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Oil extraction and processing biotechnologies
CHAPTER 9
Oil extraction and processing biotechnologies 9.1
9.2 9.3 9.4
Olive oil extraction technologies 9.1.1 Traditional method 9.1.2 Modern methods 9.1.3 Extraction by selective filtration (Sinolea) 9.1.4 Summary Bio-enzymatic technology for improved olive oil extraction Negev olive oil extraction studies 9.3.1 Oil potential of Negev-grown olives Decantation, bottling and shelf-life, with an emphasis on Negev Desert conditions
223 223 225 229 230 231 233 235 239
Olive oil has a privileged position among edible vegetable oils because it is the juice of a fruit, and as such is consumed almost entirely in its natural state. Therefore, it contains (among other important constituents) phenolic compounds, which are usually removed from seed oils during the refining process. The olive oil extraction process refers to extracting the olive oil that is present in the olive fruits, botanically termed “drupes,” for food use. In olive fruits the oil is produced in the mesocarp cells, and stored in a particular type of vacuole called a lipovacuole (i.e., every cell contains an olive oil droplet). Hence, olive oil extraction can be said to be a process of separating the oil from the other fruit contents (vegetation tissue, water and solid material), and this separation is done only by physical means. This is totally different from other vegetable oils, which are extracted with chemical solvents (generally hexane). Since olive oil is present in the form of minute drops in the vacuoles of mesocarp cells in the olive fruit, as well as being scattered to a lesser extent in the colloidal system of the cell’s cytoplasm and, to a still lesser extent, in the epicarp and endosperm, it is possible for all the oil in the vacuoles to be released during processing. It is difficult to obtain oil dispersed in the cytoplasm, so this oil generally remains in the olive pomace – a by-product of processing (Kiritsakis, 1998). The olive oil extraction process is normally divided into crushing, malaxation (mixing), and separation of the oil. Below, we describe the main tasks that need to be performed, after harvesting the olives, in order to obtain good-quality olive oil. 221
222 Chapter 9 Delivery of the olive fruits After the olives have been picked in the proper manner, they are delivered to the mill and stored there in the yard in collection boxes. This offers the opportunity to determine the quality and rate of yield, and serves as the basis for the settlement of accounts between the oil mill and olive farmer. At harvest, there is generally 18–32 percent oil, 40–55 percent water and 23–35 percent seeds and vegetable tissue in olive fruits. Washing and cleaning of the olive fruits Stems, twigs and leaves should be removed, and the olive fruits may or may not be cleaned with water to remove pesticides, dirt, etc. Rocks and sand will quickly wear out a centrifugal decanter or oil separator, reducing its lifespan. Grinding, mixing or malaxation of olive pulp After cleaning, the olive fruits are ground up together with their stones. For grinding, either stone rollers or a metal tooth grinder or hammer mill can be used. Each has its own advantages and disadvantages. After this process, mixing or malaxation should be performed. Malaxation is the action of slowly churning or mixing milled olives in a specially designed mixer for 20–40 minutes. This mixing allows the smaller droplets of oil that were released by the milling process to combine into larger ones that can be more easily separated. The paste is normally heated to around 27 ◦ C during this process. It is now possible, with newer equipment, to use a blanket of inert gas (such as nitrogen or carbon dioxide) over the olive paste, which greatly reduces oxidation; this allows for an increased yield without compromising the quality of the oil. After malaxation is complete, the paste is then sent to a phase separator. Nearly all producers use a decanter centrifuge for this phase. The most common mixer is a horizontal trough with spiral mixing blades. Malaxation of the pulp is carried out, dependent on the oil extraction procedure – i.e., warm water may be added. To aid in further breakdown of the olive cells, and to create large oil droplets, the pulp is beaten. Salt is often added at this stage, to aid the osmotic breakdown of cells in the olives and thus ease the separation of the oil from the water. The olives are beaten several times. Separation of the oil and water In this step, separation of oil and water from the olive paste is carried out. Many procedures can be used to achieve this, and will be discussed later in the chapter; centrifuge/decanters, modern di- and tri-phase centrifuge, and percolation-Sinolea methods are common. In small oil mills, the olive oil is extracted in a batch operation using the traditional press method. The oil extracted is collected in containers and clarified by sedimentation. In a big oil extraction system,
Oil extraction and processing biotechnologies 223 large amounts of olive fruits are used for extraction by more modern systems. To improve the separation of oil and pulp, biological or chemical aids that attack the cell walls can be added. Separation of oil from the water This is a very important process, where the olive oil is separated from the olive’s water. As in cream separation in a dairy, the liquid is spun to separate the heavier water from the oil. Gravity decanter or vertical centrifuges with perforated conical discs are commonly used in this process. Extraction of residual oil After extraction of the first oil – generally virgin olive oil – the residual waste or pomace derived after first pressing or centrifugation undergoes further extraction by different means. The process of extraction of the residual oil differs according to the system originally used. The solid waste from oil extraction by pressing still contains about 6 percent oil or, using the continuous decanter, 4 percent olive oil. The oil content in the solid–liquid mixture from the dual-phase decanting process contains 2.5–3.5 percent oil. In specialized extraction plants, a solvent extraction process is used for this purpose. First, the waste is completely dried; hexane is then used as a solvent to extract further oil, known as olive-pomace oil. The utilization of by-products of olive oil extraction The dry residues can be used as concentrated fodder. In some extraction plants, the stones are separated from the pulp after extraction and used as fuel for heating the driers. The pulp is sold as fertilizer or fodder. In some oil mills, the solid waste from the press is directly used as fuel for the heating of water. Vegetable water, the liquid phase obtained as a result of centrifugation, is abundant in the threephase extraction method due to water injection into the paste before centrifugation. As vegetable water still contains oil, it is treated a second time in order to remove the maximum amount of oil. However, since the result is a combination of water and fat, it is difficult to recycle it. Vegetable water is highly polluting and has a negative effect on underground water. The most serious ecological problem in olive oil production is the recycling of vegetable water.
9.1 Olive oil extraction technologies 9.1.1 Traditional method The traditional method of olive oil extraction is by using an olive press. The olive press works by applying pressure to olive paste to separate the liquid oil and vegetation water from the solid materials. In this method, after separating the oil and water from the fruit paste, the water and the oil themselves are then separated from one another by a standard decantation procedure. This method has been in use since the Greeks first began pressing olives over 5000 years ago,
224 Chapter 9
Figure 9.1: Traditional olive oil press. Source: Wikipedia (www.wikipedia.org), 2007.
and is still widely used today, with some modification. Even now, it is considered a valid way of producing high-quality olive oil if adequate care is taken (Figure 9.1). However, this type of method is suitable only for small growers. In this method, after cleaning the olive fruits, the olives are ground into an olive paste using large millstones. The olive paste generally remains between the stones for 30–40 minutes in order to guarantee that the olives are well ground; this allows enough time for the olive droplets to join to form large drops of oil, and for the fruit enzymes to produce some of the oil aromas and taste. Some traditional oil presses also use modern crushing methods. After grinding, the ground olive paste is spread on fiber disks, which are stacked on top of each other, then placed into the press. Traditionally the disks were made of hemp or coconut fiber, but nowadays they are made from synthetic fibers, which are easier to clean and maintain. These disks are then threaded onto a hydraulic piston, forming a pile. Pressure is applied on the disks, thus compacting the solid
Oil extraction and processing biotechnologies 225 phase of olive paste and percolating the liquid phase (oil and vegetation water). In this system, the applied hydraulic pressure can be as high as 400 atm. To facilitate the liquid phase, water is run down the sides of the disks to increase the speed of percolation. The liquid is then separated either by a standard process of decantation or by the faster vertical centrifuge. The main advantages of the traditional method are that it produces good-quality oil by more efficient grinding of the olives, reducing the release of oil oxidation enzymes, and that it also reduces added water, which minimize the washing of polyphenols. The pomace obtained from this method has a low water content, so it is easier to manage. The disadvantages of this method include the difficulty of cleaning the disks. Also, since this method is not a continuous process, it means that there are periods when the olive paste is exposed to oxygen and light, which can cause deterioration of oil quality. Finally, the traditional method requires more manual labor, and also needs a longer time period between harvest and pressing. This traditional method, also called discontinuous extraction, is an ancient procedure that only distinguishes two phases by pressing or centrifugation. The liquid phase is filtered later in order to obtain oil. In this case the by-product is a plastic paste, which has the advantage of avoiding the production of vegetation water. However, although it is better ecologically, this technique provides a lower yield, which is not always seen as an advantage by the main producing countries. Nearly all olive oil producers in the traditional olive-growing regions, such as Spain, changed from the traditional presses to modern systems in the mid-1980s.
9.1.2 Modern methods The modern method of olive oil extraction uses an industrial decanter to separate all the phases by centrifugation. In this method, olives are crushed to a fine paste by a hammer crusher, a disk crusher, a depitting machine or a knife crusher. The paste is then malaxed for 30–40 minutes to allow the small olive droplets to agglomerate and the fruit enzymes to create aromas. After this the paste is pumped into an industrial decanter, where the phases will be separated. Depending upon the system, water might be added with the paste to facilitate the extraction process. The decanter is a large-capacity horizontal centrifuge rotating at around 3000 rpm; the high centrifugal force created allows the phases to be readily separated according to their different densities (solids > vegetation water > oil). The most common industrial processing method is a continuous extraction system with two centrifuges (first horizontal and then vertical). Over the years, a few technological variations have been introduced to the industrial decanter. In the three-phase decanter (oil, pomace and vegetable water), as shown in Figure 9.2, some of the oil polyphenols are washed out due to the higher quantity of water added, which produces a large quantity of vegetable water that needs processing.
226 Chapter 9
Figure 9.2: Flow diagram of the olive oil extraction process. Source: Unctad (www.unctad.org (2004).
