There are three fundamental methods for extracting the essential oil (Di Giacomo and Mincione, 1994):
1 essential oil extraction from the whole fruit, preceding juice extraction (Figure 6.1);
2 juice extraction precedes that of essential oil (Figure 6.2);
3 extraction of essential oil and juice takes place simultaneously (Figure 6.3).
In their turn, all the procedures for essential oil extraction from the whole fruit can be classified as follows, according to the way in which the surface of the fruit is treated:
• superficial rasping, generally not a drastic operation, and one which causes minimal damage to the tissues;
• total abrasion or peeling of the exterior layer containing the essential oil glands.
Furthermore, with regard to the machines that operate on the peel after the juice has been extracted, the most widespread process is that of 'slow-folding' or sfumatura, employing a mechanism which largely reproduces the manual sponge process. The extraction can also be carried out in a more drastic form, by means of continuously working presses.
Juice extraction can also be carried out on half-fruit using machines based on diverse principles. In some, extraction is carried out using revolving rosettes, without causing much deformation of the peel, whereas others use a more vigorous action on the half-fruit, extracting a notable amount of essential oil, as well as other liquids from the peel, above all in cases where juice extraction precedes that of oil.
During the FMC-in line process, the juice and the oily emulsions are conveyed separately; also the pulp and seeds are passed down a separate spiral channel. The finishing system for essential oils consists of a vibrating screen to separate the oily emulsion, then a centrifugal clarifier to concentrate the emulsion and after that a centrifugal polisher to completely separate the essential oil from the liquid phase. The latter is returned into circulation and re-sprayed onto the interior of the machines, washing off deposits of extracted oil to form another emulsion. From the extractors the juice goes into special collectors which pass it on to the finisher. Here the juice is separated from a certain aliquot of pulp, fiber and seeds. The system, which functions like a screw, together with the continual advance of the product through the machine, ensure that the sieves are kept clean and the juice flows quickly. A further reduction in pulp content can be obtained through a centrifugal separator.
The juice obtained, especially if earmarked for immediate packaging, must be deoiled and deaerated. A high essential oil content in the juice can impair the quality, following the alteration of some of its components. Amongst other things, deaeration allows the heat exchangers to work better, and allows the final containers to be uniformly filled.
After deaeration, the juice must be pasteurized. So that the action of the heat does not damage the organoleptic characteristics of the juice, flash-pasteurization is necessary; that is, pasteurization completed in less than a minute. Pasteurization is generally carried out using plate heat-exchangers, that work by passing the juice in
a thin layer between two specially designed plates. The heat exchange is effectuated using stainless steel plates heated by a fluid (hot water or low-pressure steam) that circulates in countercurrent. Apart from the liquids mentioned above, cold water is also used, to bring the juice back down to room temperature; an economy in the thermal balance is obtained by using the heat given off by the just pasteurized juice to start the heating of the juice about to be pasteurized. When the juice is earmarked for canning, the juice does not pass through the cooling phase: this allows the juice to be canned at around 90 °C. Obviously, once the lid has been clamped on, the containers are cooled rapidly.
The vacuum concentrators can be grouped in two categories:
1 concentrators working at low temperature, in which the consumption of steam and water is given relative importance;
2 rapid concentrators working at high temperature, with reduced consumption of steam and water.
In recent years, the second category of concentrators has had more widespread use. In particular, the FMC TASTE evaporator (thermally accelerated short time evaporator) is, perhaps, the only evaporator designed and built for citrus juices. The natural juice is pumped into the evaporator where, after a pre-heating cycle and an initial evaporation stage, it is pasteurized and stabilized. During evaporation the juice is nebulized inside the Tube Nest at a very high speed, expanding in the separator very quickly. Juice evaporation occurs in a single passage through a number of stages. The machine can also be equipped with an Aroma Recovery System: aromas are extracted from the juice and concentrated 150 times before pasteurization. Before leaving the evaporator, the concentrated juice passes through the flash cooler, which reduces the temperature to 10 °C.
An improvement in the quality of the juice is obtained by operation cut-back: the just produced concentrate is immediately chilled to a temperature close to freezing point, is put in tanks with a double cooling jacket and then freshly squeezed juice is added to reach 42° Brix.
Before the filling of the final containers (e.g. plastic drums, or metal drums with a plastic lining), the juice is sent to tanks where standardization is carried out if necessary, and the juice is further cooled to around —6°C, either in the same tank or in a continual cooler of the 'Votator' type.
Finally, the concentrated juice is put in the drums by an accurately controlled system with preset weight scales.
The production of essential oil, natural and concentrated juice, and peel make up the fundamental phases of what is commonly called primary processing of the citrus fruit.
To these above mentioned phases should be added the processing that may be carried out either in the same plant, or in external plants. Above all, the complete utilization of the fruit and, particularly, of the refuse material and the peel, from which, by various procedures, other important products can be obtained (Braddock, 1999). These products will be described in a separate chapter.
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