PROSEA, Introduction to Essential oils
- 1 Definition and selection of species
- 2 Role of essential oils
- 3 Production techniques
- 4 Properties
- 4.1 Odours and odour description
- 4.2 Physical characteristics
- 4.3 Chemical components
- 4.4 Quality control
- 4.5 Quality standards
- 4.6 Adulterations
- 5 Classification of perfumes
- 6 Production and international trade
- 7 Botany
- 8 Ecology
- 9 Agronomy
- 10 Genetic resources and breeding
- 11 Prospects
Definition and selection of species
An essential oil is a mixture of fragrant, volatile compounds, named after the aromatic plant material of a single type and identity from which it has been derived by a physical process and whose odour it has. This definition indicates that a given essential oil is always derived from a single species or variety. The opposite is not necessarily true: a single species may yield several essential oils because it may consist of several genetically defined chemotypes. Cinnamomum camphora (L.) J.S. Presl, for example, comprises several chemotypes each yielding a specific essential oil (Jantan & Goh, 1992). Also, various parts of a plant may yield different essential oils. The best example is Citrus L., where the flowers, fruits and leaves of many species and cultivar groups yield essential oils, each with its own characteristic olfactive qualities (Boelens, 1991). To add to the variability, ecological conditions influence many aspects of plant growth and may influence the composition of an essential oil. In trade, the origin of an oil is therefore often indicated by its name, e.g. Rose oil Bulgaria and Geranium oil Bourbon.
The definition given above states that an essential oil is derived by a physical process, i.e. it has not been purposely changed chemically. In industry, some essential oils are separated into chemical constituents that are either used directly or may be processed into different aroma chemicals. Such chemically isolated or changed products will not be considered further here. However, the physical process by which essential oils are obtained may also influence the chemical composition of an essential oil. Water distillation, steam distillation, hydrodiffusion, expression and solvent extraction produce different essential oils from the same plant material, because not all components are extracted equally well by each process or because individual components may undergo changes during the process. Such differences are generally minor, but are important for the quality of the essential oil, especially in luxury perfumery.
No definition is without exceptions. Oil of wintergreen, obtained from Gaultheria spp., for instance, is much richer in methyl salicylate than the living plant. After harvesting, the plant material is fermented for some time and during this process enzymes convert precursors into methyl salicylate (Arctander, 1960).
Essential oils have many functions for people. Most obvious is their role in fragrance materials, but they are equally important as flavouring materials and in medicine. The distinction between these uses can be vague. Perfumes are often used to favourably influence mood; aromatherapy goes further, by exploiting this to create a soothing, tranquillizing or healing effect on a patient (Lis-Balchin, 1997). In traditional medicine, essential oils are important in driving out diseases. In medical practice in Asia they are also widely applied, but in Western medicine they are currently used mainly to improve the taste of drugs and as household preparations e.g. as disinfectants, against colds and in liniments against muscle pains. However, essential oils are also sources of many constituents that are used for more specific purposes. Linalool, one of the most widely ocurring compounds in essential oils, is an important source material for the production of vitamin E; citral, another important component of several essential oils, has long been used to produce vitamin A. The distinction between spices and fragrance materials is clearer. Spices have primarily a culinary use, to enhance the tastiness of foods, whereas fragrance materials appeal more directly to the sense of smell. However, many essential oils obtained from plants primarily used as a spice e.g. clove, ginger and cinnamon, also play a major role in luxury perfumery, cosmetics and functional perfumery. The reverse is almost equally true. Lavender leaves in a salad may add to its attractiveness, but the essential oil distilled from the flowering branches is primarily used to compose perfumes, cosmetics and industrial aroma products. Lemongrass oil is an important aroma product, whereas the leaves and leaf bases have an important culinary use in Indonesia and elsewhere. Citrus oils are indispensable for perfumery, but also for the flavouring of soft drinks. Rose petals are the source of one of the most important and costly essential oils, but because of their delicious smell and taste they are also made into confectionaries (Weiss, 1997).
Fragrant woods are important sources of aroma materials. Some of them yield fragrant exudates, such as the resins opopanax and myrrh from Commiphora spp., benzoin from Styrax benzoin Dryander and balsam from Myroxylon spp. Other fragrant woods such as Cinnamomum camphora and Santalum album L. yield their aromatic constituents on distillation and are thus considered sources of essential oils.
It is the plants used primarily for their essential oil that are covered in this volume. Plants yielding essential oil but with another primary use are dealt with in various other volumes of the Prosea Handbook, including the volumes: Edible fruits and nuts, Medicinal and poisonous plants and Spices (which includes culinary herbs), while a small number of species is treated in the volume: Plants producing exudates. The present volume deals with essential-oil plants that are grown or could be grown in South-East Asia. It includes a few plants that play a central role in the fragrance industry worldwide but are currently not grown for their essential oil in South-East Asia. At first sight, the selection of species for this volume may seem imbalanced: 2 groups of essential-oil yielding plants of major economic importance, Mentha and Citrus spp., appear to have been omitted or not dealt with properly. However, all Mentha spp. are covered in the volume on Medicinal and poisonous plants, and most Citrus spp. appear in the volume on Edible fruits and nuts. On the other hand, Jasminum, Lavandula and Rosa spp., which are mainly grown as ornamentals in South-East Asia, are included here because of the great importance of their essential oils on a world scale.
In total, 29 genera, species or cultivar groups are dealt with in Chapter 2, covering the major essential-oil plants grown or used in South-East Asia. Chapter 3 covers the essential-oil plants that are grown on a minor scale only, and also a few plants that do not currently occur naturally or in cultivation in South-East Asia, but are potentially suitable. Chapter 4 lists species that produce essential oil but have another primary use and have been assigned to other volumes of this Handbook.
Role of essential oils
History of essential oils and their production
The use of essential-oil plants for their pleasant fragrance is as old as human civilization. Incense and myrrh are the oldest known aromatic materials; they are mentioned in Assyrian clay tablets. Aromatic materials were widespread in Pharaonic Egypt as fragrance materials and to embalm corpses. Their role in embalming is well documented and the compounds used can be detected even today. The early use of fragrance materials is also well documented in the Indian Vedic literature. Hundreds of aromatic substances and their uses (mainly religious and medicinal) have been listed. Other classical sources are the Gilgamesh Epic, the Bible and Greek authors such as Herodotus and Hippocrates. Theophrastus (372-287 BC) gave the first detailed description of the procedure for preparing perfumes using extraction with fat. It is to Paracelsus (1493-1541) that we owe the term "essential" in essential oil. He expounded the theory of the "quinta essentia", believing that this quintessence was the truly effective element in a medical preparation; its isolation is an important goal of pharmaceutical science to this day (Teisseire, 1994).