The procedure shown in Figure 9.2 is the most widely used in intensive production areas, and dates back to the 1970s and 1980s. The main disadvantages of this process are the huge amount of water needed, which leads to production of large quantities of waste and the resultant pollution. In an attempt to solve this problem, a two-phase oil decanter has been created; here, no water injection is used and just the oil and a plastic paste are produced. Since no water is added in this two-phase system, there is less phenol washing and oil and hard pomace are obtained. Instead of three exit points, for oil, water and solids, this type of decanter has only two (Figure 9.3). The water is expelled by the decanter coil together with the pomace; resulting in a wetter pomace; this is much harder and needs to be processed industrially, leading to high energy costs so that the pomace can be dried in order to extract further oil from it. In practice, therefore, the two-phase decanter solves the phenol washing problem but increases the residue management problem. This has led to the creation of a new two-and-a-half phase oil decanter (sometimes still referred to as a two-phase decanter system). This separates the olive paste into the standard three phases, but has a lesser need for added water and thus a smaller
Oil extraction and processing biotechnologies 227
Figure 9.3: Three- and two-phase centrifugation system of olive extraction (Alburquerque et al., 2004).
vegetable-water output. This system is considered to be more economically and environmentally viable than the others. This new industrial technique of continuous extraction is commonly practiced these days and, because it needs less water, is being used more and more widely. A detailed comparison of results obtained from a two-phase extraction and a three-phase extraction is represented in Table 9.1. The study involved different varieties of olives, composed of 48–51 percent water and 19–23 percent oil. Olive oil obtained from the two-phase decanter system also contained higher amounts of polyphenols, artho-diphenols, hydroxytryrosol, tocopherols, trans-2-hexenal (the most important aroma component, with a herbaceous aroma) and total aroma volatile compounds than that obtained from the three-phase decanter. Moreover, the oil obtained from the two-phase decanter showed higher oxidative stability, lower turbidity, and lower contents of pigments, steroid hydrocarbons, waxes, and aliphatic and triterpenic alcohols than did the oil from the three-phase decanter. Some of the differences in the quality of oil obtained from two- and threephase decanter systems, as found by Di Giovacchino (1996) are shown in Table 9.2. This shows that the two-phase decanter system produced better-quality oil than the three-phase system.
228 Chapter 9 Table 9.1: A detailed comparison of results obtained from two- and three-phase olive oil extraction systems
Parameters Oil extraction capacity Pomace Quantity (kg/100 kg of olives) Moisture (%) Oil (%) Oil (% dry matter) Oil (kg/100 kg of olives) Dry pomace (kg/100 kg of olives) Vegetable water Quantity (l/100 kg of olives) Oil (g/l) Oil (kg/100 kg of olives) Dry residual (kg/100 kg of olives) Oil in by-products (kg/100 kg of olives)
Two-phase extraction
Three-phase extraction
86%
85%
72.5 a 57.5 a 3.16 a 7.44 a 2.28 a 30.7 a
50.7 b 52.7 b 3.18 b 6.68 a 1.60 b 23.9 b
8.30 a 13.4 a 0.14 a 1.20 b 2.42 a
97.2 b 12.6 a 1.20 b 8.3 b 2.80 a
Values followed by identical letters are not statistically different at P < 0.05. From Amirante et al. (1993).
Table 9.2: Quality characteristics of olive oils obtained from the two- and three-phase decanter system (Di Giovacchino, 1996)
Quality characteristic
Two-phase decanter
Three-phase decanter
Acidity (%) Peroxide value (meq/kg) Total polyphenols (mg/l gallic acid) O-diphenols (mg/l caffeic acid) Rancimat stability (h) (induction time at 120 ◦ C 20 l/h air-flow rate) Chlorophyll pigments (ppm) K232 K270 Sensory evaluation
0.35 3.8 333 342 15.3
0.34 4.3 220 165 11.6
6.3 1.548 0.105 7.1
6.6 1.438 0.091 7.2
However, apart from the yield and the level of polyphenols, other basic oil-quality parameters, such as the acidity level, fatty acid composition, triacylglycerol molecular species and stability, were found to be similar whether the oil was extracted using the discontinuous (pressing) or continuous (centrifuging) procedure (Ben Miled et al., 2000). That said, the quality and the quantity of oil obtained from the different varieties did show differences.
Oil extraction and processing biotechnologies 229
Figure 9.4: Percolation of the olive oil from the olive paste in a Sinolea unit.
9.1.3 Extraction by selective filtration (Sinolea) This is considered to be the most up-to-date method of extracting oil from olives. In this method, selective filtration combined with centrifugation is used for the separation of olive oil from the olive paste. The most common selective filtration system is the Sinolea process (Figure 9.4). This process is the opposite of pressing, since no pressure is applied to the paste. In Sinolea, rows of metal plates are dipped into the olive paste; the oil preferentially wets and sticks to the metal, and is removed with scrapers in a continuous process. This process is based on the different interferential tensions of oil and water coming into contact with steel plates. A steel plate will be coated with oil when plunged into the olive paste, since the interfacial tension of the oil is less than that of the water. These different physical behaviors allow the olive oil to adhere to the steel plate while the other two phases remain behind. In this system, 350–750 kg of olive paste can be handled at one time. Sinolea works by continuously introducing several hundred steel plates into the paste, thus extracting the olive oil; the moving plates slot through the slits in the grating units, and slowly penetrate the paste in a reciprocating motion. In this process, the oil does not suffer any physical damage, so its quality is very high. Furthermore, the temperature can be kept very low, which can help to protect many oil-quality parameters – especially the polyphenols, aroma and flavor. The process is automated, so labor costs are low. However, it is not completely efficient, and a large quantity of oil remains in the
230 Chapter 9
Figure 9.5: Flow diagram of combined processes of olive oil extraction (selective filtration and centrifugation).
paste. Therefore, the remaining paste has to be processed by the standard modern method (industrial decanter), and generally many commercial oil-extracting companies use Sinolea processing combined with the decanter method, as shown in Figure 9.5. Furthermore, the Sinolea equipment is complicated and requires frequent cleaning, maintenance of the stainless steel blade mechanism, and a constant heat source to keep the paste at an even temperature. Extraction is stopped when vegetable water begins to appear in the oil.
9.1.4 Summary It is widely accepted that, apart from polyphenols, most of the other olive oil-quality parameters (such as free fatty acids, peroxide values, ultraviolet absorption and sensory character) are not
Oil extraction and processing biotechnologies 231 Table 9.3: Characteristics of virgin olive oil obtained from good-quality olives by three different processing systems
Characteristic
Pressure
Sinolea
Centrifugation/decanter
Free fatty acid (%) Peroxide value (meq/kg) Total polyphenols (mg/l gallic acid) o-diphenols (mg/l caffeic acid) Induction time (h) Chlorophyll pigments (ppm) K232 K270 Sensory evaluation (panel taste)
0.23 4.0 158 100 11.7 5 1.93 0.120 6.9
0.23 4.6 157 99 11.2 8.9 2.03 0.129 7.0
0.22 4.9 121 61 8.9 9.1 2.01 0.127 7.0
generally affected by the different modes of extraction, provided high-quality olive fruits are used and harvesting is carried out at the optimum time period (Di Giovacchino et al., 1994) so that there is minimal enzymatic alteration in the olives before they are processed (Kiritsakis, 1991; Di Giovacchino, 1996). The oil-quality parameters of these three systems of oil extraction – pressure, Sinolea and centrifugation/decanter – are presented in Table 9.3.
9.2 Bio-enzymatic technology for improved olive oil extraction Modern industrial olive oil extraction systems, either discontinuous (pressing) or continuous (centrifugation or decanter), both provide a better yield compared to traditional processing; however, these systems are still not optimal regarding either the yield or the quality of the olive oil produced. In fact, these mechanical systems are capable of extracting no more than 80– 90 percent of the oil contained in the olive fruit (Ranalli et al., 2003). Furthermore, the overall residual oil content in the byproducts (olive pomace and vegetable water) can be equivalent to the percent of the olives pressed. Thus, there is still a great economic and environmental loss to the olive oil sector. Although the modern double-phase system has been found to be much more effective than the three-phase system, this method remains less than optimal and in fact, despite the increased production costs and the positive effect only on the yield, there is little improvement in the quality of the oil produced. In this context, efforts have been made to use natural enzymes or enzyme preparations (vegetable extracts) to enhance the mechanical extraction system during olive fruit processing. Enzymes are biocatalysts that facilitate the breakdown of cellulose and pectin, thereby helping to release the droplets of oil by reducing the stability and resistance of the cell walls. The addition of enzymes during grinding provides better results; however, it has become more common to add enzymes during the malaxation step.
