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WHAT MAKES AN ESSENTIAL OIL SMELL THE WAY IT DOES
See Scent and Fragrances by Ohloff

8.1. Citrus Oils

            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.

8.1.1 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.

8.1.2 Mandarin Oil

            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%.

8.1.3 Lemon Oil

            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.

8.1.4 Grapefruit Oil

            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.

8.1.5 Bergamot Oil

                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.

8.1.6 Lime 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!

8.2 Petitgrain Oil     

            Originally, Petitgrain, which means “small seed” was actually distilled from the immature and small, round green fruits of the Citrus trees.  Of course, if you distill the fruits then there will be no mature fruit to eat or preserve.  So eventually, the distillation was limited to the leaves and small branchlets but the oil is still called Petitgrain.

                        Petitgrain is produced by steam distilling the leaves and small branches of certain citrus trees.  The yields is 0.25-0.5%.  The most important of the Petitgrain oils comes from the Bitter Orange tree. Paraguay is a big producer of low end Petitgrain.  Higher quality Petitgrain comes from France and Italy.  Petitgrain has a strong, bittersweet, floral and somewhat woody odor, sometimes vegetative (bad), dry and a bit leathery. 80% of the yield is made up of linalyl acetate and linaloöl in a proportion of 2:1.  The woody smell is from the sesquiterpene alcohol, spathulenol.  Aldehydes contribute to the odorous principle, even though they are in small amounts.  Another important constituent is the pyrazines with their Galbanum-like green notes.

            It is important to mention that the trace components in Petitgrain leads to a good scent and especially in perfumery where they are important when the scent needs to be reconstituted.

             Blossoms of the true bitter  (sour) orange tree, Citrus aurantium Linnaeus, subsp. amara L., on being distilled yield Neroli bigarade oil. If, on the other hand, the leaves and petioles (leaf stalk) are distilled, oil and hydrosol of Petitgrain Bigarade is obtained.  The word Petitgrain comes from a French word, meaning the 'little' and 'seed, fruit or berry'. Oil of Petitgrain is also distilled in Paraguay.  However, in Paraguay, the plant that is distilled is a hybrid of the sweet and the bitter Orange that has gone wild and thus the oil and hydrosol is of lower quality. 

            True Petitgrain oil and hydrosol should originate exclusively from the true bitter Orange tree. These distillations show that the oil will have a relatively high laevorotation and the presence of leaves from sweet Orange trees will result in oils of lower laevorotation, or even dextrorotation.

            The quality of Petitgrain depends on various factors:

1. The leaf material should originate exclusively from the bitter or sour Orange tree.

2. The leaf material should not contain any wooden branches, nor any small unripe fruit (in spite of the name).

3. The leaf material should be distilled rapidly and with direct steam that is generated in a separate boiler. The leaves must not be immersed in water, as this will cause hydrolysis of the linalyl acetate, which is the most important constituent. Properly distilled the oil and hydrosol will have high ester content.

4. The plants of Southern France bloom in May and June and this cutting is used for Neroli production.  The leaves and petioles are harvested from the pruning after the Neroli harvest, which is from late June to October.

            Petitgrain oil and hydrosol is thus produced from the Bitter Orange tree after the harvest of the flowers (for Neroli); leaves and stalks are freshly picked in July-October and freshly and immediately distilled for the best product.          

            The quality if Petitgrain oil can be evaluated by its physicochemical properties. Here are the properties of genuine Petitgrain distilled in Southern France as outlined in Guenther's work.

 

Specific Gravity @ 15º 0.891 to 0.896
Refractive Index at 20º 1.4574 to 1.4584
Optical Rotation —4º50' to —5º38'
Ester Content, calculated as Linalyl Acetate 64.4 to 68.8%
Solubility in 70% alcohol Soluble in 3.5 to 4.5 vol. and more, sometimes with opalescence

            It should be emphasized that American producers wishing to produce Petitgrain oil and hydrosol be very careful as to the citrus variety they use, time of harvesting and the type of distillation.         

8.3 Neroli Oil

            850 kg of carefully picked Orange flowers yields 1 kg of Neroli oil after steam distillation.  Annually 2-3 tons is produced, mainly in North Africa.  The oil possesses a strong. Floral, powdery aldehydic odor, very fresh with a warm base note that resembles freshly dried hay.  It does resemble Petitgrain in its odor and often Petitgrain is used to adulterate Neroli oil.  Ocimenes are higher in Neroli than Petitgrain. Also limonene, linaloöl and linalyl acetate occur in larger amounts than Petitgrain.  Indole which possesses a powerful exotic floral note at high dilution and a somewhat fresh breast-fed baby shit odor when not diluted separates and differentiates Neroli from Petitgrain.  This indole odor is sometimes very prevalent in the Neroli hydrosol. Methoxypyrazine contributes to a green character, which also is the interesting green note in Galbanum and Green Peppers.  Nootkatone is not present in either Petitgrain or in Neroli.

OTHER INTERESTING CITRUS:  Yuzu, a Grapefruit type; Blood Orange with blackish flesh, dark essential oil and raspberry flavored juice; Fingered Citron whose rind is candied and eaten, essential oil is not known; Brain fruit whose leaves are called Keffir Lime leaves, not known what juice, if any, is used for, essential oil; and Limequats ?                     

-The end-

Bibliography:
1. Ohloff, Günther:  SCENT AND FRAGRANCES: Springer-Verlag. 1990. Translated by Pickenhagen and Lawrence {this is the main source}
2. Guenther: THE ESSENTIAL OILS, volume III, Citrus oils: Krieger. 1949.
3.  Rose, Jeanne: 375 ESSENTIAL OILS AND HYDROSOLS; Frog, Ltd. 1999.  
4. Williams, David G.: THE CHEMISTRY OF ESSENTIAL OILS: Micelle Press. 1996.

Citrus Notes:  Citrus oils are used in the perfumery business to impart a fresh, sparkling note to any blend.  They are usually not overpowering.  They can be used in up to 25% as the base for classic type of eau de cologne.  Citrus oils harmonize with a large number of other essential oils and they are used in different concentrations in almost all scent blends and modern perfumes. 

In combination with Lavender oil, citrus oils are the basis for English Lavender, which is an 1826 creation.   High concentrations of citrus oils are in Chanel No. 5(1921).  Also of importance are the citrus oils in pop drinks like Coca-Cola and others.

©All Rights Reserved 2003, 2004. No part of this article may be used
without prior permission from The Aromatic Plant Project.
©Author's Copyright and Jeanne Rose, info@aromaticplantproject.com



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