The recovery of essential oil from the emulsion obtained during the extraction phase is a crucial step in the processing cycle.
When the extraction is performed manually, either by the sponge process or the ecuelle method, water is not used and the tissues of the peel are only subjected to superficial grazing. In this case the liquid extracted separates rapidly after standing, into three layers: the upper layer is practically pure essential oil which merely needs to be filtered through paper; the intermediate layer contains emulsified oil and includes colloidal particles in suspension; the lower aqueous layer contains no oil and cannot be used. In order to recover oil from the intermediate layer a suitably absorbent material (sponge or wool) is impregnated with the emulsion and squeezed in a press. In this way the emulsion is easily broken, allowing the recovery of the essential oil.
When mechanised extraction processes are adopted, varying amounts of water are used to wash away the essential oil from the surface of the fruit or the peel to prevent it being reabsorbed and no spontaneous separation of the essential oil occurs. In this case the liquid obtained tends to separate very slowly into two layers. The upper layer, containing essential oil, is highly viscous, rich in mucilage and detritus and tends to form foam on its surface. The lower layer is coloured but contains no oil and, being of no value, can be eliminated. The emulsion is collected in a series of Florentine vessels. These are kept filled to the same level and linked up by connecting the lower part of each to the upper part of the next one, thereby allowing the accumulation of the emulsion in the upper part of the vessels. The aqueous phase from the last Florentine oil-collecting vessel is sent back into the extractor.
The absorption method of oil recovery, using sponge or wool followed by pressing and separation via hydraulic press, was used for a long time. This system has been
practically abandoned because as well as being lengthy and costly, the quality of the oil may be severely compromised by its prolonged contact with the aqueous phase.
Today, the essential oil is separated using centrifuges (Figure 8.13) for the following reasons: higher yields, speed of separation and thus reduced contact of the essential oil with the aqueous phase, and the possibility of utilising continuous operating cycles.
Centrifugation is preceded by the elimination or drastic reduction of the coarsest waste particles. The crude oil emulsion is passed through a finisher (screw or paddle type), having sieves with 0.5-1 mm openings, or over a shaker screen (20-80 mesh). Excessive finisher pressure should be avoided, otherwise the emulsion becomes enriched with pectic substances and suspended solids, causing increased viscosity which makes breaking the emulsion in the centrifuge more difficult. To this end it is convenient to apply the technique known as the Bennet process (1931). This consists of adding 2 per cent sodium bicarbonate with sodium sulphate to the water in circulation. The use of this additive offers undoubted advantages which facilitate separation in the centrifuge: firstly, to increase the difference between the specific gravity and the surface tension of the oil and the aqueous phase; secondly, sodium bicarbonate gives rise to the formation of sodium pectate, which is a less powerful emulsifier than pectin, and also neutralises the acidity present which has a negative effect on the quality of the essential oil; lastly, sodium sulphate tends to neutralise the electrical charge of the colloidal micelles.
A number of commercial enzyme preparations, added to the circulating water, can also have positive effects in the process of separation of the essential oil (reduction of viscosity and breaking the colloidal oil emulsion). The benefits obtained are: improved centrifugal performance; higher oil yields; lower consumption of fresh water and reduction in the quantity of waste water produced. The increase in oil is obtained indirectly, in the sense that the losses of oil during processing are reduced by using the selected enzyme preparation. The amount of enzyme used will depend on such factors as: variety, type and maturity of the fruit; amount of water recycled in the system; the length of time the emulsion remains in the system; pH and temperature of the emulsion.
A screen or filter at the point of discharge of the oil emulsion into the emulsionholding tank is a good safeguard against contaminating an entire tank in the event that a finisher screen is ruptured. The recovered oil emulsion will normally contain a small but significant amount of sand. This can cause undue wear on the centrifugal equipment unless it is removed. However, its removal can be accomplished quite readily and satisfactorily by using a liquid cyclone in the line to the desludger feed tank from the small surge tank next to the oil finisher.
The finished emulsion contains from 1 to 2.5 per cent oil, with suspended solids not exceeding 4 per cent.
The centrifugal process includes a number of different stages which can vary according to the size of the processing plant and the prevailing practice in the different citrus production areas.
In the USA, especially in Florida (Kesterson and Braddock, 1976), the finished emulsion is fed to a desludger centrifuge to produce a concentrated oil emulsion or 'cream' (70-80 per cent or higher oil content). The aqueous discharge effluent should not contain more than 0.1—0.25 per cent oil under optimum operating conditions. The oil-rich emulsion is fed to a self-thinking type polisher centrifuge (16,000-18,000 rpm) without adding water. The feed rate to the polisher should not exceed 1—1.5 gallons per minute, depending on the capacity of the polisher and the oil concentration in the feed. The aqueous discharge from a self-thinking type polisher should contain no more than 5-7 per cent oil. If the polisher does not have the self-thinking feature, water should be added to the polisher feed at a rate of between 3 and 20 parts to 1 part of oil-rich emulsion from the desludger. This water is not needed to break the emulsion, but is used primarily to keep the polisher clean.
In Mediterranean countries the recovery of essential oil from emulsion is carried out by two-stage centrifugation: a primary separation, which removes 50—70 per cent of the aqueous phase from the emulsion, and a second centrifugation to achieve complete separation of the oil phase from the residual water.
It has been demonstrated (Di Giacomo etal., 1999) that the temperature increases during the second centrifugation, and that some alteration of the components most liable to deterioration occurs, because of the close contact of the essential oil with air and the acidity of the emulsion. This problem can be avoided by using the following devices:
• a pressure tank for storage of the emulsion under nitrogen;
• a cooling apparatus to refrigerate the emulsion before centrifugation;
• centrifugal separation equipment operating in hermetic conditions.
All of these contribute to prevent or significantly reduce the molecular transformation phenomena occurring in the standard process for separating the rich emulsion which has been used in industry to date.
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