232 Chapter 9 Studies by Montedoro and Petruccioli (1972) and Petruccioli et al. (1988) have reported an increase in olive oil yield and a decrease in processing time with the use of the enzymes pectine depolymerase, papain, cellulose, hemicellulase and acid proferase during olive oil extraction. Use of an endo-polygalacturonase has also been found to increase the oil yield and lead to a better aroma. Olivex – an enzyme preparation produced by the fungus Aspergillus acculeatus (Novo Nordisk Ferment Ltd., Dittiingen, Switzerland) – has been found to increase olive oil yield, and has subsequently been recommended for use during the malaxation process. Olivex (Olivex Ltd., Turkey) has also been found to increase the concentration of phenolic compounds in olive oil when used during the mechanical extraction process of virgin olive oil production (Vierhuis et al., 2001). Although these endogenous enzymes are natural products and increase both the quality and the yield of olive, studies have shown that the enzymes are largely inactivated during the critical crushing step, which could be due to oxidized phenols binding to their prosthetic group (Ranalli et al., 2003). In order to overcome these problems, a new complex enzyme preparation called Rapidase addax D (Gist-Brocades, Seclin City, France) that degrades the uncrushed vegetable cell walls and promotes the release of functional components has been developed (Ranalli et al., 2003). 9.2.1.1 Features of the complex enzyme preparation Rapidase Adex D This enzyme preparation essentially comprises pectolytic, cellulolytic and hemicellulolytic enzyme species. The activity is not less than 200 units/ml, where the activity of the enzyme is defined as the amount of enzyme complex that liberates 1 mol of reducing sugars per unit from pectins. The Rapidase Adex D degrades the vegetable colloids (pectins, hemicellulose, proteins, etc.) that emulsify the minute oil droplets. This enzyme preparation is water soluble and comes out in the liquid effluent (waste water) during the final step (oily must centrifugation) of the extraction cycle. When Ranalli and colleagues (2003) used this enzyme preparation in extraction of virgin olive oil from three typical Italian olive species, namely Dritta, Leccino and Coratina, cultivated organically, they obtained the results shown in Table 9.4. These clearly show that although values were quite different among the three varieties tested, most of the oil-quality parameters were meaningfully and positively increased when the enzyme was used during oil extraction. The enzyme-treated oils showed an increase in the content of total phenols, o-diphenols and secoiridoid derivatives, such as major free phenols. They also had higher phenol/polyunsaturated fatty acid ratios, which are considered to be of great importance in predicting shelf-life (Ranalli and Angerosa, 1996). Enzyme treatment also led to substantial increases in the oil yield, which ranged from 11.3 to 16.7 kg/ton of olives, regardless of the olive cultivars. The evidence shows that, with the use of the enzyme, greater amounts of oil can be freed from the vegetable tissue and in addition the coalescence phenomenon occurred regarding the minute oil droplets. Furthermore,
Oil extraction and processing biotechnologies 233 Table 9.4: Values of the major analytical and quality parameters in three enzyme-treated virgin olive oils as compared to the controls
Characteristic
Cv. Dritta
Cv. Leccino
Cv. Coratina
Enzyme Control Enzyme Control Enzyme Control Acidity (as oleic acid, g/kg) Peroxide value (mequiv of O2 /kg) Chlorophyll pigments (mg/kg) Carotenes (mg/kg) K232 K270 Sensory scoring (panel taste) Pleasant volatilesa (as nonan-1-ol, mg/kg) Phenols (as caffeic acid, mg/kg) O-diphenols (as caffeic acid, mg/kg) Secoiridoid derivativesb (as resorcinol, mg/kg) Phenolic antioxidants/polyunsaturated fatty acids Tocopherols (␣ + ␥, mg/kg)
8.0 14.3 18.3 30.7 1.80 0.10 7.5 305
7.1 14.1 15.4 29.6 1.71 0.11 7.0 218
16.1 12.5 21.5 27.7 1.70 0.11 7.0 440
19.2 14.0 18.7 23.6 1.82 0.10 6.5 283
8.1 19.2 20.4 38.3 1.90 0.20 7.6 969
7.2 19.2 16.0 34.4 2.01 0.21 7.2 858
97 49
64 36
89 52
60 31
128 73
93 37
36.3
24
33
20
66
46
13.9
10.3
8.4
100
5.8
86
8.8
242
6.1
185
210
185
Source: Ranalli and Angerosa (1996). a Includes trans-2-hexenal, trans-2-hexen-1-ol, cis-3-hexenyl acetate, cis-3 hexenyl acetate, cis-3-hexen-1-ol, penta-1-en-3-one, cis-2-pentenal, trnas-2-pentenal, and other. b Free tyrosol and hydroxytyrosol and their aglycons.
the enzyme preparation was found to aid in producing a more environmentally friendly liquid waste.
9.3 Negev olive oil extraction studies Until the past decade, most of the olive production in Israel and, consequently, the oil presses were located in the northern and central parts of the country. However, following the development of advanced saline water irrigation technology, specifically engineered and approved for olive trees (Wiesman et al., 2004; Weissbein, 2006; Weissbein et al., 2008), a significant increase in olive cultivation has taken place in the southern Negev region. The rapidly increasing trend for olive oil cultivation in the Negev Desert area may soon lead to the area producing approximately 30 percent of Israel’s total olive oil output. As the cultivation of olives has become more intensive in the Negev Desert area (Figure 9.6), the oil extraction process has also advanced. A study
234 Chapter 9
Figure 9.6: The main environmental factors affecting olive oil production in the Negev Desert.
on olive oil extraction was initiated and has been further developed in several units located in various parts of the Negev. An experimental research station, the Ramat Negev Desert AgroResearch Center (RNDARC), established by pioneers such as Yoel DeMalach and others many years ago, provides basic horticultural and cultivation facilities, mainly using saline irrigation and drip irrigation technologies for efficient development of the olive industry. Ben-Gurion University, which is located in the capital of the Negev area at Beer-Sheva, has initiated a long-term research and development study of desert olive oil extraction technologies and oil analyses. Until recent years, the main olive variety cultivated in the Negev region was Barnea. However, other varieties from different sources of origin have been introduced and were used in these
Oil extraction and processing biotechnologies 235 olive oil studies. Among these are varieties originating in Israel (Barnea, Souri, Maalot), Italy (Frantio, Leccino), Spain (Arbeqina, Pecual, Picudo), Greece (Kalamata, Koroneiki), France (Picholine), Morocco (Picholin di Morocco) and many others, as described in Chapter 7.
9.3.1 Oil potential of Negev-grown olives In 1997, the Phyto-Lipid Biotechnology Laboratory of the Ben-Gurion University of the Negev initiated a study to try to gain an indication regarding the potential oil production from olives cultivated in the Negev Desert region, and to develop an efficient method of oil extraction. The study continued until 2007. Initially, the basic systems for oil extraction and determination of oil content were organized (a brief description is given below; a more detailed discussion is provided in Chapter 8). r
r
r
r
Soxhlet extraction. This is quite an old method, which detects the oil content using solvents. In this method, all traces of oil are removed from the olive material. Soxhlet apparatus is generally used, and hexane is the most common solvent. The extracted oil does not meet the requirements for extra-virgin olive oil, so this method is employed only in the production of refined pomace oil and for analyzing the amount of oil accumulated in the olive fruits. Soxhlet extraction is generally agreed to provide very accurate results in comparison to most of the other common chemical oil analysis methods; however, the method is time-consuming, which limits the number of olive samples that can be analyzed. Near infrared (NIR) analyzer. NIR technology can measure the oil and moisture in milled olives and fatty acids, and the moisture in oil. This instrument works by passing near-infrared light through the oil sample and measuring the emerging energy, which relates to the concentration of oil, moisture and fatty acids. This method only determines the percentage oil content. IR spectroscopy. In this method, a small sample of olives is crushed and dissolved in strong solvent, and then analyzed using infrared absorbance – a non-dispersive infrared spectrophotometric technique which is specific to hydrocarbons such as oil. The HORIBA’s OCMA-350 oil content analyzer is commonly used for this purpose (Weissbein, 2006). IR spectroscopy is also used to detect the percentage of oil in a particular olive fruit. Once a good oil calibration curve has been achieved, this technique gives very similar results to those from the soxhlet extraction method. Low resolution nuclear magnetic resonance (NMR). Low resolution NMR is a rapid, non-destructive and highly reliable technique for detecting the oil content in olive fruits (Nordon et al., 2001; MARAN ULTRA, 2006). This is a new technique, based on a 23-MHz low resolution NMR system that has recently (2006) been introduced
236 Chapter 9
Figure 9.7: Oliomio mini-olive mill system: (a) main system; (b) olive oil waste material; (c) pure olive oil drainage.
and used in the Phyto-Lipid Biotechnology Laboratory at Ben-Gurion University of the Negev. Following two years’ experience of its use, the system appears to be highly reliable and as accurate as the soxhlet system, but the oil content can be determined in just 16 seconds. r
Oliomio mini-olive oil mill. The Oliomio mill is a small mill suitable for olive oil extraction (Figure 9.7). This mini olive-oil system provides high-quality olive oil in a similar way to the large-scale industrial olive oil mill systems. The proximity of this system to the experimental plots in the Negev Desert enabled production of the best olive oil, soon after harvesting, from trees of the various olive varieties being cultivated in the region using the advanced cultivation technologies developed over the past decade (Wiesman et al., 2004). Subsequently, two main long-term studies were carried out simultaneously. One was dedicated to evaluating the olive oil potential of various olive varieties tested in the Negev Desert area, and the second was focused on developing proper and efficient oil extraction in the same region. These studies were based on advanced laboratory olive oil analysis and used the Oliomio mini olive oil mill system for extraction.
The olive oil content of Souri olives as determined by various systems is shown in Table 9.5. The level is relatively similar in the three systems tested, although the percentage obtained using the Horiba system is slightly lower than with the soxhlet system or low resolution NMR.
Oil extraction and processing biotechnologies 237 Table 9.5: Determination of percentage olive oil content of Souri olives cultivated in the Negev Desert region in the 2006 season, measured by three different methods
Analysis date Nov 21 Nov 28 Dec 7 Dec 12 Dec 19 Dec 29
Olive oil percentage Soxhlet
Horiba 350
LR NMR
12.9 13.8 14.5 18.3 18.8 18.6
11.9 12.7 13.9 17.6 18.1 18.0
12.2 13.7 14.9 18.1 18.7 18.7
These values are the means of five samples for the Soxhlet and Horiba systems, and of 50 samples for the LR NMR system.