Until the Middle Ages, distillation of fragrant plant materials was mainly used to prepare fragrant waters, the essential oil often being considered a waste product. Production technology was developed probably simultaneously in the Middle East and in India. In the 12th Century, primitive distillation equipment was improved by the addition of a condenser and the invention of steam distillation. The condenser greatly improves the efficiency of a still, whereas steam distillation prevents the plant material overheating and possibly leading to the formation of unwanted by-products. The first description of distillation of a true essential oil is generally attributed to Arnold de Villanova in the late 13th Century. Three centuries later, the publication of 2 books in Germany marked an important step forward in the understanding of perfume production. The first gave a detailed account of the distillation of spike lavender in France; the second added descriptions of the distillation of lavender and mentioned the use of "exotic" essential oils from anise, cinnamon, clove, mace, and nutmeg. In the Middle East and the Mediterranean several other processes for the production of essential oil were developed or perfected: enfleurage, extraction and expression. In India an independent technology was developed for the production of "attars".
The development of the modern perfumery industry started in Grasse (France). This town had long been an important link in the trade between Italy and France and was also a centre of leather production and manufacturing of gloves. The fashion of perfuming gloves in the 17th Century led to the establishment of a perfume industry in Grasse, first on an artisanal scale but later as a modern industry. The area around the town became a centre of cultivation of cassie flower (Acacia farnesiana (L.) Willd.), geranium (Pelargonium L'Herit), jasmine (Jasminum grandiflorum L.), lavender (Lavandula L.), rose (Rosa L.), sour or bitter orange (Citrus aurantium L.) and tuberose (Polianthes tuberosa L.). Important contributions were made here to the development of industrial enfleurage and distillation equipment. Later, production of most essential-oil crops was transferred to countries where labour is cheaper and the centre of production and trade of luxury perfumes moved to Paris, which is still the world centre of perfumery. Further refinements in production technology also took place in France. At the beginning of the 19th Century it became possible to distil and purify alcohol to a product free of off-odours on an industrial scale. This made possible the development of alcohol-based or luxury perfumery as it is known today. Around 1870, Louis-Maximin Roure used this purified alcohol to extract the fragrant principles from "pomades" and became the first to produce "absolutes". Just before his death in the 1880s he was the first to build an installation for hydrocarbon extraction of fragrant plant material on an industrial scale. He also planned to produce absolutes from the "concrete" obtained by this process.
The development of organic chemistry from the end of the 19th Century completely changed the fragrance industry. Fragrance compounds were a major research subject in organic chemistry, as can be seen from the Nobel prizes awarded for research involving aroma chemicals. Kékulé (1873) coined the term "terpene" to describe hydrocarbons of molecular formula C10H16 to which many aroma chemicals conform. Wallach, who received a Nobel prize for his work in 1910, identified many of the terpenes and recognized isoprene as the basic building block of terpenes. A second Nobel prize, for the work on terpenes, was awarded in 1947 to Robinson, who studied the condensation of isoprene. During the same period another group of aroma compounds was investigated by Ruzicka, who studied macrocyclic compounds with a characteristic musk odour and who received a Nobel prize for his work in 1939.
Organic chemistry made possible the isolation of individual chemicals from essential oils, their synthesis and the development of many new aroma compounds. The synthetic compounds not only allowed the production of cheap aroma materials which greatly extended their use into everyday products such as soaps, detergents and air fresheners, but also enhanced the development of new perfumes by subtly changing the fragrance of natural products. The development, for instance, of the famous perfume Chanel No 5 in 1921 was made possible by the application of synthetic fatty aldehydes. Important discoveries have been the syntheses of: piperonal or heliotropin whose odour is reminiscent of the flowers and essential oil from heliotrope (Heliotropium peruvianum L.) and coumarin, which occurs naturally e.g. in Tonka bean (Dipteryx odorata (Aubl.) Willd.) from salicylic acid. Ionones can be synthesized from citral or hydroxycitronellal. The odour of ionones from citral resembles that of orris (Iris spp.) and violet (Viola odorata L.), while those synthesized from hydroxycitronellal are reminiscent of lily of the valley (Convallaria majalis L.) (Teisseire, 1994).
While fresh fragrant flowers are widely grown and traded and the odour of burning incense pervades any temple and shrine, especially in Asia, it is the use of essential-oil plants in fragrance materials that is most important worldwide. There is a vast range of uses of fragrance products. The most appealing use is in luxury perfumery, where the use of the most exquisite, rare and costly materials is combined with all the cunning of product promotion to create products that are associated with the best moments in life. However, most uses of essential oils are more down to earth and are part of everyday life. Perfumery can be divided into several types, each using a range of products with respect to their application. The main groups are: luxury or alcoholic, cosmetic, functional and industrial or technical perfumery (Table 1). In luxury perfumery the main purpose is to enhance one's personal fragrance. Cosmetics or body and hair care products have another primary use, but imparting a pleasant personal fragrance is an essential quality. In functional perfumery the role of the fragrance compound is less pronounced; it mainly adds to the perceived quality of products such as soaps and detergents. In technical perfumery, odour may be added to a product to make it easier to recognize (e.g. natural gas) or to mask an unpleasant odour (e.g. plastics).
In industry, the purpose of adding fragrance to cosmetics, soaps and detergents is to promote their sales through the sense of smell. The odour should not only be pleasant, but should enhance the perceived performance of the product. Advertising plays a major role in our perception of odours. Such is the power of advertising that associating an odour with famous personalities or with exotic holiday destinations seems more important for the success of certain fragrance materials than the characteristics of the odour itself. So strong is this associative power that several essential oils once widely used in luxury perfumes have fallen out of fashion through their current association with soaps and detergents, as has happened with citronella oil.
In cosmetics, soaps and other utility products, only cheaper natural essential oils are used and these are almost always supplemented with synthetic aroma chemicals. This contrasts with the use of essential oils as flavour materials. In foods, regulations on the use of synthetic compounds are much stricter and the premium paid for natural products much higher.
Three distinct processes are used to produce essential oils: solvent extraction, expression and distillation. Solvent extraction of essential oils developed from the much older practice of mixing fragrant flowers with fatty oils to extract their fragrance. This principle led to the development of the "enfleurage" technique, which allows efficient extraction of essential oil from delicate flowers that continue to produce aroma compounds for a long time. Solvent extraction is an industrial process in which highly purified, volatile hydrocarbons are used to dissolve aroma compounds from plant material, followed by the removal of the solvent by distillation.
Expression is used to obtain the essential oils from the peel of citrus fruits. It was originally a household industry using only very simple tools, but it has been superseded by large-scale industrial processes. Most citrus peel oils are produced as by-products of the citrus juice industry; only Citrus bergamia Risso & Poiteau and some cultivars of Citrus aurantium are grown especially for their peel oil.
Several forms of distillation are applied to produce essential oils, the most important being water distillation, steam distillation and hydrodiffusion. Water distillation or hydrodistillation is a very old process for the production of essential oil and has undergone centuries of improvement. Small-scale traditional water distillation apparatus is still being operated alongside large industrial equipment, for example in Mauritius, where Pelargonium is grown in small fields on poorly accessible hilly land. There it is more economical to operate small and simple portable stills than to move a bulky crop to a central still. The quality of the essential oil from Mauritius is maintained by traders who mix oil from several suppliers to obtain a standard quality. Steam distillation is a similar process, but hot steam is forced through the plant material to extract the essential oil. Large-scale industrial systems, e.g. continuous distillation systems and the use of harvesting containers that can double as distillation vessels, have been developed alongside small, traditional systems. Hydrodiffusion is a recent process in which low-temperature, low-pressure steam is used to extract the essential oils.
One of the oldest techniques employed to capture the true odour of the most delicate flowers is "enfleurage". The technique originated in Asia, where it was a common household practice to place fresh flowers in fat or oil to capture their fragrance. In the mid-18th Century it was developed into a large-scale, commercial process in Grasse (France) where in its heyday thousands of women were employed in large enfleurage factories. The enfleurage process is mainly advantageous for flowers that continue to produce aroma compounds for several days after they have been picked. Jasmine, for example, produces 4-5 times more essential oil than is present at any time in the fresh flower, tuberose up to 12 times more. Using enfleurage, the yield of essential oil from these flowers is higher than when using extraction processes with volatile solvents or distillation. Currently, the technique has almost died out because of the very high labour cost involved. Only for the production of the highest quality tuberose oil is the process still occasionally used, and then only on a very small scale.
In "enfleurage" a single layer flowers is placed on a tray covered beforehand with a thin layer of grease, which absorbs the volatile compounds from the flowers. Several chassis are stacked so that all aroma compounds emitted are absorbed by the grease. Pieces of cotton cloth soaked in oil and laid out on a metallic grid are used as an alternative to greased glass. The absorption takes 1-3 days, after which the flowers are replaced with fresh ones until the grease is saturated. The resulting product is called "pomade" and was either used directly in cosmetics, but more commonly washed with alcohol. The resulting solution is known as an "extrait" or "absolute de pomade".
Enfleurage was used to preserve the true odour of delicate flowers, such as cassie flower (Acacia farnesiana), heliotrope (Heliotropium peruvianum), jasmine (Jasminum grandiflorum), jonquil (Narcissus jonquilla L.), sour or bitter orange blossom (Citrus aurantium), tuberose (Polianthes tuberosa) and violet (Viola odorata).
In "hot enfleurage" flowers are placed in linen bags, which are then dipped in melted fat, vegetable oil or mineral oil heated to about 50-60 °C. After a maximum of 12 hours the bags are removed and replaced with fresh ones. When the oil is saturated it is pressed from the bags mechanically. The highest quality "pomade" is obtained when the flowers are in contact with the absorbent for the maximum time (Gildemeister & Hoffmann, 1956-1966).
Efforts to extract the true odour of flowers recently resulted in a new technique: headspace analysis. This technique is used on flowers with a very attractive scent that produce too little essential oil to extract economically, such as lily of the valley and many orchids. A live flower still attached to the plant is carefully enclosed in a flask. The air in the flask then becomes saturated with the volatile compounds the flower emits. A sample of the air is fed directly into very sensitive analysing equipment, or the volatile compounds are first concentrated by condensation or adsorption. The technique is now so sensitive that even fluctuations in character and intensity of the odour can be recorded. As the amount of volatile compounds produced by these flowers is minute, the sole objective of these analyses is to obtain accurate analytical data on the odour that can help in reconstituting the odour of these flowers artificially. However, as the human nose is still about 1000 times more sensitive than modern analysers, it is not yet possible to capture the full richness of the scent of many flowers in compounded fragrance materials. The technique has also been used to find new aroma chemicals. A large number of very interesting, previously unknown compounds have been identified, several of which can be made synthetically (Kaiser, 1991; Raguso et al., 1998).
Extraction of aromatic plant materials with volatile solvents was developed in the mid-19th Century from hot enfleurage. Large-scale experiments were conducted independently by several workers, one of whom, named Garnier, obtained a patent for a novel type of extractor that became widely used first in France and subsequently in the rest of the world. This method gradually replaced the method of enfleurage.
The principle of solvent extraction is simple. Fresh aromatic plant material, e.g. flowers or comminuted leaves, is put into an extraction vat. A carefully purified volatile solvent is gradually fed into the top of the vat and allowed to seep through the plant material. The solvent penetrates the plant material and dissolves the aroma compounds, together with waxes, albuminous and colouring matter. The solvent with the dissolved compounds is subsequently transferred to an evaporator where the solvent is distilled off at a low temperature under partial vacuum, yielding a "concrete". Repeated washing of the concrete with alcohol to remove waxes and other inert matter produces an "absolute". The extraction procedure is repeated several times until all aroma compounds have been extracted. In the processing of spices a very similar process is used. The resulting extract containing fragrance compounds, waxes, resins and dye compounds are called oleoresins.
