Most essential oils are isolated from the respective plant parts by the process of hydrodistillation or steam distillation. A few essential oils such as those present in the citrus fruit peel can be, and in large part are, obtained by pressing, which yields a product of superior quality. The long action of heat with boiling water affects some thermolabile constituents which may decompose due to hydrolysis, polymerization and resinifi cation.
Therefore, the essential oil obtained by distillation does not represent the natural oil as it originally occurred in the plant material. In such plant ma- terials, essential oil is extracted by the process of expression or solvent extraction.
Before dealing with the process of expression in citrus oil, it is important to review the structure of a citrus peel. The citrus peel contains numerous oval, balloon-shaped oil sacs and glands or vesicles (0.4-0.6 mm diameter). These are ductless glands which are irregularly distributed in the outer colored (fl avedo) portion of the peel of maturing and matured fruit, in the outer mesocarp beneath the epicarp and hypoderm and above the inner mesocarp (albedo), respectively. Albedo is made of cellulose, hemicel- lulose, lignin, pectin, sugars, glycosides, etc. On maturation, cells of albedo become elongated and branched, with large intracellular spaces which give the ripe peel its spongy texture. The spongy layer plays an important role in the expression of oil, but it easily absorbs the oil ejected from the sacs and causes some mechanical diffi culties in oil extraction. The freshness and stage of maturity affect the oil ejection from the peel. The total weight of the peel is about half that of the fruit and the oil content is 0.5%-0.7%.
The cells surrounding the oil sac contain salt (colloids) in aqueous solution.
In contact with water, the higher osmotic pressure of the cell results in diffu- sion of water into them, increasing turgor pressure and causing the oil sac to stress from every side. If the spongy tissue is not fi lled with water, it will absorb the oil as the sacs are broken and hold it with great tenacity. So the pressure exerted yields fi rst aqueous fl uid and later oil.
In Sicily, peels are immersed in water for several hours before being subjected to a “sponge” process. A dilute aqueous solution of salt acts as a carrier to prevent loss by spurting. The product of expression is not a simple mixture of oil and water but a thin emulsion which is left to stand; gradually a supernatant layer forms. Filtration through a sponge ab- sorbs the colloidal material and leaves the mixture of oil and water.
4.3.1 Process of Expression
4.3.1.1 Hand ProcessIn this process, the freshly harvested fruits are cut transversely into two halves. The pulp is removed with a sharp-edged spoon called rastrel- lo. The peel is then immersed in water for several hours and fi nally pressed by hand. The worker holds, with the left hand, either one large or two smaller fl at sponges on top of a wooden crossbar and, with the right hand, presses the peel against the upper sponge. Thinner peels can be pressed from in- side. The emulsion ejected from the oil sacs is soaked by the sponge, which retains solid matter and absorbs colloidal substances. Squeezing the con- tent of the sponge from time to time, oil is fi nally decanted and drawn off.
This process requires much labor and the yield of this method is 50%-70% of the total oil present in the peel. The quality of oil obtained from hand press- ing is near to the quality present in fruit peel. A large number of small units in Sicily and Calabria were formerly responsible for the entire Italian produc- tion of lemon and orange oil, but the process is not in use now.
4.3.1.2 Ecuelle Process
This process was common in the south of France. Ecuelle con- sists of a shallow bowl of copper with a hollow central tube with which it forms a sort of funnel. The bowl is equipped with large brass nails with blunt ends, on which the fruit is rolled by hand with some pressure until the entire surface of fruit yields its oil. The oil and aqueous cell contents drain into the central tube where they are separated by decantation. The yield is only 20%
of the total oil present in the peel.
These methods were quite laborious and, with the advance of technology, machines have been invented to do them. Nowadays, rinds are extracted for oil using centrifugal force.
4.3.1.3 Hand Machine
For expression, the peel is placed in a hollow sponge attached with other sponges to a plate actuated by the lever and fi xed below with additional sponges to the base. The sponge is fi tted with a funnel through which oil and aqueous material pass to the receiving vessel. The part that comes in contact with oil is made of brass or bronze.
4.3.1.4 Sfumatrici and Pelatrici
The machines that treat only the peel after removal of juice and pulp are called sfumatrici, while those that process the whole fruit are known as pelatrici.
4.3.1.4.1 Special Sfumatrice
This is a specially designed roller type machine in which each peel is bent to expel the maximum quantity of oil. Not much pressure is ex- erted to expel other contents of the cells. The emulsion is collected and fi l- tered through wool or sponges to yield oil and water. A number of sfumatrici have been developed and were in use in many countries.
In expression using sfumatrici, two approaches are used. In the fi rst approach (used by Ramino Sfumatrice), only the peel is treated, so halving the fruit, removing the pulp and expressing the peel are the steps involved. In the second approach, the oil is extracted by either puncturing the peel glands or cutting a superfi cial layer of peel to expose the oil glands;
this is followed by washing away the oil with a spray of water.
In whole fruit extraction with a rasping machine, the whole fruit is crushed and oil is subsequently separated from the aqueous phase (juice and cell liquid). As citrus peel oil is a byproduct of the citrus juice industry, both the oil and juice are extracted subsequently, e.g. in a rotatory juice
extractor or Pipkin peel oil press. Here, the fruit is washed automatically, sorted according to size and then cut in two halves. The halved fruit is passed between two cylinders which remove only the juice by gentle press- ing without affecting the oil glands on the surface. The residual halved peels are extracted for essential oil. In the Pipkin peel oil press, for example, oil is expressed by two close stainless steel cylinders with capillary grooves run- ning around the circumference. The expressed oil automatically comes out of the grooves and there is no need to spray water..
4.3.1.5 Modern Machines
Nowadays, complete mechanization has been achieved and whole fruit processing machines have been developed. These machines ei- ther crush whole fruit and then separate the oil from the aqueous phases by distillation or centrifugation, or they express the oil in such a way that it does not come in contact with the juice during the process. The oil extractor developed by Brown International, California, liberates the essential oil from whole citrus fruit. Oil removal is achieved by lightly puncturing the entire surface of the fruit with over three million sharp stainless steel points con- fi gured in the form of rotating rolls. An adjustable speed differential between adjacent rolls controls the amount of pressure applied to the fruit. Whole fruits roll across the brown oil extractor (BOE, Figure 1), which is made of toothed rollers partially set in a fl owing bed of water which propels the fruit across the machine and simultaneously punctures the peel to release the oil from the glands. The fruit proceeds to the extractor and the oil-water mix- ture goes to centrifugation and to the oil recovery chamber.
Figure 1: Brown oil extractor
4.3.1.6 FMC Whole Fruit Extractor
Food Machinery Corporation (FMC) of San José, California, has developed many designs and improved extractors in which both the juice and the volatile oil are extracted without getting mixed with each other. Fur- ther details are available at: http://fi les.asme.org/ASMEORG/Communi- ties/History/Landmarks/5549.pdf.
Figure 2: FMC whole fruit extractor
The FMC juice extractor has two inter-meshing jaws which encom- pass the fruit, crushing it between them. The juice exits through a mesh screen which penetrates the center of the fruit and the juice is separated from the peel, pith and seeds. This crushing action is suffi cient to force the oil out of the glands on the surface of the fl avado. The FMC machine sprays water onto the surface of the fruit, and the oil-water emulsion is subjected to centrifugation.
Traces of water and waxy material are separated by keeping low temperature.
About 75% of citrus oil production utilizes this technology. The odor of cold pressed oil is true in nature and similar to that of the oil present in fruit.