Table 9.6: Olive oil extraction from Negev Desert cultivated olive varieties using the Oliomio system and laboratory Horiba 350 analysis, 2005 season
Olive variety Barnea Souri Leccino Frantoio Picual Arbequina Kalamata Koreneiki Phicolin
Olive oil percentage Oliomio
Horiba 350
14.3 16.6 14.1 12.9 14.0 15.0 10.3 15.8 18.0
17.2 19.8 16.4 15.2 16.5 16.9 16.7 18.6 20.9
These values are the means of three separate Horiba analyses and one Oliomio extraction of each olive variety.
The data obtained in Table 9.6 indicate an average difference in oil extraction of about 2– 3 percent between the Oliomio mini-olive oil system and the Horiba system. Usually, such a difference between an olive oil mill and laboratory testing is well accepted, as laboratory analysis is supposed to provide information regarding the oil potential of olives. Furthermore, the difference is explained by the fact that for laboratory analysis only the flesh of the olive is crushed and assessed for oil content, whereas in the actual mill system the crushed paste consists of both flesh and stone, and of course a certain amount of oil is lost in separating the oil fraction from the organic waste material. Only in the case of Kalamata was a large difference in oil extraction shown; here, the Oliomio result was more than 6 percent less than that obtained by the laboratory system (10.3 versus 16.7 percent). Laboratory analysis therefore provides a
238 Chapter 9 Table 9.7: Olive oil extraction from Negev Desert cultivated olive varieties by the Oliomio system: comparison of trees cultivated with saline and with fresh water, 2004 season
Olive variety
Olive oil Saline water (4.2 dS/m)
Barnea Souri Leccino Frantoio Picual Arbequina Kalamata Koreneiki Phicolin
15.3 19.6 12.6 12.9 12.8 14.4 14.9 19.8 21.4
Percentage Fresh water (1.2 dS/m) 16.2 17.8 13.4 10.2 10.5 12.6 12.7 16.6 18.3
relatively good estimate of the oil potential of olives; however, in an actual milling system it can be difficult to separate the oil and the waste material. The relatively high levels of emulsification agents such as phospholipids mean that it is necessary to add a significant volume of water to precipitate the heavier organic material from the lighter oil fraction. In Kalamata and some other olives, this seems to be the cause of the unusually low level of oil extraction. It is not clear whether this phenomenon is directly related to desert environmental conditions or to cultural practices, or whether it is mainly related to genetics. A study to address these issues is already underway. The percentage oil content in the waste material obtained in the final Oliomio process is shown in Table 9.7. The results presented in this table show that some olive varieties produce more oil when irrigated with saline water, while others produce more oil when irrigated with fresh water. This suggests that the use of saline water does not directly affect the rate of olive oil accumulation. As discussed in Chapter 7, it appears that the main factor dominating the oil yield of each variety is genetic. However, saline water may affect the rate of maturation of each olive variety, and thus indirectly affect the oil content of olives at an early or later date. Based on experience gained over recent years, it seems that saline water irrigation combined with Negev Desert conditions delays somewhat the rate of olive maturation in comparison to other areas in the northern part of Israel that are not considered to be desert. This issue needs to be systematically studied. In any case, as discussed in Chapter 8, it is very important to monitor the level of olive oil content regularly. Another study carried out in the Negev Desert area regarding the effect of intensive irrigation versus dry regulated irrigation on Muchasan olive oil extraction clearly showed that the water content of the intensively irrigated olives (58 percent) was significantly higher than in olives that
Oil extraction and processing biotechnologies 239 were cultivated under supplemental irrigation only (42 percent). Obviously, the oil content was higher in the less intensively irrigated olives (26 percent per fresh weight) than in the intensively irrigated olives (21 percent). Furthermore, the efficiency of olive oil extraction in a commercial tri-phase olive oil mill was greater in the less irrigated olives than those that had been more intensively irrigated. This was concluded from the oil percentage obtained from the solid waste material (6.5 versus 9 percent, based on dry weight, respectively). These results suggest that it is important to gradually decrease the rate of irrigation in the final stages of olive maturation. This practice is indeed risky in desert environments, but may pay for itself in easier mechanical harvesting (as discussed in Chapter 8) and also the higher operational efficiency of the olive oil mill in obtaining a greater amount of oil from the olives.
9.4 Decantation, bottling and shelf-life, with an emphasis on Negev Desert conditions Premium-quality oils should be stored in stainless steel containers and maintained at a constant temperature between 8◦ and 18 ◦ C. After processing, the oil should be stored in bulk for 1–3 months to allow any particulate matter and vegetable water to settle out. Bulk storage and decantation eliminate the problems of sediment in bottles and oil contact with processing water residues that could lead to off flavors in the oil. When olive oil is too old and has oxidized, it usually becomes rancid. Rancidity is most commonly detected by taste, but can be checked chemically. The “rancimat” chemical method is mostly used for large industrial frying operations. Oil doesn’t suddenly go rancid; it slowly becomes more oxidized and, as it does, the flavor suffers. Different oils age at different rates. Some olive varieties make oil with more natural antioxidants, which resist ageing. These oils may be good for up to three to four years if properly stored in unopened containers. Other oils, particularly unfiltered, may be unpalatable in a year even if stored well. A two-year-old olive oil may taste rancid to some while not to others. Most people would be put off by the taste of any vegetable oil more than four to five years old. Rancid oil has fewer antioxidants, but is not poisonous. A good percentage of the world’s population routinely eats rancid oil because of lack of proper storage conditions, and some actually prefer the taste. In historical times, olives that had dropped to the ground or may have spoiled were made into olive oil which was stored in open-mouthed earthenware vats. Practices like these encouraged rancidity. People have come to expect non-rancid oil in the past 50 years because of chemical refining and better production and storage methods. Heat, exposure to light, and exposure to air are the three major factors that affect the oxidation and rancidity of the oil in bottles. Since desert areas are prone to all these conditions, extra
240 Chapter 9 precautions need to be taken, especially regarding processing and bottling, and storage periods. A tightly capped bottle that is full will oxidize less than a large tin that is only half-full. The longer the oil sits, the more rancid it will become. The following factors also affect the quality of the oil and its shelf-life. r
r
r
r
r
Olive variety. Polyphenols and other natural antioxidants in the oil help to keep the oil from going rancid, and some varieties have more antioxidants than others. The Tuscan varieties – Frantoio, Coratina, Pendolino and Leccino – tend to be higher in polyphenols than others. Time of picking. Olives picked earlier in the year may have more polyphenols and a longer shelf-life. Low-polyphenol olive oils often needed to blend with high-polyphenol oils to give a longer shelf-life. Picking method. In some parts of the world, nets are placed under the trees and the olives are allowed to drop for weeks as they ripen. Any remaining olives are then beaten from the tree and the nets are gathered. Some olives may therefore have been off the tree for weeks or months, spoiling in the interim. Olives that are badly bruised during picking will become more rancid if not pressed within hours. However, in intensive cultivation olive fruits are harvested only by machines, so there it is important that great care is taken during picking time, especially in desert areas. Time to milling. Live olives start to die once they have been picked, and the longer it takes to get to the mill, the more oxidized the oil will be. Producers who want to produce extra-virgin oil must get the olives to the mill within 48 hours of picking. Refrigeration and good ventilation of the orchard bins extends the olives’ life. Milling method. During the milling process, the olive paste may be exposed to air. The old-fashioned stone wheel and hydraulic press with jute mats or Rapanelli discs may be very picturesque, but expose the paste to far too much air. In modern techniques, where malaxation is carried out in specially made tanks, it is important that the paste is always enclosed with minimal contact with the air. Some mills bathe the paste with an inert gas during the mixing or malaxation step, or perform it in a vacuum. Heating the paste during malaxation will extract more oil, but will hasten oxidation. Centrifugal presses expose the paste to zero air.
Extra-virgin olive oil is not filtered, because it can lose some aromatic and taste properties during the procedure, so it is decanted. This process takes about 40 days; the olive oil is left to rest at a temperature of about 18 ◦ C, and the residues in suspension begin to sediment on the bottom of the tank. It is then only necessary to remove the oil via an outlet situated above the level of the solid residues.
