The Practical Planter

The Extraction Of Oil

From Citrus Fruits

The essential oils extracted from the peel of citrus fruits are valuable by-products produced, for the most part, in the processing of the whole fruit for juice. The fruits commonly extracted, which include orange, lemon, lime, grapefruit, tangerine, mandarin and bergamot, are usually either sub-standard or superflous fruit as far as the fresh-fruit market is concerned. Of these only bergamot is normally processed for the oil alone.

The essential oil of citrus fruits :curs in glands or sacs which are mated in the outer layer (or ivedo) of the fruit, above the hite, spongy portion of the peel ilbedo). These glands vary in size om 0.4 mm. to 0.6 mm. in ameter and occur at different ;pths in the flavedo. The sacs do >t have well-defined cell walls, but e merely spaces bounded by the :bris of cells which have become :graded in the formation of the oil ivity.

Amount of oil

The amount of oil present in the uit is very small, of the order of 5 to 0.7 per cent., and the aximum amount of oil can only : recovered by water distillation of e finely divided fruit; but as the 1 deteriorates considerably in this ocess the method is little used itside the laboratory, except in the se of distilled lime oil.

The actual yield of oil obtained immercially will depend on the uit to be processed, its condition and e method of extraction used. Yields ay be as low as 0.05 per cent, for apefruit, from 0.1 to 0.3 per cent, r orange and lemons, and as high 0.5 per cent, for bergamot.

While small quantities of so-called stilled oils are produced as by oducts in various stages of the ocessing of the fruit, by far the eater part of citrus oils are ob ined by rupturing the oil sacs, either

This article was supplied by the ntish Government’s Tropical Pro icts Institute.

by puncturing them, or by abrasion of the flavedo, or by expression.

In such methods the yield of oil is always less than that theoretically possible, often considerably so, owing to absorption of the oil by the spongy albedo.

A decision as to the method of extraction most suited to a pro ducer’s particular needs will depend on several factors. They include: • The quantity of fruit to be processed. • The availability and cost of Islands labour, • The amount of capital avail able for the purchase of equipment.

The numerous methods available for the extraction of citrus peel oil, which in the majority of cases are inextricably bound up with the extraction of the fruit juices, fall into two main groups—the manual and the mechanical methods.

Manual methods were originally used in the citrus industries of Italy and Southern France, and would almost certainly prove to be quite uneconomical today, unless an abundance of very cheap labour was readily available.

However, although they have been almost completely superseded by modern mechanical methods, the principle of extraction has, in many instances, been embodied in the modern machinery; for this reason a brief description of two of these methods is given. a. The sponge process. The oldest method, now obsolete, is the sponge method, developed in Italy. The fruits are halved, the pulp, removed with a sharp, spoon-shaped knife (“rastrello”), and the peels steeped in water for some hours. The swollen

peels are then pressed by hand over a sponge which absorbs the expelled oil together with water and tissue fluids. The sponge is squeezed out from time to time into an earthen ware pot in which the oil separates from the aqueous fluid. The sponge process gives an oil of better quality than can be obtained by any other method, but it is slow and costly, requires skilled labour, and has therefore been superseded. b. The ecuelle process. Another hand method, said to have originated in Southern France, is that of the ecuelle. This consists of a shallow copper bowl attached centrally to a tube closed at the lower end, and re sembling a funnel. The bowl is equipped with a large number of brass spikes, on which the fruits are rolled, thus puncturing the oil sacs.

The liberated oil, together with tissue fluids and fragments of peel, collects in the tube, from which it is decanted from time to time.

Mechanical methods

Mechanical methods of oil extrac tion are used throughout the world.

Whatever method of mechanical extraction is used, the fruits are first washed, either in a spray of water, or preferably by a series of revolv ing brushes in a tank of water, after which they are transferred to a cutting machine if the juice is to be extracted first (this is often integral with the juice extracting machine) or direct to the oil extractor.

The mechanical methods of ex traction may be conveniently clas sified into three main groups: 1, machines treating separated peels; 2, superficial working of whole fruit

Xtraction Of Citrus Oils

' rasping machines; 3, methods jating crushed whole fruits. 1. Machines treating separated els : All these methods, which were iginally developed in Italy, involve e preliminary separation of the el from the flesh of the fruit. The fits are cut in half transversely id either the pulp is removed by nd (rastrello), or the halved fits are squeezed for juice mechanic y by means of rosettes (reaming).

The hand machines are based on ; original sponge process. Working the principle of the lever, these ichines merely dispense with the duous and specialised hand-expres >n. They do not effect any great ring of time or expense and are little practical use today.

Numerous automatic machines r treating separated peels were veloped by the early Italian dustries and are known as : umatrici”. They are, for the most rt, simple and relatively inex nsive machines, depending upon ; same principle in having a wing member (e.g. revolving drum belt) mounted within or against stationary member.