The solvent used should be carefully selected. It should quickly dissolve the odoriferous compounds, yet as little as possible of inert matter such as waxes, pigments and albumen. It must not absorb water, because this is difficult to remove from the extract. Frequently used solvents are petroleum ether (a mixture of mainly pentane and hexane obtained by fractionation of crude petroleum), benzene and alcohol, while for the extraction of spices hexane and dichloromethane are commonly applied. One solvent that has gained importance in recent years is carbon dioxide in liquid or supercritical form. Extraction with carbon dioxide is a costly process but has several advantages over extraction with other solvents. It is odourless, tasteless and non-toxic, and non-combustible. Because of its low viscosity it readily penetrates plant material and because of its low boiling point it is easily removed from the extract. The selectivity of the carbon dioxide extraction can be influenced by varying the temperature and pressure.
Solvent extraction is generally a more costly process than water or steam distillation. The process does not lend itself to simple, small-scale processing units, but requires large factories. Many solvents are highly flammable and costly when highly purified; they should be carefully recycled to avoid environmental pollution. Extracted flower oils are usually darker in colour and are more difficult to dissolve in alcohol than distilled ones. Advantages of extraction are that the process can be influenced more precisely and that, in general, the concretes and absolutes obtained, match the olfactive characteristics of the plant material more closely (Arnaudo, 1991; Schügerl, 1994).
Expression is used to obtain citrus peel oils, e.g. bergamot, grapefruit, lemon, lime, orange, tangerine. Many components of the essential oils from citrus fruits are delicate and suffer significantly from heat degradation when exposed to steam distillation. A "cold expression" process is used therefore to obtain essential oils from citrus fruits; distillation is only used to valorize the residues of juice production, and even these distillations occur under vacuum at a maximum temperature of only 50 °C, to minimize degradation. The only exception is lime (Citrus aurantifolia (Christm.) Swingle). The oil distilled from the rind of lime is commonly traded and its fragrance has become accepted as the typical lime odour. Cold pressed lime oil is a minor product only.
In all "cold expression" methods the fruit peel is compressed, lacerated or abraded to rupture the oil cells in the exocarp and to release the essential oil. Two methods, both developed in Italy, have been used since early times: the "spugna" or "sponge method" and the "scodella" or "spoon method". Modern expression methods are based on the same principles as these traditional methods.
In the "spugna" method, fruits were halved and the juicy pulp was removed with a spoon-shaped knife. The peel was placed in warm water; the albedo or oilless mesocarp then absorbed water, effectively toughening it against fracture. This toughening was necessary to preserve the integrity of the peel during the second part of the process. This started with retrieving the peels from the water and pressing them individually against a small sponge with sufficient force to turn the peel inside out. Pressing and turning the peel ruptured the oil glands, which released the essential oil into the absorbent sponge. To retrieve the essential oil, the sponge was periodically squeezed over a collecting vessel or "concolina".
The "scodella" method used a funnel-shaped or inverted bell-shaped bowl. The wide part of the bowl was covered with points, the narrower part was used as a funnel. Entire fruits were turned and pressed against the points, tearing the exocarp with the oil cells. The mixture of essential oil released together with cell contents and raspings was collected in the bottom of the funnel, from which it was periodically removed into a receptacle where separation of the oil would take place.
The "pellatrice" method was developed from the "scodella" method. A slowly turning Archimedean screw with an abrasive surface takes entire fruits from a container to a set of high speed rollers also covered with abrasive spikes. En route, the fruits are washed by a spray of constantly recycled water. The mixture of water, essential oil and detritus is collected and passed through a separator where any solids are removed. The mixture is then separated in a series of centrifuges to obtain the pure essential oil. The "pellatrice" method is commonly employed in the production of bergamot oil. It has been refined into several continuous methods. In one of these, the "Brown Process", the equipment consists of an elevator that feeds whole fruits to an extractor. The extractor comprises a tank in which a series of rollers covered with sharp points are placed. The rollers all rotate in the same direction, alternatingly at a high and a moderate speed, and also move laterally. As the fruits are moved through the tank by the rollers, they become thoroughly lacerated, which liberates the oil from the glands. The oil and debris are washed off with water flowing countercurrently through the tank, forming an emulsion of oil and water. Subsequently, the oil and water clinging to the fruits is removed by special rollers in a drying unit. The emulsion collected from the tank and the dryer is filtered and separated in a centrifuge (Arnaudo, 1991; Lawrence, 1995).
The "sfumatrice" system is a refinement of the "spugna" method. Fruits that have been halved and whose juice has been expressed are immersed in a solution of calcium carbonate and water for 24 hours to harden the peel and to facilitate expression of the oil. This also reduces degradation of the oil which occurs in an acid medium. The peels are then squeezed and contorted between a transport belt with elastic elements and ribbed rollers. As in the "pellatrice" method, the essential oil is removed by sprays of water and subsequently separated by centrifuges.
A further refinement of the "sfumatrice" method is the continouously operating "F.M.C. in line extractor" of the Food Machinery and Chemical Co. Currently over half of all citrus peel oil is produced by this method. The equipment processes whole fruits and simultaneously extracts the fruit juice and the peel oil. The fruit is placed between 2 cups consisting of metal fingers. A small disk of peel is then cut from the bottom of the fruit. When the top cup is forced down, the fingers of the 2 cups interlock and squeeze the fruit, pressing out the juice. The pressure of the interlocking metal fingers also bursts the oil glands in the peel. The oil is washed off the fruit by strong water jets. Finally, the mixture of oils and water is collected and separated in a centrifuge (Arnaudo, 1991; Lawrence, 1995).
The oldest distillation equipment known dates from the 4th Century AD. It used the familiar process of condensation of vapours on the lid of a cooking pot; the main modification was a rim around the inside of the lid, to collect and remove the condensate. A mixture of water and the material to be distilled was heated in the pot or vat by direct fire. Where this material came into contact with the hot wall of the pot, charring occurred, causing some of the compounds to decompose. In the 11th Century, the Iranian physician Abu Cina (known in Europe as Avicenna) added a frame to the vat, fixed above the level of the water, on which the material to be distilled was placed. In this way the material came into contact with steam only, and fewer degradation products were formed. This improvement later led to the development of steam distillation. A final major improvement made in the 12th Century was the addition of a condenser in which the vapour could be cooled and condensed rapidly. This greatly improved the efficiency of the distillation process.