Oil extraction and processing biotechnologies 241
References Alburquerque, J. A., Gonz´alvez, J., Garc´ıa, D., & Cegarra, J. (2004). Agrochemical characterisation of “alperujo,” a solid by-product of the two phase centrifugation method for olive oil extraction. Bioresource Technol, 92, 195–200. Amirante, P., Di Renzo, G. C., Di Giovacchino, L., Bianchi, B., & Catalano, P. (1993). Evoluci´on tecnol´ogica de las instalaciones de extracci´on del aceite de oliva. Olivae, 48, 43–53. Ben-Miled, D. D., Smaoul, A., Zarrouk, M., & Cherif, A (2000). Do extraction procedures affect olive oil quality and stability? Biochem Soc Trans, 28, 929–933. Di Giovacchino, L. (1996). Influence of extraction systems on olive oil quality. Olivae, 63, 52–55. Di Giovacchino, L., Solinas, M., & Miccoli M, M. (1994). Effect of extraction systems on the quality of virgin olive oil. J Am Oil Chem Soc, 71, 1189–1194. Kiritsakis, A. (1991). Extraction of olive oil. In: A. Kiritsakis (Ed.), Olive Oil (pp. 61–69). American Oil Chemists Society, Champaign, IL. Kiritsakis, A. K. (1998). Olive Oil from the Tree to the Table (2nd edn.). Food & Nutrition Press, Inc., Trumball, CT, 26. MARAN-ULTRA (2006). MARAN-ULTRA SFC User Manual V6.2. Oxford: Oxford Instruments, Molecular Biotools Ltd. Montedoro, G., & Petruccioli, G. (1972). Enzymatic treatments in the mechanical extraction of olive oil. Acta Vitam et Enzym, 26, 171. Nordon, A., Macgill, C. A., & Littlejohn, D. (2001). Process NMR spectrometry. Analyst, 126, 260–272. Petruccioli, M., Servili, M., Montedoro, F., & Federici, F (1988). Development of recycle procedure for the utilization of vegetation waters in the olive oil extraction process. Biotechnol Letts, 10, 55–59. Ranalli, A., & Angerosa, F. (1996). Integral centrifuges for olive oil extraction. The qualitative characteristics of products. J Am Oil Chem Soc, 73, 417–422. Ranalli, A., Gomes, T., Dicuratolo, D., Contento, S., & Lucer, L. (2003). Improving virgin olive oil quality by means of innovative extracting biotechnologies. J Agric Food Chem, 51, 2597–2602. Vierhuis, E., Servilli, M., Baldioli, M., Schols, H. A., Voragen, A. G. J., & Montedoro, G. F. (2001). Effect of enzyme treatment during mechanical extraction of olive oil on phenolic compound and polysaccharides. J Agric Food Chem, 49, 1218–1223. Weissbein, S. (2006). Characterization of new olive (Olea europae L.) varieties’ response to irrigation with saline water in the Ramat Nege area. MSc Thesis, Ben-Gurion University of the Negev, Beersheva, Israel. Weissbein, S., Wiesman, Z., Efrath, Y., & Silverbush, M. (2008). Physiological and reproductive response olive varieties under saline water. Hort Sci, 43, 320–327. Wiesman, Z., Itzhak, D., & Ben Dom, N. (2004). Optimization of saline water level for sustainable Barnea olive and oil production in desert conditions. Sci Hort, 100, 257–266. Ranalli, A., & Martinelli, N. (1995). Integral centrifuges for olive oil extraction at the third millennium threshold. Transformation yield. Grasas y Aceites, 46, 255–263. Suarez, M. J. M. (1975). Preliminary operations. In: J. M. Moreno Martinez (Ed.), Olive Technology. FAO, Rome.
Oil extraction and processing biotechnologies 9.1
9.2 9.3 9.4
Olive oil extraction technologies 9.1.1 Traditional method 9.1.2 Modern methods 9.1.3 Extraction by selective filtration (Sinolea) 9.1.4 Summary Bio-enzymatic technology for improved olive oil extraction Negev olive oil extraction studies 9.3.1 Oil potential of Negev-grown olives Decantation, bottling and shelf-life, with an emphasis on Negev Desert conditions
223 223 225 229 230 231 233 235 239
Olive oil has a privileged position among edible vegetable oils because it is the juice of a fruit, and as such is consumed almost entirely in its natural state. Therefore, it contains (among other important constituents) phenolic compounds, which are usually removed from seed oils during the refining process. The olive oil extraction process refers to extracting the olive oil that is present in the olive fruits, botanically termed “drupes,” for food use. In olive fruits the oil is produced in the mesocarp cells, and stored in a particular type of vacuole called a lipovacuole (i.e., every cell contains an olive oil droplet). Hence, olive oil extraction can be said to be a process of separating the oil from the other fruit contents (vegetation tissue, water and solid material), and this separation is done only by physical means. This is totally different from other vegetable oils, which are extracted with chemical solvents (generally hexane). Since olive oil is present in the form of minute drops in the vacuoles of mesocarp cells in the olive fruit, as well as being scattered to a lesser extent in the colloidal system of the cell’s cytoplasm and, to a still lesser extent, in the epicarp and endosperm, it is possible for all the oil in the vacuoles to be released during processing. It is difficult to obtain oil dispersed in the cytoplasm, so this oil generally remains in the olive pomace – a by-product of processing (Kiritsakis, 1998). The olive oil extraction process is normally divided into crushing, malaxation (mixing), and separation of the oil. Below, we describe the main tasks that need to be performed, after harvesting the olives, in order to obtain good-quality olive oil. 221
222 Chapter 9 Delivery of the olive fruits After the olives have been picked in the proper manner, they are delivered to the mill and stored there in the yard in collection boxes. This offers the opportunity to determine the quality and rate of yield, and serves as the basis for the settlement of accounts between the oil mill and olive farmer. At harvest, there is generally 18–32 percent oil, 40–55 percent water and 23–35 percent seeds and vegetable tissue in olive fruits. Washing and cleaning of the olive fruits Stems, twigs and leaves should be removed, and the olive fruits may or may not be cleaned with water to remove pesticides, dirt, etc. Rocks and sand will quickly wear out a centrifugal decanter or oil separator, reducing its lifespan. Grinding, mixing or malaxation of olive pulp After cleaning, the olive fruits are ground up together with their stones. For grinding, either stone rollers or a metal tooth grinder or hammer mill can be used. Each has its own advantages and disadvantages. After this process, mixing or malaxation should be performed. Malaxation is the action of slowly churning or mixing milled olives in a specially designed mixer for 20–40 minutes. This mixing allows the smaller droplets of oil that were released by the milling process to combine into larger ones that can be more easily separated. The paste is normally heated to around 27 ◦ C during this process. It is now possible, with newer equipment, to use a blanket of inert gas (such as nitrogen or carbon dioxide) over the olive paste, which greatly reduces oxidation; this allows for an increased yield without compromising the quality of the oil. After malaxation is complete, the paste is then sent to a phase separator. Nearly all producers use a decanter centrifuge for this phase. The most common mixer is a horizontal trough with spiral mixing blades. Malaxation of the pulp is carried out, dependent on the oil extraction procedure – i.e., warm water may be added. To aid in further breakdown of the olive cells, and to create large oil droplets, the pulp is beaten. Salt is often added at this stage, to aid the osmotic breakdown of cells in the olives and thus ease the separation of the oil from the water. The olives are beaten several times. Separation of the oil and water In this step, separation of oil and water from the olive paste is carried out. Many procedures can be used to achieve this, and will be discussed later in the chapter; centrifuge/decanters, modern di- and tri-phase centrifuge, and percolation-Sinolea methods are common. In small oil mills, the olive oil is extracted in a batch operation using the traditional press method. The oil extracted is collected in containers and clarified by sedimentation. In a big oil extraction system,
Oil extraction and processing biotechnologies 223 large amounts of olive fruits are used for extraction by more modern systems. To improve the separation of oil and pulp, biological or chemical aids that attack the cell walls can be added. Separation of oil from the water This is a very important process, where the olive oil is separated from the olive’s water. As in cream separation in a dairy, the liquid is spun to separate the heavier water from the oil. Gravity decanter or vertical centrifuges with perforated conical discs are commonly used in this process. Extraction of residual oil After extraction of the first oil – generally virgin olive oil – the residual waste or pomace derived after first pressing or centrifugation undergoes further extraction by different means. The process of extraction of the residual oil differs according to the system originally used. The solid waste from oil extraction by pressing still contains about 6 percent oil or, using the continuous decanter, 4 percent olive oil. The oil content in the solid–liquid mixture from the dual-phase decanting process contains 2.5–3.5 percent oil. In specialized extraction plants, a solvent extraction process is used for this purpose. First, the waste is completely dried; hexane is then used as a solvent to extract further oil, known as olive-pomace oil. The utilization of by-products of olive oil extraction The dry residues can be used as concentrated fodder. In some extraction plants, the stones are separated from the pulp after extraction and used as fuel for heating the driers. The pulp is sold as fertilizer or fodder. In some oil mills, the solid waste from the press is directly used as fuel for the heating of water. Vegetable water, the liquid phase obtained as a result of centrifugation, is abundant in the threephase extraction method due to water injection into the paste before centrifugation. As vegetable water still contains oil, it is treated a second time in order to remove the maximum amount of oil. However, since the result is a combination of water and fat, it is difficult to recycle it. Vegetable water is highly polluting and has a negative effect on underground water. The most serious ecological problem in olive oil production is the recycling of vegetable water.
9.1 Olive oil extraction technologies 9.1.1 Traditional method The traditional method of olive oil extraction is by using an olive press. The olive press works by applying pressure to olive paste to separate the liquid oil and vegetation water from the solid materials. In this method, after separating the oil and water from the fruit paste, the water and the oil themselves are then separated from one another by a standard decantation procedure. This method has been in use since the Greeks first began pressing olives over 5000 years ago,
224 Chapter 9
Figure 9.1: Traditional olive oil press. Source: Wikipedia (www.wikipedia.org), 2007.
and is still widely used today, with some modification. Even now, it is considered a valid way of producing high-quality olive oil if adequate care is taken (Figure 9.1). However, this type of method is suitable only for small growers. In this method, after cleaning the olive fruits, the olives are ground into an olive paste using large millstones. The olive paste generally remains between the stones for 30–40 minutes in order to guarantee that the olives are well ground; this allows enough time for the olive droplets to join to form large drops of oil, and for the fruit enzymes to produce some of the oil aromas and taste. Some traditional oil presses also use modern crushing methods. After grinding, the ground olive paste is spread on fiber disks, which are stacked on top of each other, then placed into the press. Traditionally the disks were made of hemp or coconut fiber, but nowadays they are made from synthetic fibers, which are easier to clean and maintain. These disks are then threaded onto a hydraulic piston, forming a pile. Pressure is applied on the disks, thus compacting the solid
Oil extraction and processing biotechnologies 225 phase of olive paste and percolating the liquid phase (oil and vegetation water). In this system, the applied hydraulic pressure can be as high as 400 atm. To facilitate the liquid phase, water is run down the sides of the disks to increase the speed of percolation. The liquid is then separated either by a standard process of decantation or by the faster vertical centrifuge. The main advantages of the traditional method are that it produces good-quality oil by more efficient grinding of the olives, reducing the release of oil oxidation enzymes, and that it also reduces added water, which minimize the washing of polyphenols. The pomace obtained from this method has a low water content, so it is easier to manage. The disadvantages of this method include the difficulty of cleaning the disks. Also, since this method is not a continuous process, it means that there are periods when the olive paste is exposed to oxygen and light, which can cause deterioration of oil quality. Finally, the traditional method requires more manual labor, and also needs a longer time period between harvest and pressing. This traditional method, also called discontinuous extraction, is an ancient procedure that only distinguishes two phases by pressing or centrifugation. The liquid phase is filtered later in order to obtain oil. In this case the by-product is a plastic paste, which has the advantage of avoiding the production of vegetation water. However, although it is better ecologically, this technique provides a lower yield, which is not always seen as an advantage by the main producing countries. Nearly all olive oil producers in the traditional olive-growing regions, such as Spain, changed from the traditional presses to modern systems in the mid-1980s.
