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| 1-p-menthene-8-thiol | nootkatone |
The citrus family is one of enormous commercial importance not only for its nutritious and flavoured edible fruits but also for several others valuable aromatic products derived from the leaves and flowers of these plants (e.g., Oil of Petitgrain, Oil of Neroli).
Citrus fruits have a common structure and consist of:
In citrus juice processing the segment membranes and the inner core tissue are called rag.
The following citrus fruits are of major importance in flavouring:
The characteristic aroma and flavour of freshly expressed juice is due mainly to aromatic constituents of the juice but also, in part, to minute droplets of skin oil which inevitably are present. The aromatic profile of citrus juices changes considerably shortly after the juice is expressed. Fresh juice is commercialized in bottles, which are shelf dated for a maximum storage time at cool temperatures of up to 17 days. There are also commercially available juices, with preservatives and antioxidants, shelf dated for a longer storage time.
Citrus juices destined to manufacture into processed products undergo heat treatment to inactivate pectic enzymes and destroy spoilage microorganisms. The required heat treatment depends on the container selected and the product's storage environment.
Citrus oils comprise both essential oils obtained from the peels of citrus fruits and essence oils obtained by concentrating citrus juice.
Oils isolated from other parts of citrus plants (blossoms and leaves) are not classified as citrus oils because the former show marked differences in composition and organoleptic properties.
Production of Citrus Peel Oils: apart from distilled lime oil, pressing produces citrus peel oils. Pressing of the peels for oil is often combined with juice production.
Production of Citrus Essence Oils: distillate concentration of citrus juices yields essence oils, which separate from the aqueous phase in the receiver when the distillate condenses. The composition of essence oils is similar to that of peel oils, but the essence oils usually contain a larger amount of aliphatic ethyl esters. Thus, their aroma resembles that of a particular juice more than that of peel oils.
Citrus oils contain up to 95% monoterpene hydrocarbons. Limonene is the major component of citrus oil. The important aroma-determining components of citrus oils are functionlized terpenes and aliphatic compounds (predominantly carbonyl compounds and esters) present only in very low concentrations. (+)-Limonene, with its low odour threshold of 10 ppm, contributes to the aroma of the citrus fruits. Traces of hydrogen sulphide and dimethyl sulphide are present in all citrus fruit juices and also contribute to their aromas.
Grapefruit oil is obtained by cold pressing of the outer peels of the fruits. It is a greenish-yellow liquid, with an odour resembling that of sweet orange oil, but more herbaceous and bitter. The typical aroma note of grapefruit is due to 1-p-menthene-8-thiol and to nootkatone, which are about 5% of the grapefruit oil.
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| 1-p-menthene-8-thiol | nootkatone |
Lemon oil is obtained by pressing peel from the fruits. It is a pale yellow to pale greenish-yellow liquid with a characteristic lemon peel odour. Citral, which is actually a mixture of two stereoisomers, geranial and neral, is the character-impact compound of lemon oil.
Lime oil may be either pressed or distilled, but the distilled oil is produced on a much larger scale. Pressed lime oil is a yellow to greenish-yellow liquid with a strong, characteristic odour, reminiscent of lemon. Distilled lime oils are produced by steam distillation of an oil-juice emulsion that is obtained by chopping the whole fruit; it is a colourless to pale yellow liquid, with a characteristic odour, which differs from that of the fresh fruit and the cold-pressed oil. Acid-catalyzed cyclization and dehydration of citral and linalool give rise to several compounds that occur at comparatively high concentrations and contribute to the typical aroma distilled lime oil (e.g., 1,4-cineole, 1,8-cineole, 2,2,6-trimethyl-6-vinyltetrahydropyran and 2-(2-buten-2-yl)-5,5-dimethyltetrahydrofuran).
Mandarin oil is obtained by cold pressing the peel of mandarin orange. The oil is a greenish-yellow to reddish-orange liquid, with a pale blue fluorescence and a characteristic odour, reminiscent of mandarin peel. The aroma of mandarin oranges is due to: thymol, N-methylanthranilic acid methyl ester, g-terpinene and a-pinene.
Orange oil bitter is obtained by pressing fresh peel from the fruits of Citrus aurantium L. subsp. aurantium (Rutaceae). It is a pale yellow to yellowish-brown liquid with a slightly mandarin-like odour and a somewhat bitter aroma. Its composition differs from that of sweet orange oil (its aldehydes content is lower and its ester content is higher).
Orange oil sweet is obtained from the peel of the fruit of Citrus sinensis (L.) Osbeck. It is a yellow to reddish-yellow liquid with the characteristic odour of orange peel and may become cloudy when chilled. The oils have a high terpene hydrocarbon content (mainly (+)-limonene), but their content of oxygen-containing compounds differs and effects their quality. Important for aroma are aldehydes, mainly decal and citral, and aliphatic and trepanned esters. The sesquiterpene aldehydes a-sinensal and b-sinensal, which contribute particularly to the sweet orange aroma, also occur in other citrus oils, although in lower concentration.
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| a-Sinensal | b-Sinensal |
The oils of different fruit have different applications:
