using more-traditional systems

• Stainless-steel materials decrease the risk of contamination by other metals

• No diaphragms are used, thus improving the hygiene of the process and lowering the risk of contamination

Some of the disadvantages are:

• The initial capital investment is high.

• The process requires large amounts of water, which pollutes the environment and removes a significant amount of natural antioxidants.

• The energy consumption is high.

• The organoleptic characteristics of the oil may be adversely affected.

A variant consisting of a two-phase centrifugal continuous system reduces the use of water and eliminates the production of waste water (alpechin) (see Fig. 2).

Classification of Olive Oil Grades

Virgin Olive Oil. Oil extracted from olives by mechanical or other methods that do not modify its basic properties is called virgin olive oil. It is a completely natural product that maintains the taste as well as the chemical and biological characteristics of the olive. Within the virgin grade are three recognized quality levels:

Extra. Oil with the best organoleptic characteristics and with an acidity level not exceeding 1% is classified as extra virgin. The highest-quality extra virgin olive oils have an acidity level of at least 0.4 to 0.5% to maintain the organoleptic characteristics and lower than 0.7 to 0.8% so as not to exceed the maximum legal level. Its production requires the greatest care and attention from the cultivation stage all the way through to final processing.

The olives, unaffected by parasites, need to be harvested at the ideal point of maturation. If the fruit is still unripe, the extracted oil will be too sour. If the fruit is too ripe, the oil will be too sweet. The olives need to be harvested by hand to avoid any damage from mechanical processes. They should not be piled but should be arranged in layers of 15 to 30 cm in rigid containers. They should not be stored; pressing should take place immediately after removal of leaves and other detritus. If storage is required, it should be done only for the briefest time at low relative humidity (50%) and temperature (10-15°C). The use of water needs to be limited, and any rise in temperature should be avoided during the extraction process to ensure the highest-quality oil. Extra virgin oil is extracted during the first phase of pressing and therefore comes directly from the pulp of the fruit, whereas any oil that comes out later will also contain oil from the pit of the fruit. The oil extracted from the press needs to be separated immediately from the sludgy dregs and should be stored in a cool place away from direct light. Laboratory analysis is insufficient to detect the organoleptic qualities of extra virgin olive oil; taste tests are necessary. Olive oil tasters are highly qualified experts who use strict and sophisticated methods (26,44-55).

The cost of production is higher than that of other vegetable oils. This is due to the high amount of manual labor required for the cultivation and harvest of the olives. A person can pick from 60 to 100 kg of olives in a day, and from these about 13 to 20 L of oil can be extracted. The high cost is more than compensated, however, by the high yield and the oil's distinctly superior properties.

Average. Average oil has a good taste and acidity levels not exceeding 3.3%. This may be classified as:

• Superfine virgin olive oil: Obtained by mechanical extraction from olives and having undergone washing, sediment removal, and filtering with no chemical manipulations. It should not contain more than 1.5% acidity.

• Fine virgin olive oil: Obtained by mechanical extraction from olives and having undergone no chemical manipulations but only washing, sediment removal, and filtering. It should not contain more than 3.3% acidity.

Strong (Lampante). Strong virgin olive oil has inadequate taste, or acidity levels above 3.3%.

Refined Olive Oil. Refined olive oil is obtained by refining virgin oil whose taste, acidity levels, or both make it unsatisfactory for direct consumption. This is a healthy and perfectly acceptable food product, but it does not have the full taste of virgin olive oil.

Olive Oil. Olive oil is made by blending both refined and virgin olive oil. This type of oil is very much a standard in the marketplace; its properties fall somewhere between those of its two components.

Adulteration of Olive Oil

The high market price and the well-known health benefits attributed to olive oil make this product a target for different types of fraud, such as adulteration and mislabeling. This is especially true for extra virgin olive oil, which in the international trade market reaches a much higher price than any other vegetable oil (see Table 5).

Several physical, chemical, chromatographic, and spectroscopic tests are in use to detect adulteration of high-quality olive oil with low-grade olive oil (56,57), hazelnut oil (57), and seed oils (58,59). These have classically included determination of iodine value, saponification value, viscosity, density, refractive index, ultraviolet absorbance, fluorescence, and colorimetric reactions. More recently, FT-Raman spectra (60,61), mid-infrared spectroscopy (62), C02 laser infrared optothermal spectroscopy (58), reversed-phase liquid chromatography coupled to gas chromatography (63), and NMR (36) have been reported to detect the presence of <1% of other oils in extra virgin olive oil. These tests rely on the detection in the olive oil of unusual amounts of certain fatty acids or other compounds

Three-phase process Olives Water

Two-phase process Olives Water

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