Rodano (1930) identified three distinct types of phenomena that occur during the cold-press extraction of essential oils from citrus fruit:
1 rupture of the epidermis and of the oil glands that contain the oil;
2 creation in the peel, of compressed areas, surrounded by areas which are under less pressure, through which the oil can be expressed;
3 abrasion of the peel, producing small pieces of debris.
Essential oil can be extracted either from the whole fruit or from the peel. When the whole fruit is processed, all three of the above-mentioned phenomena occur, while only the first two take place when the peel alone is processed. In addition, one of the extraction procedures in widespread use today (the FMC recovery system) involves the simultaneous extraction from the whole fruit, of both juice and essential oil.
The readiness with which the oil spurts from the oil glands is affected by the freshness and ripeness of the fruit. Essential oil issues more violently from unripe fruit.
In the case of bergamots, however, it is often helpful to store the fruit for a time prior to processing; it is easier in this way to separate the oil from the emulsion.
Opinions differ greatly over the relative merits of extracting the essential oil by processing the whole fruit or by processing the de-pulped peel. Processing of the whole fruit is better suited to continuous production (partly because an easier and fuller separation and clarification of the essential oil is achieved) and therefore to the industrial processing of large quantities of fruit. Processing of the peel, on the other hand, involves a discontinuous production cycle and requires a lot of space because the peel has to be treated with lime and left to stand for a period prior to extraction. However, the essential oil obtained from peel (especially that extracted by the sfumatura (slow-folding) method) is of higher quality than that obtained from the whole fruit. As a result, the method chosen will depend on the circumstances in each case. Large plants processing hundreds of metric tons of fruit every day will choose to work on the whole fruit with highly automated production systems, paying a price in terms of essential oil quality. Conversely, small companies processing limited quantities (tens of metric tons) of fruit per day are better off working on the peels using the sfumatura method, which produces higher quality oils.
In order to provide a full picture of the advantages and disadvantages of using the two different oil extraction processes discussed so far, the main implications of these methods for the production of other important and related citrus derivatives, namely juice and peel, are summarised below:
• if the essential oil is extracted before the juice, the quality of the juice is less likely to be compromised by the presence of high levels of oil. However, it is always good practice in any case to de-oil the juice prior to packaging;
• whole fruit which has undergone drastic surface treatment for oil extraction is more likely to release bitter substances from the albedo into the juice. Moreover, the recycled water used to wash the oil away from the surface of the fruit may contaminate the juice with micro-organisms. These two drawbacks can be minimised by washing the fruit again with water after the oil has been extracted. It is also a good practice to add an antiseptic to the water cycled through the extraction machinery and the system used to separate the emulsion;
• pulpless peel can only really be used satisfactorily to produce brined peel, since the lime used prior to the extraction of the essential oil is detrimental to pectin production (Di Giacomo and Lo Presti, 1956).
To conclude this brief discussion, we should point out that for many years, a number of Sicilian citrus processing plants have used the sfumatura method to extract oil from the peel without the pre-treatment with lime, which was considered indispensable in the past (Di Giacomo and Signorino, 1971). With improvements in technology, this last approach is certain to become ever more widespread and will lead to better yields and higher quality oils, and therefore the disadvantages of oil extraction from the peel will no longer apply. With this technique, a continuous production process can be used to extract the oil, since it is no longer necessary to let the peel stand prior to extraction. Furthermore, after extraction, the peel will still be suitable for pectin production.
Machinery for the treatment of whole fruit
The procedures for essential oil extraction from whole fruit can be classified as follows, according to the way the surface of the fruit is treated:
• superficial grazing, generally an operation which is not too drastic and which causes minimal damage to the tissues of the fruit;
• total abrasion of the surface layer containing the essential oil glands.
Both procedures are mechanised versions of manual prototypes:
• the ecuelle method, which originated in the area around Nice (France) and was used in the West Indies;
• the circular grater process, which in the past was used in Spain.
The manual extraction procedures and the famous 'Calabrian Machine' (still in limited use today) have already been discussed in our earlier chapter on the historical development of the citrus industry.
Brown oil extractor (BOE) (Brown International Corporation, Covina, CA, USA)
The BOE (Figure 8.3) is installed in the fruit stream after the washing stage and before the juice extractors. Citrus fruit is delivered to the BOE by a metering elevator which ensures a steady flow. An interruptor gate is incorporated into the inlet chute feeding the BOE. This brief pause in fruit flow allows additional time for the fruit to settle in the machine for maximum performance. The BOE extracts essential oil from whole citrus fruit. Oil removal is achieved by lightly puncturing the entire surface of the fruit with sharp stainless steel spiked rollers, rotating in a pool of water. An adjustable speed differential between adjacent rollers controls the extent to which the fruit is punctured. For complete coverage of an elongated fruit such as a lemon, the rollers not only rotate, but oscillate horizontally in opposite directions, causing the entire peel surface to be treated, releasing the oil. The puncturing takes place beneath the surface of a shallow pool of water to ensure that there is no loss of oil to the atmosphere. Drying rollers remove any water and oil clinging to the fruit surface after its discharge from the machine. This liquid is added to the oil-water stream leaving the BOE. The oil-water mixture then passes through a fine screen in preparation for centrifugation. A first centrifuge produces a stream of rich oil and the aqueous phase from this centrifuge is recycled back to the BOE to eliminate losses. The rich oil stream passes through a second centrifuge which continuously produces pure polished oil.
Figure 8.3 Brown oil extractor (BOE).
This process is described in detail by Waters (1993), who reports that it is possible to recover more than 50 per cent of the oil from Valencia oranges.
Polycitrus extractor (F.lli lndelicato, Giarre, Catania, Italy)
The Polycitrus extractor (Figure 8.4) is equipped with an automatic selecting device to expel rotten fruit through a screw conveyor, so that only sound fruit is squeezed. It can process simultaneously fruit of any size. The machine is composed of two units arranged in a cascade: one for the extraction of essential oil by rasping the surface of whole fruit, the other for the extraction of juice. All parts in contact with the products, including the covers, are made of AISI 304 stainless steel. The fruit arrives on the hopper where an automatic device regulates the flow to maintain a constant level. A batcher draws the fruit into the machine, where it is pushed forward by paddles between pairs of rotating rasping cylinders. The speed of the paddles can be regulated, so that the fruit remains on the cylinders for the time required for a complete rasping. This varies according to fruit variety and ripeness. The essential oil is collected by jets of water and the mixture flows into a finisher, where the solid particles are eliminated. The clarified mixture can then be conveyed to the centrifugal separators to recover the essential oil.
Figure 8.4 Polycitrus (F.lli Indelicato, Giarre, Italy).
After oil extraction, the fruit falls into the juice extracting section, where every piece of fruit is cut in half and each half is squeezed against a stainless steel screen.
Different models of this machine are available. For these the maximum working capacity with oranges are the following:
Type ZX2-M6 8,000 kg/h Type ZX2-M10 12,000kg/h Type ZX2-M15 19,800kg/h
With some modifications, the Polycitrus extractor can be operated without using water to remove and carry away the oil extracted from the fruit. The raspings obtained by the rotating action of the graters are collected in the appropriately-shaped lower section of the machine and from there are channelled out of the machine by two screw conveyers. The raspings are then pressed to extract the essential oil. The yields are fairly modest, but the oil obtained from oranges has an intense colour due to its high carotenoid content. This characteristic is much appreciated by producers of soft drinks, especially in those countries where the use of artificial colourings is not permitted.
Pelatrice speciale (Fdli Speciale, Giarre, Catania, Italy)
The 'Pelatrice Speciale' (Figure 8.5) is completely automatic and does not require any manual input while in operation. It can process oranges, lemons, bergamots, mandarins, limes and grapefruits, all in optimal yields. The machine can process roughly-washed fruit of any shape, size or degree of ripeness (including very ripe). As a result, no additional machinery is required for sorting, washing or brushing of the fruit prior to extraction.
The extractor consists of a series of abrasive rollers and a screw conveyor mechanism with an abrasive surface. The fruit is fed automatically from a drawer in the bottom of
the hopper into the processing boxes defined by the abrasive rollers and the screw conveyor. The extraction takes place under a constant mist of water from spray nozzles placed along the axis of the machine. The water is recycled and a special device, operated by an external winch, keeps the spray nozzle holes clear while the machine is in operation. The extractor is equipped with three variable speed drives which control: the rotation of the abrasive rollers (according to the hardness of the fruit); the screw conveyor (according to the kind of product desired); the feeding roller. All parts of the machine which come into contact with the oil-water mixture are made of stainless steel. The external surfaces of the rollers and the screw conveyor are covered with stainless steel sheets with hard-wearing abrasive spikes. The machine is usually supplied together with a filtering and pressing machine, to filter and press the solid
residues produced by the abrasion of the epidermis of the fruit, and with a waterrecycling pump.
Avena extractor (Giuseppe and Placido Avena, Pistunina, Messina, Italy)
In his famous book on essential oils, Guenther (1949) wrote that the Avena extractor (Figure 8.6) 'today represents a most efficient and attractive machine of general application'.
The extractor, which exploits the centrifugal principle, is very flexible and is able to handle different varieties of citrus at different degrees of ripeness (Braverman, 1949). As a result, it was widely used for many years, especially in Italy, Spain, Brazil and Argentina.
It resembles a large sugar centrifuge, about 1.2 m in diameter, in which the rotating portion is not the basket but two horizontal circular plates, the upper of which has a large central opening; the surrounding walls are stationary. The upper surfaces of the plates are fitted with stainless steel segments that have replaceable, pyramidal, abrasive spikes covering the peripheral area. The fruit is loaded by means of a hopper which is divided into two compartments, each of which has a discharge opening at the bottom.
Each compartment contains one plate-load (ca 25 kg). At intervals, the shutters at the bottom open and fruit is discharged onto the two plates. Because of the centrifugal force, the fruit is slammed to a greater or lesser extent, according to the speed setting used, the length of the treatment and the length of the spikes on the plates. Inside the machine, water is sprayed to carry away the essential oil and the debris produced. When the pre-set abrasion time has elapsed, the abraded fruit is expelled and a new cycle starts.