There are three methods of distillation: water distillation, steam distillation and hydrodiffusion. The principle of water distillation is to boil and vaporize a suspension of aromatic plant material and water in a vat so that its vapours can be condensed and collected. The essential oil, which is immiscible with water, is then separated by gravity in a "Florentine flask". The water in the still must be kept in motion to prevent the plant material from clogging together and settling at the bottom of the still. This would result in a low yield of essential oil, charring of the plant material and degradation of thermo-instable compounds, resulting in "still odours".
Water distillation is still applied in traditional field stills, but is mainly used for the distillation of floral materials such as flowers of Cananga odorata (Lamk) Hook.f. & Thomson, Rosa spp., Iris spp. and Citrus spp. that clog together in other distillation procedures. The main drawback of water distillation is that large amounts of water have to be heated. For roses, for example, 400 kg of flowers are added to 1600 l of water in a 3000 l still.
A special form of water distillation is used in India to produce "attars". It is a large and mostly traditional industry. Direct-fired stills of 100-160 kg capacity are used to process floral or herbal material. The peculiarity of the process is that the distillate is not cooled in a condenser, but collected directly in a receiver containing a base material. The mixture of base material, aroma materials and water is left to cool and the water is drained off. The base material with dissolved aroma materials is stored in leather containers. Leather is used, as it retains the oils but allows any remaining water to evaporate. For high quality attars sandalwood oil is the preferred base material; liquid paraffin is used for cheaper products. Attars typically contain the extract of a single plant; only "hina attar" is a compounded perfumery product. Popular attars are made of Anthocephalus cadamba (Roxb.) Miquel, Jasminum sambac (L.) Aiton, Lawsonia inermis L., Pandanus odoratissimus L.f., Rosa L. cv. group Damascena and the wild form of Vetiveria zizanioides (L.) Nash. A special attar is distilled from the baked earth of the region around Kannauj near Lucknow in Uttar Pradesh, the main centre of attar production (Kapoor, 1991).
Water and steam distillation (also named "wet steam" distillation) is a method that has characteristics of both water distillation and steam distillation. With this method, a metal grid is placed in the still above the level of the water and the plant material is placed on the grid. Direct contact between the water and the plant material is thus avoided. As only the water is heated, the risk of charring and the formation of "still odours" is reduced, but the hot walls of the still may cause some damage. Water and steam distillation are used for many types of plant material, e.g. lavender, thyme and peppermint. Cohobation is a procedure that can be used with water distillation and with water and steam distillation. After removal of the essential oil, the distillate water which still contains water-soluble aroma compounds is returned to the still and is used again. This may be repeated several times. When the concentration of aroma compounds has reached the desired level, the water is drained from the still. It is either traded as such or the aroma compounds are removed from the water by extraction. Cohobation increases the yield of partially water-soluble compounds, but increases the risk of hydrolysis and degradation of aroma compounds. It is common practice in the distillation of rose flowers, where the distillate water is an important product of the distillation, and in the production of "attars".
In steam distillation (sometimes called "dry steam" distillation), a separate steam generator is attached to the still. As in steam and water distillation, plant material is placed on a grid in the distillation vat, but no water is added. Steam produced in the generator is forced through the material to be distilled. High pressure steam is often used, e.g. steam of 5-10 bar pressure at 150-200 °C. The duration of the distillation process depends on the steam temperature and the ease with which the essential oil can be removed from the plant material. Plants in which the oil is stored in hair glands can be distilled very easily; those in which the oil is stored in or below the epidermis require more intensive distillation. The main advantages of steam distillation are that the amount of steam used and its temperature can be readily controlled. As the vat walls do not become hotter than the temperature of the steam, the risk of charring is minimal. Steam distillation is suitable for the production of most essential oils, except those from delicate flowers. The only precaution necessary when distilling leafy material is to ensure that it is not cut too fine, since this may cause "channelling", resulting in poor distillation yields. Channelling occurs when the plant material becomes too compact. The steam then forces its way through via a few large channels, instead of moving through the entire mass of plant material. Steam distillation is sometimes conducted under reduced pressure, to lower the distillation temperature (Lawrence, 1995).
Equipment that overcomes the time-consuming loading and emptying of the still was developed during the 1980s and 1990s. In the United States, a system has been built in which the container that is used to collect mint (Mentha spp.) in the field doubles as a distillation vat in the distillery. This avoids the labour intensive operations of discharging the container and loading the still. In France, the Russian Federation and the United States, installations for "continuous distillation" have been developed. In these, plant material is moved slowly through a distillation unit, while steam is forced through it in the opposite direction. The flow of steam and the feeding and removal of plant material are carefully coordinated to ensure a high extraction efficiency and a low consumption of steam. Auxiliary equipment e.g. to clean the oil and to dry the spent plant material is often integrated in the system.
Hydrodiffusion is a distillation method developed in the 1980s. Low pressure steam (<0.1 bar) is used and the volatile components are extracted from the plant material mainly by osmosis. In this method a distillation vat is filled with comminuted plant material. Steam is fed to the still but, unlike steam distillation, it is fed from the top of the still and moves downwards through the plant material by gravity. After passing through the plant material, the steam and volatile compounds flow through a condenser placed at the bottom of the still and are collected in an oil separator. Hydrodiffusion has shown excellent results under experimental conditions: short distillation times, low steam consumption, high yields of high-quality oil and absence of high temperatures. Under commercial conditions, however, performance has been less impressive (Boelens at al., 1987; Lawrence, 1995).
After distillation, the distillate is collected in a "Florentine flask", where the water and essential oil fractions are allowed to separate. The flask has a spout at the top and the bottom so that the water and oil can be tapped off separately. Subsequently, traces of water are often removed from the oil and the oil may be rectified, i.e. unwanted compounds may be removed by fractional distillation. Vacuum distillation and molecular distillation may be used, to avoid degradation of thermo-instable compounds. Rectification is used e.g. in the production of terpeneless citrus peel oil, geranium oil and bay leaf oil. From these oils the terpenes are removed, leaving the more strongly odorous oxygenated terpene derivatives behind at a higher concentration.
For industrial purposes, specific compounds may be distilled from the essential oil, e.g. the essential oils from Litsea cubeba (Lour.) Persoon and Cymbopogon citratus (DC.) Stapf are major sources of natural citral and the oil of Corymbia citriodora (Hook.) K.D. Hill & L.A.S. Johnson of citronellal.
Odours and odour description
The sense of smell is crucial in the study of flavours and fragrances. This sense is unique among the senses because it is very subjective and lacks objective standards. Smells are often described in terms of sensations related to other odours or to experiences of the other senses. The memory of these odours and associated experiences is very personal and an odour that is repulsive to some may be attractive to others. However, classifications of odours have been developed. They are based either on comparison with common fragrance materials or on odour concepts that have to be acquired by experience.
The sense of smell is one of two discriminatory chemical senses through which information about the chemical composition of the environment can be obtained. Whereas the second chemical sense, taste, recognizes only 4 conditions (bitter, salty, sour, sweet), the sense of smell recognizes an immense number of odours and odour compounds. For perfumery, it is unfortunate, but at the same time challenging, that odour sensations cannot be described in absolute terms, but have to be related to memories of the same odour or to sensations from the other senses. Odours may be described in relation to touch (soft, harsh), sight (the smell of a forest), or even hearing (an aroma associated with the sound of the sea). Descriptive language is the only means of communicating odour sensation from one person to another because odour quality cannot be measured or expressed independently of individual human experiences.
Although "artificial noses" linked by computer to automated perfumery laboratories are in use today, most fragrance compounds continue to be created by perfumers. To communicate with colleagues, technicians, marketing staff and with their customers, perfumers need an odour vocabulary in which each term conveys the same meaning to all its users.
Systems of characterization of odours can be developed by two different methods. A qualitative description of an odour or odour pattern can be obtained either by a "reference procedure" i.e. by direct comparison with the odour of a series of known chemicals or by a "semantic procedure" i.e. in a verbal descriptive way. The latter procedure is preferred by perfumers, flavourists and food technologists, such as wine, tea and coffee tasters. A standardized vocabulary to descibe odours is used in which each term is precisely defined, in stead of spontaneous but subjective everyday language. In these classifications, terms such as "animal", "green" and "metallic" have a specific connotation that anyone dealing with odours has to acquire. Although it takes years of practice to become a perfumery expert or "nose", some conscious attention to odours and their description will quickly put one on the right track. Several systems have been developed. Linnaeus was the first to propose a logical and objective classification of odours using 7 classes based on the odours of selected plants. The American Society for Testing and Material (ASTM) designed a system using a large number of classes that was later simplified. A classification of odour qualities and similarity coefficients is given in Table 2. In this table, most terms describe an odour that has little in common with other odours described by the other terms. However, terms that have some similarity with neighbouring ones are linked, the link representing the statistically established degree of similarity. The descriptions used in this volume largely follow this classification. (Harper, Bate Smith & Land, 1968; Müller & Lamparsky, 1994; Ohloff, 1990).
Until a few decades ago, measurements of physical characteristics supplemented with simple chemical analyses were the only means to characterize samples of essential oils and to compare them with standard samples. The origin, quality, possible adulterations and extensions had to be detected by these methods. It is only since the 1980s that the use of capillary gas-liquid chromatography and mass spectrometry has enabled more detailed analyses to be done.
The physical characteristics most commonly used to characterize essential oils are relative density, refractive index, miscibility and optical rotation.
Relative density is the ratio of mass and volume of a substance. Refractive index refers to the property of transparent materials to deflect light by a specific degree when it enters such materials at an oblique angle from another material with a different density. Miscibility of essential oils refers to the solubility of an essential oil in a solvent. It is usually measured with aqueous alcohol as the solvent. The amount of essential oil soluble in a given amount of alcohol of a given concentration is recorded. An oil may be fully soluble in pure alcohol, but only slightly soluble in a mixture of alcohol and water.
Molecules with an asymmetrical structure rotate the plane of polarization of polarized light. Pure, optically active compounds deflect the plane of polarized light by a characteristic angle. Optical isomers, which are identical molecules but each other's mirror images, rotate the plane of polarization in opposite directions. Optical rotation provides a measure of the relative concentration of isomers of optically active compounds. In synthetic asymmetric compounds, optical isomers are almost always present in equal amounts, resulting in an optical rotation of 0°. The optical rotation of essential oils and of individual compounds of the oils is often highly characteristic of the oil and even of its origin.
Several quality control institutes have specifications with which essential oils must comply. The most important ones are the ISO standards of the International Organization for Standardization and EOA standards of the Essential Oil Association of the United States. Standards organizations use rigorously defined methods of analysis. As each organization uses is own methods, slight differences in values may occur, making direct comparisons of data difficult. An overview of standard values established for the essential oils covered in this volume is given in the Table on standard physical properties of some essential oils (see p. 221).
Essential oils are complex mixtures of sometimes hundreds of chemical compounds. To give an indication of their composition and complexity, examples are given in Composition of essential-oil samples (see p. 205). Most of these compounds can be grouped into a few major classes, but there are also many components of essential oils that bear little resemblance to these classes. In the overview of important and characteristic components given below compounds are classified into 4 major groups: aliphatic compounds, terpenes and terpene derivatives, benzene derivatives and miscellaneous compounds.
Aliphatic compounds are acyclic organic compounds. The chain of C-atoms in these compounds may be straight or branched and some of the bonds between C-atoms may be unsaturated. Aliphatic hydrocarbons occur abundantly in foodstuffs such as fruit, but contribute to their odour to a limited extent only. The highly unsaturated hydrocarbons 1,3-trans-5-cis-undecatriene and 1,3-trans-5-trans-undecatriene, however, contribute significantly to the odour of galbanum oil.
The odour of most aliphatic alcohols is weak and their role as components in fragrance compositions is limited. An exception is 3-octanol, occurring in mushrooms and imparting their characteristic smell. A few unsaturated alcohols are more important. Cis-3-hexen-1-ol or leaf alcohol, which has the characteristic odour of freshly cut grass, forms a large proportion of the essential oils from the leaves of Robinia pseudoacacia L., mulberry (Morus spp.) and from green tea. Its isomer cis-2-hexen-1-ol occurs in many fruits and has a sweeter aroma. Lavender oil and mushrooms contain 1-octen-3-ol, a compound with an intense mushroom and earthy-forest odour.
Aliphatic aldehydes are important compounds in perfumery and flavouring. The series n-octanal, n-nonanal, n-decanal and n-undecanal, for instance, occurs in citrus oils. The unsaturated trans-2-hexenal, or leaf aldehyde, occurs in many leaf oils and has a sharp herbal-green, somewhat pungent, odour.
Of the aliphatic ketones, only 3-hydroxy-2-butanone (acetoin) and diacetyl (2,3-butanedione) are widely occurring natural isolates that play a role in flavouring. Both have a buttery aroma. Aliphatic esters are important flavour and fragrance compounds occurring widely in nature. In perfumery, acetates of alcohols up to C6 are used for their fruity notes, C8-C12 acetates for their blossom fragrance, C12 acetate also for its conifer notes, while esters of acids of longer chain length have a more fatty-soapy odour. Structural formulas of some aliphatic fragrance compounds are given in Figure 1.
Terpenes and terpene derivatives
Terpenes constitute a widely represented group of substances. Although they show wide structural diversity, they share a common characteristic: they are built from 2 (monoterpenes), 3 (sesquiterpenes) or more isoprene (C5H8) units. Isoprene is one of the basic compounds in animal and plant biochemistry from which carotenoids, steroids and rubber are also formed. It is formed from acetyl-CoA that plays a role in the synthesis and oxidation of sugars. The terpene hydrocarbons contribute to the odour and taste of essential oils to a limited extent only, but their oxygenated derivatives are among the most important aroma chemicals.
Monoterpenes conform to the molecular formula C10H16 and can be acyclic, monocyclic, or bicyclic. There are even a few tricyclic monoterpenes: cyclofenchene and tricyclene. Acyclic monoterpenes are relatively unstable and some have a slightly agressive odour, because of their strongly unsaturated structure. Examples of acyclic monoterpenes include myrcene and ocimene.
Cyclic monoterpenes occur in essential oils, sometimes in considerable amounts. By themselves they generally contribute relatively little to the odour of a fragrance or flavour product, but often serve as starting materials for the biological or chemical synthesis of flavour and fragrance compounds. Examples of monocyclic terpenes include α-terpinene, γ-terpinene or para-menthadiene, limonene, α-phellandrene, β-phellandrene, and terpinolene. There are 5 types of bicyclic terpenes, characterized by: thuyene, carene, pinene, camphene and fenchene. Of the bicyclic terpenes, the α-pinene and β-pinene are technologically the most important by far. Structural formulas of some monoterpenes are given in Figure 2.
Sesquiterpenes are compounds generated from 3 isoprene units and conforming to the formula: C15H24. As there are so many, it is impossible to generalize about their molecular structure. Farnesene is perhaps the most simple, acyclic example; many sesquiterpenes are bicyclic, having two C6-rings or a C6 and a C5 ring; an extreme example is the monocyclic humulene with a C11-ring.
In fragrance products in which the essential oil is dissolved in diluted alcohol, e.g. "eau-de-Cologne", the terpenes present would give rise to problems of separation. To overcome this difficulty, terpeneless oils, i.e. essential oils from which the terpenes have been removed, are used. Terpeneless oils have the added advantage of a stronger characteristic odour. Structural formulas of some sesquiterpenes are given in Figure 3.
Oxygenated derivatives of monoterpenes and sesquiterpenes are more important than the terpene hydrocarbons as aroma chemicals. The characteristic odour of many essential oils is representative of the combined odours of the oxygenated compounds. Important groups of oxygenated compounds are alcohols, aldehydes and ethers, ketones, acids and esters.
Acyclic monoterpene alcohols and acyclic sesquiterpene alcohols occur in many essential oils and contribute strongly to their characteristic odour. Some of them, such as citronellol, geraniol, linalool and nerol are also synthesized from turpentine on an industrial scale. In many parts of the world these alcohols take up the production capacity of entire chemical factories. The synthetic products may differ in their odour qualities from compounds isolated from plant sources, since it is difficult to separate the desired products from compounds with similar physical properties but different odour characteristics, such as enantiomers. Lavandulol differs in structure from most terpene alcohols, as the isoprene units are not arranged in the usual head-to-tail manner. Structural formulas of several important acyclic terpene alcohols are given in Figure 4.
The most important aldehydes derived from acyclic monoterpenes and sesquiterpenes are citral and citronellal. They are major components of essential oils from Cymbopogon spp. and Litsea cubeba and hold key positions in many flavour and fragrance materials. Both are important starting materials for the synthesis of other aroma compounds. Both optical isomers of citronellal occur in nature; (+)-citronellal occurs in Cymbopogon spp., (-)-citronellal in Backhousia citriodora F. Muell., whereas Corymbia citriodora oil contains a racemic mixture. One of its derivatives is hydroxydihydrocitronellal (also named hydroxycitronellal). It is a widely used fragrance compound, but has not yet been found in nature. Structural formulas of some acyclic terpene derivatives are given in Figure 5.
Cyclic terpene derivatives are so numerous and diverse that only a few important examples can be given here. Whereas cyclic terpene aldehydes occur in essential oil in low concentrations only, cyclic ketones are more important. Menthone and carvone, which both have the para-menthane structure, are commercially important flavour and fragrance compounds. Menthone is a constituent of the essential oil of Mentha ×piperita L. and Pelargonium cv. group Rosat, carvone is found in Carum carvi L. and Mentha spicata L., Anethum spp. and Cymbopogon martini (Roxb.) J.F. Watson. Camphor is the main constituent of the essential oil from Cinnamomum camphora and formerly a source material for the synthesis of celluloid and smokeless gunpowder. Ionones and homologeous compounds are an important group of terpenoid aroma chemicals. They are derived from carotenoids and are present in essential oils in small amounts only. Several ionones have odour notes reminiscent of Viola odorata. Damascones are isomers of ionones: β-damascenone is one of the characteristic components of Bulgarian rose oil.
The cyclic sesquiterpenoid nootkatone is one of the characteristic or "character impact" components of the grapefruit aroma. Structural formulas of examples of cyclic terpene derivatives are given in Figure 6.