9.1.2 Modern methods The modern method of olive oil extraction uses an industrial decanter to separate all the phases by centrifugation. In this method, olives are crushed to a fine paste by a hammer crusher, a disk crusher, a depitting machine or a knife crusher. The paste is then malaxed for 30–40 minutes to allow the small olive droplets to agglomerate and the fruit enzymes to create aromas. After this the paste is pumped into an industrial decanter, where the phases will be separated. Depending upon the system, water might be added with the paste to facilitate the extraction process. The decanter is a large-capacity horizontal centrifuge rotating at around 3000 rpm; the high centrifugal force created allows the phases to be readily separated according to their different densities (solids > vegetation water > oil). The most common industrial processing method is a continuous extraction system with two centrifuges (first horizontal and then vertical). Over the years, a few technological variations have been introduced to the industrial decanter. In the three-phase decanter (oil, pomace and vegetable water), as shown in Figure 9.2, some of the oil polyphenols are washed out due to the higher quantity of water added, which produces a large quantity of vegetable water that needs processing.
226 Chapter 9
Figure 9.2: Flow diagram of the olive oil extraction process. Source: Unctad (www.unctad.org (2004).
The procedure shown in Figure 9.2 is the most widely used in intensive production areas, and dates back to the 1970s and 1980s. The main disadvantages of this process are the huge amount of water needed, which leads to production of large quantities of waste and the resultant pollution. In an attempt to solve this problem, a two-phase oil decanter has been created; here, no water injection is used and just the oil and a plastic paste are produced. Since no water is added in this two-phase system, there is less phenol washing and oil and hard pomace are obtained. Instead of three exit points, for oil, water and solids, this type of decanter has only two (Figure 9.3). The water is expelled by the decanter coil together with the pomace; resulting in a wetter pomace; this is much harder and needs to be processed industrially, leading to high energy costs so that the pomace can be dried in order to extract further oil from it. In practice, therefore, the two-phase decanter solves the phenol washing problem but increases the residue management problem. This has led to the creation of a new two-and-a-half phase oil decanter (sometimes still referred to as a two-phase decanter system). This separates the olive paste into the standard three phases, but has a lesser need for added water and thus a smaller
Oil extraction and processing biotechnologies 227
Figure 9.3: Three- and two-phase centrifugation system of olive extraction (Alburquerque et al., 2004).
vegetable-water output. This system is considered to be more economically and environmentally viable than the others. This new industrial technique of continuous extraction is commonly practiced these days and, because it needs less water, is being used more and more widely. A detailed comparison of results obtained from a two-phase extraction and a three-phase extraction is represented in Table 9.1. The study involved different varieties of olives, composed of 48–51 percent water and 19–23 percent oil. Olive oil obtained from the two-phase decanter system also contained higher amounts of polyphenols, artho-diphenols, hydroxytryrosol, tocopherols, trans-2-hexenal (the most important aroma component, with a herbaceous aroma) and total aroma volatile compounds than that obtained from the three-phase decanter. Moreover, the oil obtained from the two-phase decanter showed higher oxidative stability, lower turbidity, and lower contents of pigments, steroid hydrocarbons, waxes, and aliphatic and triterpenic alcohols than did the oil from the three-phase decanter. Some of the differences in the quality of oil obtained from two- and threephase decanter systems, as found by Di Giovacchino (1996) are shown in Table 9.2. This shows that the two-phase decanter system produced better-quality oil than the three-phase system.
228 Chapter 9 Table 9.1: A detailed comparison of results obtained from two- and three-phase olive oil extraction systems
Parameters Oil extraction capacity Pomace Quantity (kg/100 kg of olives) Moisture (%) Oil (%) Oil (% dry matter) Oil (kg/100 kg of olives) Dry pomace (kg/100 kg of olives) Vegetable water Quantity (l/100 kg of olives) Oil (g/l) Oil (kg/100 kg of olives) Dry residual (kg/100 kg of olives) Oil in by-products (kg/100 kg of olives)
Two-phase extraction
Three-phase extraction
86%
85%
72.5 a 57.5 a 3.16 a 7.44 a 2.28 a 30.7 a
50.7 b 52.7 b 3.18 b 6.68 a 1.60 b 23.9 b
8.30 a 13.4 a 0.14 a 1.20 b 2.42 a
97.2 b 12.6 a 1.20 b 8.3 b 2.80 a
Values followed by identical letters are not statistically different at P < 0.05. From Amirante et al. (1993).
Table 9.2: Quality characteristics of olive oils obtained from the two- and three-phase decanter system (Di Giovacchino, 1996)
Quality characteristic
Two-phase decanter
Three-phase decanter
Acidity (%) Peroxide value (meq/kg) Total polyphenols (mg/l gallic acid) O-diphenols (mg/l caffeic acid) Rancimat stability (h) (induction time at 120 ◦ C 20 l/h air-flow rate) Chlorophyll pigments (ppm) K232 K270 Sensory evaluation
0.35 3.8 333 342 15.3
0.34 4.3 220 165 11.6
6.3 1.548 0.105 7.1
6.6 1.438 0.091 7.2
However, apart from the yield and the level of polyphenols, other basic oil-quality parameters, such as the acidity level, fatty acid composition, triacylglycerol molecular species and stability, were found to be similar whether the oil was extracted using the discontinuous (pressing) or continuous (centrifuging) procedure (Ben Miled et al., 2000). That said, the quality and the quantity of oil obtained from the different varieties did show differences.
Oil extraction and processing biotechnologies 229
Figure 9.4: Percolation of the olive oil from the olive paste in a Sinolea unit.
9.1.3 Extraction by selective filtration (Sinolea) This is considered to be the most up-to-date method of extracting oil from olives. In this method, selective filtration combined with centrifugation is used for the separation of olive oil from the olive paste. The most common selective filtration system is the Sinolea process (Figure 9.4). This process is the opposite of pressing, since no pressure is applied to the paste. In Sinolea, rows of metal plates are dipped into the olive paste; the oil preferentially wets and sticks to the metal, and is removed with scrapers in a continuous process. This process is based on the different interferential tensions of oil and water coming into contact with steel plates. A steel plate will be coated with oil when plunged into the olive paste, since the interfacial tension of the oil is less than that of the water. These different physical behaviors allow the olive oil to adhere to the steel plate while the other two phases remain behind. In this system, 350–750 kg of olive paste can be handled at one time. Sinolea works by continuously introducing several hundred steel plates into the paste, thus extracting the olive oil; the moving plates slot through the slits in the grating units, and slowly penetrate the paste in a reciprocating motion. In this process, the oil does not suffer any physical damage, so its quality is very high. Furthermore, the temperature can be kept very low, which can help to protect many oil-quality parameters – especially the polyphenols, aroma and flavor. The process is automated, so labor costs are low. However, it is not completely efficient, and a large quantity of oil remains in the
230 Chapter 9
Figure 9.5: Flow diagram of combined processes of olive oil extraction (selective filtration and centrifugation).
paste. Therefore, the remaining paste has to be processed by the standard modern method (industrial decanter), and generally many commercial oil-extracting companies use Sinolea processing combined with the decanter method, as shown in Figure 9.5. Furthermore, the Sinolea equipment is complicated and requires frequent cleaning, maintenance of the stainless steel blade mechanism, and a constant heat source to keep the paste at an even temperature. Extraction is stopped when vegetable water begins to appear in the oil.
9.1.4 Summary It is widely accepted that, apart from polyphenols, most of the other olive oil-quality parameters (such as free fatty acids, peroxide values, ultraviolet absorption and sensory character) are not
Oil extraction and processing biotechnologies 231 Table 9.3: Characteristics of virgin olive oil obtained from good-quality olives by three different processing systems
Characteristic
Pressure
Sinolea
Centrifugation/decanter
Free fatty acid (%) Peroxide value (meq/kg) Total polyphenols (mg/l gallic acid) o-diphenols (mg/l caffeic acid) Induction time (h) Chlorophyll pigments (ppm) K232 K270 Sensory evaluation (panel taste)
0.23 4.0 158 100 11.7 5 1.93 0.120 6.9
0.23 4.6 157 99 11.2 8.9 2.03 0.129 7.0
0.22 4.9 121 61 8.9 9.1 2.01 0.127 7.0
generally affected by the different modes of extraction, provided high-quality olive fruits are used and harvesting is carried out at the optimum time period (Di Giovacchino et al., 1994) so that there is minimal enzymatic alteration in the olives before they are processed (Kiritsakis, 1991; Di Giovacchino, 1996). The oil-quality parameters of these three systems of oil extraction – pressure, Sinolea and centrifugation/decanter – are presented in Table 9.3.