The average capacity of the most common model is about 1 metric ton of fruit per hour, but higher capacity models have also been made. Speed regulators allow the operator to vary the speed of rotation of the plates and the length of the treatment, according to the quality of the fruit and the kind of product desired. Good quality essential oil can be produced with this machine if care is taken to ensure that the length of the abrasion treatment never exceeds 90 seconds, with a plate rotation speed of about 70rpm. Under these conditions, 'normal' yields are obtained. The quantity of oil extracted can be increased by applying a more vigorous treatment, but the quality of the oil is inevitably lower in this case.
The Avena extractor is better suited to processing hard-skinned citrus fruit. It can also be used on thinner-skinned and more delicate fruit, providing that the abrasive spikes are changed to avoid causing excessively deep incisions.
The use of the Avena extractor has been steadily decreasing in recent years and, in fact, it is hardly ever employed today. The main reason for this decline is undoubtedly its non-continuous mode of operation, which makes it difficult to incorporate this machine into continuous production cycles.
Pelatrice Moscato (CMA, Pellaro, Reggio Calabria, Italy)
It works on the same principle as the Avena extractor.
This machine, currently used for the bergamot essential oil extraction, consists of three rotary parts that will force the fruit to change direction of rotation, from that at the entrance of the extractor to that at its end.
Fraser-Brace excoriator (Fraser-Brace Engineering Co., Tampa, FL, US)
The Fraser-Brace extractor consists of a corridor formed by four horizontal, abrasive, carborundum-covered rolls revolving at high speed. As fruit passes through the extractor it is rapidly turned over and over on the abrasive rolls. A fine spray of water plays upon the fruit and rolls to carry away the oil, wax and grated peel. The oil and wax emulsion with suspended solids is pumped to a screen to remove the coarse particles and then it flows into settling tanks, where it is held from 3 to 12 hours. It is then decanted and passed into a centrifuge to separate the oil. The machine is completely enclosed and very little loss of oil occurs.
According to Guenther (1949), orange oil recovered by the Fraser-Brace extractor exhibits a dark red colour and a high evaporation residue, specific gravity, refractive index and ester content because of the high wax content. The optical rotation is low. The oil is low in aldehydes because of the very large amount of spray and carrying water required to remove the emulsion and cell detritus (100 parts of water per part of oil) from the fruit. Moreover, settling of the oil and cell detritus mixture for a period ranging from 3 to 12 hours results in major changes in the composition of the oil, due to enzyme action, etc.
AMC scarifier (American Machinery Corporation, Orlando, FL, USA)
The scarifier consists of a tilted frame on which 22 revolving cylinders are mounted. The cylinders are covered with stainless steel sheets which have been pierced, causing sharp points of the metal to protrude, similar to the points in a grater. These points puncture the oil glands of the peel. The punched holes are of such size and spacing to prevent clogging with peel while, at the same time, ensuring excellent piercing action. The cylinders are mounted at right angles to the flow of the fruit. A variable speed drive controls the speed of rotation. Inside the machine, a mist of water coming from fog-type spray nozzles washes the released oil from the fruit and saturates the flavedo with moisture to minimise reabsorption of the oil by the peel.
A similar method of extraction is also used by the Israeli Koffler machine (Taglith Ltd, Tel Aviv, Israel) and by the Italian Citrorap (Bertuzzi, Brugherio, Milan, Italy), models 700, 1400 and CK series (Figure 8.7).
Indelicato MK extractor (Fdli lndelicato, Giarre, Catania, Italy)
The Indelicato MK extractor (Figure 8.8) consists of two vibrating platforms, one above the other, made up of fine-pointed spikes, which prick the peel of the fruit whilst leaving the surface apparently intact. The flow of fruit onto the vibrating platforms is regulated by a hopper at the back of the machine. The fruit is pushed through the machine by paddles made of stainless steel and plastic. The operation of the paddles is controlled by a variable speed drive which can be adjusted according to the citrus variety and degree of ripeness of the fruit being processed. Processing time can therefore vary from 50 to 200 seconds. Because of the vibration, the fruit bounces and turns around in all directions, allowing the spikes to prick and burst the oil glands. The expressed oil is washed away by sprays of water and passed through a screen to eliminate any detritus. This allows a fairly clean emulsion to be sent to the centrifuge.
The working capacity of this extractor is 4—4.5 and 1.8-2.5 metric tons per hour of oranges and lemons respectively.
The ideal term of comparison for all the machines that extract essential oils from peel is the manual technique of the sponge (sfumatura). In fact even the best machinery is not able to produce oil of a quality which even remotely approaches that produced manually. At best, as Cultrera (1954) put it, we descend 'from the noble to the merely good'. It seems therefore appropriate briefly to describe this method.
Peel which has been manually de-pulped with a special knife (rastrello), which is spoon-shaped with cutting edges, is thoroughly washed in limewater. It is then left to drip dry on woven mats or on special baskets. According to the degree of ripeness of the fruit, this takes from 3-4 hours (in hot or humid weather) to 24 hours (in cold weather).
Was this article helpful?