Citrus oils are obtained from various Citrus species found within the family of Rutaceae and the subfamily Aurantioideae. Citrus oils are located as glands within the peel and the cuticles of the fruits. Some citrus oils are also found within the leaves (Petitgrain) and flower parts (Neroli). Peel and cuticle oils are removed mechanically by cold pressing and since cold pressing yields a watery emulsion, this emulsion is then subjected to the centrifuge to separate out the essential oil. The resultant product will then always have bits of the wax and other organic matter left in it which is why this essential is subject to age and when chilled will become murky. Cold-pressing is the favored method of extraction because steam distillation yields an oil of inferior quality (with the exception of West Indian Lime peel oil).
Depending on the variety of the fruit, peel oils yield an essential oil content of 0.5-5.0%.
Main producing areas are the Mediterranean countries of Italy , Corsica , Sicily , etc. and California , Florida and South America , Each of these countries produce citrus oils that are organoleptically (1) identifiable. Orange oil from California may be Orange , while greenish color from Florida and deeper Orange from Italy . [Organoleptically = the subjective testing of food or products via the organs of sense; taste, smell, looks, feel.]
No single compound is responsible for the characteristic odor of any citrus oil. Odor is due to the fact that citrus odor is a complex mixture of related metabolites of terpenes and secondary metabolites of unsaturated fatty acids. The individual odors of the different citrus fruits and their various cultivars are not due to various different chemicals but rather to the proportions of the various chemical components. To identify the odor of any one citrus over another, it is as important to know the odor by your senses (strength, quality and perception), as it is to have a good chemical analysis.
Two components are necessary for a citrus odor, that is, limonene and citral. The major component of most citrus odor is (+)-limonene which can be up to 97% of the oil. It is responsible for the base sensory character of the citrus oils. (-)-limonene is another terpene that possesses a different odor such as that found in the pines, peppermint and eucalyptus oils. Fatty aldehydes also contribute to the aroma odor of citrus, especially octanal and decanal. Citral, an aldehyde that is also part of the chemistry of citrus oils, always occurs as a mixture of its stereoisomers geranial and neral. (A stereo or optical isomer is identical mirror-image forms of a component, one occurring in d or dextro=right or clockwise form and the other l or laevo=left in counter-clockwise form. Think of looking at your hand and then in the mirror.)
The sesquiterpene ketone (+)-nootkatone possesses a citrus-like aroma and a bitter taste and occurs in all citrus peel oils. It contributes a lot to the overall aroma character of any oil in which it is found. Its other (-)-nootkatone-ent lacks the citrus aroma.
The stereoisomeric farnesenes occur in all citrus oils. They are very important for apple flavor.
The isomeric sinensals are aldehydes of the corresponding farnesenes and are found in all citrus oils. Even trace amounts of these aldehyde components have high aroma value because of their strength of odor.
Cold-pressed oils also contain up to 4% of low volatile constituents, such as flavonoids or triterpenes that are the bitter principle.
Orange peel oil
Sweet Orange peel oil [(Citrus sinensis (L.) Osbeck]. is the most important of the citrus oils. It yields up to 0.5% by cold-pressing. It is mainly produced in the south of Italy, Spain, Portugal, Florida and Brazil. (+)-limonene is the main constituent of this oil. The aldehyde content of Sweet Orange oil is the measure of the oil. The preferred Valencia oil possesses up to 3% aldehydes. One of which is decadienal with an extremely high aroma value. Other constituents that contribute to the character and quality of Orange oil are the sinensals. x-sinensal has a high orange aroma scent and low odor threshold while b-sinensal has a metallic-fishy note that can be very objectionable.
The difference between Orange and Grapefruit oil can be as simple as the amount of (+)-valencene. When the amount of a-terpineol exceeds normal level, off-notes occur. This terpineol forms during the aging or oxidation of orange juice. (Some essential oil of Orange is indeed produced from Orange Juice). The acetates contribute to the floral notes of Orange oil.
The bitter Orange tree (Citrus aurantium L.) is used to produce the expensive Neroli flower oil while Petitgrain of Bigarade is produced from the leaves and small branches. The bitter peel oil differs from Sweet Orange oil less by its volatile composition then by its bitter taste that is caused by non-volatile constituents.
Mandarin (Citrus reticulata Blanco) and the cold-pressed oils of Tangerine and Clementine contain considerable amounts of methyl N-methyl anthranilate. According to some, if you mix this component with thymol in correct proportions you can duplicate a scent that is reminiscent of Mandarin. Add the terpenes of y-terpinene and --b-pinene and you can get an even more natural scent. a-sinensal is abundant in Mandarin oil up to 0.2%.
Lemon oil (Citrus limon (L.) Burm. f.), mostly of Sicilian or South American origin, is very much used in flavors and fragrances. It seems to have originated in southeastern China. Others consider the Lemon to be a native of India. The ancient Roman despite the vast extent of their empire did not know the Lemon.
Lemon juice oil contains up to 300 compounds. It is mainly (+)-limonene and also (-)-b-pinene and with terpinene-4-ol is responsible for the green, peely odor that one associates with Lemon oil. Other components are citral that is an aldehyde that is very important as far as odor and flavor are concerned. The difference between some types of Lemon oil can be the ratio between citronellal and citral.
The peel of (Citrus paradisi Macfaden) or Grapefruit oil is cold pressed. The fresh, fruity top-note is due to p-menth-1-en-8-thiol. This component is present only in very low amounts. Grapefruit oil is sesquiterpene rich, which is unusual in citrus oils. Nootkatone is mainly responsible for the odor of the Grapefruit and contributes to the bitter flavor of the juice. Linaloöl oxides, which are found in many essential oils, constitute the second most important class of compounds. Also found in the essential oil is epoxycaryophyllene, first found in Verbena oil, possesses a pleasant woody, balsamic odor.
The unripe peel of (Citrus bergamia Risso & Poit.) is cold-pressed. The fruit is bitter and inedible, however, it is available candied and eaten with bitter Coffee as a sweetmeat in Greece. The fruit yields about 0.5% essential oil. The tree is kept at about 15 feet in height and grown in Calabria, an Italian province. Bergamot oil is the only Citrus oil in which limonene is not the dominant component. It is however, rich in linaloöl and linalyl acetate up to 50%. Oxygenated derivatives of the hydrocarbons of caryophyllene, germacrene D, farnesene and bisabolene contribute to the typical odor of Bergamot. The strong woody odor of the aldehyde bergamotenal has been also known in Costus root oil.
The so-called distilled lime is prepared by steam distillation of whole fruits of (Citrus aurantifolia (Christm. & Panz.) Swingle). This steam-distilled oil is more important in the fragrance and perfumery industry than the cold-pressed oil. The more expensive cold-pressed oil is more like Lemon oil than what we associate organoleptically to Lime peel oil. Germacrene B has a woody-spicy odor that contributes to the fresh odor of Lime Peel oil. Lime, ummm!