9.2 Bio-enzymatic technology for improved olive oil extraction Modern industrial olive oil extraction systems, either discontinuous (pressing) or continuous (centrifugation or decanter), both provide a better yield compared to traditional processing; however, these systems are still not optimal regarding either the yield or the quality of the olive oil produced. In fact, these mechanical systems are capable of extracting no more than 80– 90 percent of the oil contained in the olive fruit (Ranalli et al., 2003). Furthermore, the overall residual oil content in the byproducts (olive pomace and vegetable water) can be equivalent to the percent of the olives pressed. Thus, there is still a great economic and environmental loss to the olive oil sector. Although the modern double-phase system has been found to be much more effective than the three-phase system, this method remains less than optimal and in fact, despite the increased production costs and the positive effect only on the yield, there is little improvement in the quality of the oil produced. In this context, efforts have been made to use natural enzymes or enzyme preparations (vegetable extracts) to enhance the mechanical extraction system during olive fruit processing. Enzymes are biocatalysts that facilitate the breakdown of cellulose and pectin, thereby helping to release the droplets of oil by reducing the stability and resistance of the cell walls. The addition of enzymes during grinding provides better results; however, it has become more common to add enzymes during the malaxation step.
232 Chapter 9 Studies by Montedoro and Petruccioli (1972) and Petruccioli et al. (1988) have reported an increase in olive oil yield and a decrease in processing time with the use of the enzymes pectine depolymerase, papain, cellulose, hemicellulase and acid proferase during olive oil extraction. Use of an endo-polygalacturonase has also been found to increase the oil yield and lead to a better aroma. Olivex – an enzyme preparation produced by the fungus Aspergillus acculeatus (Novo Nordisk Ferment Ltd., Dittiingen, Switzerland) – has been found to increase olive oil yield, and has subsequently been recommended for use during the malaxation process. Olivex (Olivex Ltd., Turkey) has also been found to increase the concentration of phenolic compounds in olive oil when used during the mechanical extraction process of virgin olive oil production (Vierhuis et al., 2001). Although these endogenous enzymes are natural products and increase both the quality and the yield of olive, studies have shown that the enzymes are largely inactivated during the critical crushing step, which could be due to oxidized phenols binding to their prosthetic group (Ranalli et al., 2003). In order to overcome these problems, a new complex enzyme preparation called Rapidase addax D (Gist-Brocades, Seclin City, France) that degrades the uncrushed vegetable cell walls and promotes the release of functional components has been developed (Ranalli et al., 2003). 9.2.1.1 Features of the complex enzyme preparation Rapidase Adex D This enzyme preparation essentially comprises pectolytic, cellulolytic and hemicellulolytic enzyme species. The activity is not less than 200 units/ml, where the activity of the enzyme is defined as the amount of enzyme complex that liberates 1 mol of reducing sugars per unit from pectins. The Rapidase Adex D degrades the vegetable colloids (pectins, hemicellulose, proteins, etc.) that emulsify the minute oil droplets. This enzyme preparation is water soluble and comes out in the liquid effluent (waste water) during the final step (oily must centrifugation) of the extraction cycle. When Ranalli and colleagues (2003) used this enzyme preparation in extraction of virgin olive oil from three typical Italian olive species, namely Dritta, Leccino and Coratina, cultivated organically, they obtained the results shown in Table 9.4. These clearly show that although values were quite different among the three varieties tested, most of the oil-quality parameters were meaningfully and positively increased when the enzyme was used during oil extraction. The enzyme-treated oils showed an increase in the content of total phenols, o-diphenols and secoiridoid derivatives, such as major free phenols. They also had higher phenol/polyunsaturated fatty acid ratios, which are considered to be of great importance in predicting shelf-life (Ranalli and Angerosa, 1996). Enzyme treatment also led to substantial increases in the oil yield, which ranged from 11.3 to 16.7 kg/ton of olives, regardless of the olive cultivars. The evidence shows that, with the use of the enzyme, greater amounts of oil can be freed from the vegetable tissue and in addition the coalescence phenomenon occurred regarding the minute oil droplets. Furthermore,
Oil extraction and processing biotechnologies 233 Table 9.4: Values of the major analytical and quality parameters in three enzyme-treated virgin olive oils as compared to the controls
Characteristic
Cv. Dritta
Cv. Leccino
Cv. Coratina
Enzyme Control Enzyme Control Enzyme Control Acidity (as oleic acid, g/kg) Peroxide value (mequiv of O2 /kg) Chlorophyll pigments (mg/kg) Carotenes (mg/kg) K232 K270 Sensory scoring (panel taste) Pleasant volatilesa (as nonan-1-ol, mg/kg) Phenols (as caffeic acid, mg/kg) O-diphenols (as caffeic acid, mg/kg) Secoiridoid derivativesb (as resorcinol, mg/kg) Phenolic antioxidants/polyunsaturated fatty acids Tocopherols (␣ + ␥, mg/kg)
8.0 14.3 18.3 30.7 1.80 0.10 7.5 305
7.1 14.1 15.4 29.6 1.71 0.11 7.0 218
16.1 12.5 21.5 27.7 1.70 0.11 7.0 440
19.2 14.0 18.7 23.6 1.82 0.10 6.5 283
8.1 19.2 20.4 38.3 1.90 0.20 7.6 969
7.2 19.2 16.0 34.4 2.01 0.21 7.2 858
97 49
64 36
89 52
60 31
128 73
93 37
36.3
24
33
20
66
46
13.9
10.3
8.4
100
5.8
86
8.8
242
6.1
185
210
185
Source: Ranalli and Angerosa (1996). a Includes trans-2-hexenal, trans-2-hexen-1-ol, cis-3-hexenyl acetate, cis-3 hexenyl acetate, cis-3-hexen-1-ol, penta-1-en-3-one, cis-2-pentenal, trnas-2-pentenal, and other. b Free tyrosol and hydroxytyrosol and their aglycons.
the enzyme preparation was found to aid in producing a more environmentally friendly liquid waste.
9.3 Negev olive oil extraction studies Until the past decade, most of the olive production in Israel and, consequently, the oil presses were located in the northern and central parts of the country. However, following the development of advanced saline water irrigation technology, specifically engineered and approved for olive trees (Wiesman et al., 2004; Weissbein, 2006; Weissbein et al., 2008), a significant increase in olive cultivation has taken place in the southern Negev region. The rapidly increasing trend for olive oil cultivation in the Negev Desert area may soon lead to the area producing approximately 30 percent of Israel’s total olive oil output. As the cultivation of olives has become more intensive in the Negev Desert area (Figure 9.6), the oil extraction process has also advanced. A study
234 Chapter 9
Figure 9.6: The main environmental factors affecting olive oil production in the Negev Desert.
on olive oil extraction was initiated and has been further developed in several units located in various parts of the Negev. An experimental research station, the Ramat Negev Desert AgroResearch Center (RNDARC), established by pioneers such as Yoel DeMalach and others many years ago, provides basic horticultural and cultivation facilities, mainly using saline irrigation and drip irrigation technologies for efficient development of the olive industry. Ben-Gurion University, which is located in the capital of the Negev area at Beer-Sheva, has initiated a long-term research and development study of desert olive oil extraction technologies and oil analyses. Until recent years, the main olive variety cultivated in the Negev region was Barnea. However, other varieties from different sources of origin have been introduced and were used in these
Oil extraction and processing biotechnologies 235 olive oil studies. Among these are varieties originating in Israel (Barnea, Souri, Maalot), Italy (Frantio, Leccino), Spain (Arbeqina, Pecual, Picudo), Greece (Kalamata, Koroneiki), France (Picholine), Morocco (Picholin di Morocco) and many others, as described in Chapter 7.
9.3.1 Oil potential of Negev-grown olives In 1997, the Phyto-Lipid Biotechnology Laboratory of the Ben-Gurion University of the Negev initiated a study to try to gain an indication regarding the potential oil production from olives cultivated in the Negev Desert region, and to develop an efficient method of oil extraction. The study continued until 2007. Initially, the basic systems for oil extraction and determination of oil content were organized (a brief description is given below; a more detailed discussion is provided in Chapter 8). r
r
r
r
Soxhlet extraction. This is quite an old method, which detects the oil content using solvents. In this method, all traces of oil are removed from the olive material. Soxhlet apparatus is generally used, and hexane is the most common solvent. The extracted oil does not meet the requirements for extra-virgin olive oil, so this method is employed only in the production of refined pomace oil and for analyzing the amount of oil accumulated in the olive fruits. Soxhlet extraction is generally agreed to provide very accurate results in comparison to most of the other common chemical oil analysis methods; however, the method is time-consuming, which limits the number of olive samples that can be analyzed. Near infrared (NIR) analyzer. NIR technology can measure the oil and moisture in milled olives and fatty acids, and the moisture in oil. This instrument works by passing near-infrared light through the oil sample and measuring the emerging energy, which relates to the concentration of oil, moisture and fatty acids. This method only determines the percentage oil content. IR spectroscopy. In this method, a small sample of olives is crushed and dissolved in strong solvent, and then analyzed using infrared absorbance – a non-dispersive infrared spectrophotometric technique which is specific to hydrocarbons such as oil. The HORIBA’s OCMA-350 oil content analyzer is commonly used for this purpose (Weissbein, 2006). IR spectroscopy is also used to detect the percentage of oil in a particular olive fruit. Once a good oil calibration curve has been achieved, this technique gives very similar results to those from the soxhlet extraction method. Low resolution nuclear magnetic resonance (NMR). Low resolution NMR is a rapid, non-destructive and highly reliable technique for detecting the oil content in olive fruits (Nordon et al., 2001; MARAN ULTRA, 2006). This is a new technique, based on a 23-MHz low resolution NMR system that has recently (2006) been introduced
236 Chapter 9
Figure 9.7: Oliomio mini-olive mill system: (a) main system; (b) olive oil waste material; (c) pure olive oil drainage.
and used in the Phyto-Lipid Biotechnology Laboratory at Ben-Gurion University of the Negev. Following two years’ experience of its use, the system appears to be highly reliable and as accurate as the soxhlet system, but the oil content can be determined in just 16 seconds. r
Oliomio mini-olive oil mill. The Oliomio mill is a small mill suitable for olive oil extraction (Figure 9.7). This mini olive-oil system provides high-quality olive oil in a similar way to the large-scale industrial olive oil mill systems. The proximity of this system to the experimental plots in the Negev Desert enabled production of the best olive oil, soon after harvesting, from trees of the various olive varieties being cultivated in the region using the advanced cultivation technologies developed over the past decade (Wiesman et al., 2004). Subsequently, two main long-term studies were carried out simultaneously. One was dedicated to evaluating the olive oil potential of various olive varieties tested in the Negev Desert area, and the second was focused on developing proper and efficient oil extraction in the same region. These studies were based on advanced laboratory olive oil analysis and used the Oliomio mini olive oil mill system for extraction.