Little demand

The peels are dragged round and ned and compressed against lugs, compressed in a narrow channel *med by the eccentric mounting the moving member. The oil is jally carried down to decanting ssels in a stream of water and : emulsion collected and pressed 0 wool or sponge to yield oil and ter, which are then separated.

The “sfumatrici”, although still pular in Italy have found little mand elsewhere. 2. Superficial working of whole lit by rasping machines : Machines irking on this principle have found de application outside Italy, and 1 the type of installation most ely to be suited to the needs of Dducers. These methods, which luire no preliminary cutting of ; fruit, and in which there is ;le or no contamination of the ce with oil, are best adapted to ; present requirements for the pro ction of oil and potable juice. The hausted peels may be used for : extraction of pectin, ensilaged as ;tlefood, or composted for manure.

A series of rasping machines, lose action is fundamentally related that of the ecuelle, have been Dived, many of which are only of

historical significance. For this reason only two models, in current use and likely to be suitable for the pro spective producer, are described.

The Avena machine, manufactured in Italy, has attained considerable popularity in other countries. The machine is similar in principle to a potato-peeler. The whole, washed fruits are fed, via a hopper, into a circular pot, the base of which re volves while the walls remain stationary.

Automatic extraction

The base is lined with removable segments covered with pyramidal, abrasive points of stainless steel, the walls being lined with glass plates bearing superficial non-abrasive pro jections. As the base revolves it throws the fruits against the sides of the machine, the projections serving to turn them, so that removal of the oil-bearing layer, which is effected mainly by the periphery of the revolving base, is even and com plete. The oil and detritus are carried away by a continuous spray of water from the top of the machine.

In comparison, the Koffler machine, designed for the automatic extrac tion of citrus oils and juices, was developed in Israel and utilises the “grater” principle for obtaining the essential oil. In this case, the fruits pass over a series of hollow rollers, surfaces of which are partly punched

through, as in the domestic “grater”.

The points thus formed pierce the oil sacs of the fruit, and the ejected oil is carried away by a spray of water.

This oil extractor, which is said to be capable of dealing with all kinds and sizes of citrus fruit, extracts about 80 per cent, of the total oil, and has a capacity of 7-8 tons of fruit per hour.

With the exception of the ecuelle and the sponge methods, in all oil extracting processes the oil must be separated from the emulsion and its accompanying detritus. The essential oil, together with tissue fluids and fragments of peel, forms an emulsion with the water used to carry the oil away from the machine. The emulsion is first screened to remove as much solid matter as possible; this may be accomplished by sieving or in a screw or filter press.

The emulsion and remaining solids are then separated in a centrifugal separator.

Frequently, a second high-speed centrifuge is used to “polish” the oil, that is, to remove the final traces of water from it.

Alternatively, where difficulty is experienced in separating emulsions, a second method may be used. In the abrasive processes considerable quantities of the tissue fluids and pectin are released from the peel, and these are immediately dissolved by the circulating water.

The tissue fluids are acidic and exert a damaging effect on some

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istituents of the essential oil, •ticularly citral. The pectin acts two ways: firstly, it is the main ise of the emulsification of the and secondly, it hydrolyses easily, Iding methyl alcohol, which in :ases the solvent power of the culating water for the citral.

It has been found that if the culating water is replaced by a Lite solution of sodium bicarbonate lout 2 per cent.) containing a le sodium sulphate, clear oil can separated at once from the ulsion on centrifuging. [t is claimed that lemon oils pre ed by this method are fully equal “sponge” oil as regards citral itent, and have the same com lition, flavour and odour.

Fhe bircarbonate solution can be irculated until the dissolved sub nces reach too high a proportion; the same time, the volume and mgth of the bicarbonate solution >uld not fall below certain limits, ese limiting factors are not cified quantitatively, and they will riously depend ultimately on the erity of the rasping to which the it is subjected—the deeper the ping the more tissue fluids will obtained, and therefore the shorter 1 be the “life” of the bicarbonate ution,

US methods

I. Methods treating crushed whole it: In the US there are two dis :t methods of processing citrus: i. The industry in Florida employs method, which is a mechanised sion of the Sicilian “sponge” tern in which oil is expressed m the peel alone. Rotary presses ve the fruit and remove the pulp 1 juice. The peels are then sent ough heavy rollers, the resulting jid strained and the oil separated high-speed centrifuges. ». The Californian industry ploys the crushed whole fruit thod designed especially for the ge-scale production of citrus oils 1 juices. The fruits are crushed by ivy rollers and the mixture of p, oil and juice is carefully eened. After the removal of solids oil and juice are centrifuged, but re is always a phase which is only >arated with great difficulty, rthermore, the juices so obtained tally contain considerable quantities oil which, in the course of time, ise deterioration in the flavour, is fault is now overcome by “de ing” and “de-aerating” the juice vacuum distillation at 160 deg. hr. for H to 2 minutes.