The olive oil content of Souri olives as determined by various systems is shown in Table 9.5. The level is relatively similar in the three systems tested, although the percentage obtained using the Horiba system is slightly lower than with the soxhlet system or low resolution NMR.
Oil extraction and processing biotechnologies 237 Table 9.5: Determination of percentage olive oil content of Souri olives cultivated in the Negev Desert region in the 2006 season, measured by three different methods
Analysis date Nov 21 Nov 28 Dec 7 Dec 12 Dec 19 Dec 29
Olive oil percentage Soxhlet
Horiba 350
LR NMR
12.9 13.8 14.5 18.3 18.8 18.6
11.9 12.7 13.9 17.6 18.1 18.0
12.2 13.7 14.9 18.1 18.7 18.7
These values are the means of five samples for the Soxhlet and Horiba systems, and of 50 samples for the LR NMR system.
Table 9.6: Olive oil extraction from Negev Desert cultivated olive varieties using the Oliomio system and laboratory Horiba 350 analysis, 2005 season
Olive variety Barnea Souri Leccino Frantoio Picual Arbequina Kalamata Koreneiki Phicolin
Olive oil percentage Oliomio
Horiba 350
14.3 16.6 14.1 12.9 14.0 15.0 10.3 15.8 18.0
17.2 19.8 16.4 15.2 16.5 16.9 16.7 18.6 20.9
These values are the means of three separate Horiba analyses and one Oliomio extraction of each olive variety.
The data obtained in Table 9.6 indicate an average difference in oil extraction of about 2– 3 percent between the Oliomio mini-olive oil system and the Horiba system. Usually, such a difference between an olive oil mill and laboratory testing is well accepted, as laboratory analysis is supposed to provide information regarding the oil potential of olives. Furthermore, the difference is explained by the fact that for laboratory analysis only the flesh of the olive is crushed and assessed for oil content, whereas in the actual mill system the crushed paste consists of both flesh and stone, and of course a certain amount of oil is lost in separating the oil fraction from the organic waste material. Only in the case of Kalamata was a large difference in oil extraction shown; here, the Oliomio result was more than 6 percent less than that obtained by the laboratory system (10.3 versus 16.7 percent). Laboratory analysis therefore provides a
238 Chapter 9 Table 9.7: Olive oil extraction from Negev Desert cultivated olive varieties by the Oliomio system: comparison of trees cultivated with saline and with fresh water, 2004 season
Olive variety
Olive oil Saline water (4.2 dS/m)
Barnea Souri Leccino Frantoio Picual Arbequina Kalamata Koreneiki Phicolin
15.3 19.6 12.6 12.9 12.8 14.4 14.9 19.8 21.4
Percentage Fresh water (1.2 dS/m) 16.2 17.8 13.4 10.2 10.5 12.6 12.7 16.6 18.3
relatively good estimate of the oil potential of olives; however, in an actual milling system it can be difficult to separate the oil and the waste material. The relatively high levels of emulsification agents such as phospholipids mean that it is necessary to add a significant volume of water to precipitate the heavier organic material from the lighter oil fraction. In Kalamata and some other olives, this seems to be the cause of the unusually low level of oil extraction. It is not clear whether this phenomenon is directly related to desert environmental conditions or to cultural practices, or whether it is mainly related to genetics. A study to address these issues is already underway. The percentage oil content in the waste material obtained in the final Oliomio process is shown in Table 9.7. The results presented in this table show that some olive varieties produce more oil when irrigated with saline water, while others produce more oil when irrigated with fresh water. This suggests that the use of saline water does not directly affect the rate of olive oil accumulation. As discussed in Chapter 7, it appears that the main factor dominating the oil yield of each variety is genetic. However, saline water may affect the rate of maturation of each olive variety, and thus indirectly affect the oil content of olives at an early or later date. Based on experience gained over recent years, it seems that saline water irrigation combined with Negev Desert conditions delays somewhat the rate of olive maturation in comparison to other areas in the northern part of Israel that are not considered to be desert. This issue needs to be systematically studied. In any case, as discussed in Chapter 8, it is very important to monitor the level of olive oil content regularly. Another study carried out in the Negev Desert area regarding the effect of intensive irrigation versus dry regulated irrigation on Muchasan olive oil extraction clearly showed that the water content of the intensively irrigated olives (58 percent) was significantly higher than in olives that
Oil extraction and processing biotechnologies 239 were cultivated under supplemental irrigation only (42 percent). Obviously, the oil content was higher in the less intensively irrigated olives (26 percent per fresh weight) than in the intensively irrigated olives (21 percent). Furthermore, the efficiency of olive oil extraction in a commercial tri-phase olive oil mill was greater in the less irrigated olives than those that had been more intensively irrigated. This was concluded from the oil percentage obtained from the solid waste material (6.5 versus 9 percent, based on dry weight, respectively). These results suggest that it is important to gradually decrease the rate of irrigation in the final stages of olive maturation. This practice is indeed risky in desert environments, but may pay for itself in easier mechanical harvesting (as discussed in Chapter 8) and also the higher operational efficiency of the olive oil mill in obtaining a greater amount of oil from the olives.
9.4 Decantation, bottling and shelf-life, with an emphasis on Negev Desert conditions Premium-quality oils should be stored in stainless steel containers and maintained at a constant temperature between 8◦ and 18 ◦ C. After processing, the oil should be stored in bulk for 1–3 months to allow any particulate matter and vegetable water to settle out. Bulk storage and decantation eliminate the problems of sediment in bottles and oil contact with processing water residues that could lead to off flavors in the oil. When olive oil is too old and has oxidized, it usually becomes rancid. Rancidity is most commonly detected by taste, but can be checked chemically. The “rancimat” chemical method is mostly used for large industrial frying operations. Oil doesn’t suddenly go rancid; it slowly becomes more oxidized and, as it does, the flavor suffers. Different oils age at different rates. Some olive varieties make oil with more natural antioxidants, which resist ageing. These oils may be good for up to three to four years if properly stored in unopened containers. Other oils, particularly unfiltered, may be unpalatable in a year even if stored well. A two-year-old olive oil may taste rancid to some while not to others. Most people would be put off by the taste of any vegetable oil more than four to five years old. Rancid oil has fewer antioxidants, but is not poisonous. A good percentage of the world’s population routinely eats rancid oil because of lack of proper storage conditions, and some actually prefer the taste. In historical times, olives that had dropped to the ground or may have spoiled were made into olive oil which was stored in open-mouthed earthenware vats. Practices like these encouraged rancidity. People have come to expect non-rancid oil in the past 50 years because of chemical refining and better production and storage methods. Heat, exposure to light, and exposure to air are the three major factors that affect the oxidation and rancidity of the oil in bottles. Since desert areas are prone to all these conditions, extra
240 Chapter 9 precautions need to be taken, especially regarding processing and bottling, and storage periods. A tightly capped bottle that is full will oxidize less than a large tin that is only half-full. The longer the oil sits, the more rancid it will become. The following factors also affect the quality of the oil and its shelf-life. r
r
r
r
r
Olive variety. Polyphenols and other natural antioxidants in the oil help to keep the oil from going rancid, and some varieties have more antioxidants than others. The Tuscan varieties – Frantoio, Coratina, Pendolino and Leccino – tend to be higher in polyphenols than others. Time of picking. Olives picked earlier in the year may have more polyphenols and a longer shelf-life. Low-polyphenol olive oils often needed to blend with high-polyphenol oils to give a longer shelf-life. Picking method. In some parts of the world, nets are placed under the trees and the olives are allowed to drop for weeks as they ripen. Any remaining olives are then beaten from the tree and the nets are gathered. Some olives may therefore have been off the tree for weeks or months, spoiling in the interim. Olives that are badly bruised during picking will become more rancid if not pressed within hours. However, in intensive cultivation olive fruits are harvested only by machines, so there it is important that great care is taken during picking time, especially in desert areas. Time to milling. Live olives start to die once they have been picked, and the longer it takes to get to the mill, the more oxidized the oil will be. Producers who want to produce extra-virgin oil must get the olives to the mill within 48 hours of picking. Refrigeration and good ventilation of the orchard bins extends the olives’ life. Milling method. During the milling process, the olive paste may be exposed to air. The old-fashioned stone wheel and hydraulic press with jute mats or Rapanelli discs may be very picturesque, but expose the paste to far too much air. In modern techniques, where malaxation is carried out in specially made tanks, it is important that the paste is always enclosed with minimal contact with the air. Some mills bathe the paste with an inert gas during the mixing or malaxation step, or perform it in a vacuum. Heating the paste during malaxation will extract more oil, but will hasten oxidation. Centrifugal presses expose the paste to zero air.
Extra-virgin olive oil is not filtered, because it can lose some aromatic and taste properties during the procedure, so it is decanted. This process takes about 40 days; the olive oil is left to rest at a temperature of about 18 ◦ C, and the residues in suspension begin to sediment on the bottom of the tank. It is then only necessary to remove the oil via an outlet situated above the level of the solid residues.
Oil extraction and processing biotechnologies 241
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