Esters of terpene alcohols and lower fatty acids, in particular acetates, are highly important as flavour as well as fragrance materials. The esters of acyclic terpene alcohols citronellol, geraniol and linalool occur in a large number of essential oils, sometimes in large amounts. As isolated compounds they are used in reconstituting such oils and other aroma compositions. Esters of cyclic terpene alcohols, such as α-terpinyl acetate, menthyl acetate, bornyl acetate, and a few acetates of sesquiterpene alcohols, such as guaiyl acetate, cedryl acetate and vetyveryl acetate, are also important components of essential oils and are applied extensively in flavour and fragrance materials. Alcohols are often acetylated in essential oils, to modify their olfactive characteristics, as for instance with vetiver oil. Structural formulas of some terpene esters are given in Figure 7.
In chemistry, benzene derivatives or benzenoids (often confusingly named aromatic compounds) are compounds containing a characteristic benzene ring, often represented as a C6 ring with 3 double bonds alternating with single bonds between the C-atoms. It is a very large and varied group that includes many natural and synthetic flavour and fragrance compounds. The most important hydrocarbon derived from benzene is para-cymene; it occurs in many essential oils and has a weak citrus odour when pure. Of the benzenoid alcohols and aldehydes, important components of essential oils are phenylethyl alcohol, cinnamic alcohol, cinnamic aldehyde, phenylacetaldehyde. Most aromatic ketones that are important in the flavour and fragrance industry are synthetics.
Esters of aromatic alcohols and aliphatic acids are of interest in flavours and fragrances because of their characteristic odour properties. Benzyl acetate is the main component of jasmine oil and gardenia oil, phenylethyl acetate is an aroma compound found in several essential oils and in many fruits, benzyl benzoate is a major component of Peru balsam and is a commonly used fixative and modifier of heavy blossom fragrances in perfumery. Some benzenoid fragrance coumpounds are represented in Figure 8.
Only a few of the many other fragrance compounds in other groups can be given as examples. Their structural formulas are given in Figure 9. Several nitrogen compounds impart characteristic sensory properties to essential oils, even when they are present in essential oils, concretes and absolutes in concentrations of less than 0.1%. Isolated or synthetic nitrogen compounds are used in industry to modify jasmine oil, lavandin oil and petitgrain oil. Sulphur compounds are rare in essential oils, but more common in animal odours. Examples of sulphur compounds are found in the essential oils of garlic, mustard and Ferula assa-foetida L. Synthetic sulphur compounds are used in industry to modify buchu, galbanum, blackcurrant and rose oil. (Bauer, Garbe & Surburg, 1997).
Verification of the genuineness of essential oils
Until a few decades ago, the human nose, supported by the measurement of a number of physical characteristics and a few chemical analyses, was the chief means of verifying the density, purity and naturalness of essential oils. The development of capillary gas chromatography/mass spectrometry (GC/MS) methods has greatly improved knowledge of the chemical composition of essential oils. Currently, compounds present in as little as 1 ppb (part per billion) can be detected and identified, making it easier to distinguish between a pure natural essential oil and oils to which foreign compounds have been added. As both the pathways of synthesizing chemical compounds and the by-products of these processes are well known, demonstration of their presence in an essential oil can be proof that the oil has been adulterated.
Developments in the chemistry of optical isomers further enhanced the possibilities of identifying added compounds in essential oils. Most of the assymmetric, optically active compounds in natural essential oils are represented by only a single isomer, or else both isomers are present in a proportion that may vary within a narrow range only. The addition of the compound obtained from another essential oil will often change this proportion. As chemically synthesized compounds are only rarely optically active, their presence can be accurately demonstrated.
However, the high prices paid for pure natural essential oils are encouraging increasingly sophisticated adulteration practices to be developed, and methods for detecting them have to keep pace. The adulteration of an essential oil with compounds chemically synthesized from carbon compounds derived from petroleum or coal can be detected by measuring the amount of radioactive carbon (14C) in the components of the oil. The atmosphere contains mainly 12C but also traces of 14C. The latter is produced by irradiation and subsequently decays slowly. Compounds synthesized by plants from atmospheric carbon dioxide contain 14C, whereas almost all the 14C in fossil material and in chemical compounds derived from it has decayed. Modern mass spectroscopy equipment is sufficiently accurate to measure the ratio of 14C/12C and makes a distinction between fossil and natural compounds possible.
Although this method has made it easier to detect adulteration with synthetics, it cannot reveal adulteration involving compounds derived from cheap natural linalool or pinene. Refinements in the analysis of carbon isotopes exploit differences in the photosynthetic pathway used by different groups of plants. The C4 pathway (typical of many tropical grasses) and C3 pathway (typical of temperate grasses and most dicotyledons) of photosynthesis fix different proportions of 13C and 12C. C4 plants are richer in 13C than C3 plants. The measurement of nuclear magnetic resonance (NMR) has been refined to such extent that it can identify whether a compound originates from a C3 or C4 plant.
The newest way to characterize chemical compounds is a method based on deuterium nuclear magnetic resonance spectroscopy. A small proportion of hydrogen in nature occurs as 2H or deuterium. Nuclear magnetic resonance (NMR) spectroscopy enables the magnetic resonance of individual bonds between atoms in a molecule to be studied. It appears that the 2H is very unevenly distributed over the various bonds in a molecule. As the internal distribution varies with the origin of the molecule, this method can distinguish if e.g. anethol is prepared from star anise, estragol or petroleum.
Physico-chemical methods of analysis of natural aroma products
The features characterizing (and complicating) the analysis of natural aroma materials are:
- A large number of components. Essential oils may contain hundreds of constituents.
- Large structural diversity. Several thousands of monoterpenes, sesquiterpenes and related phenylpropanoids are known, not to mention various unrelated compounds.
- The presence of many relevant components in small quantities. Often, 1-2% of a natural product contain several hundreds of compounds, which may belong to many different groups.
- The possible presence of relevant individual compounds in extremely small quantities. The effective threshold concentration in water at which β-ionone becomes detectable is 0.007 ppb or 7.10-12g per g water.
Rapid advances in chromatographic and spectroscopic analysis methods have revolutionized the knowledge of essential oils. However, in many cases this knowledge is still inadequate, as the human sense of smell is even more sensitive.
Probably the oldest techniques to separate components from a mixture are the chemical ones. The extraction of essential oils with acidic or alkaline aqueous solutions allows respectively the basic complexes and the acids and phenols to be isolated. Carbonyl compounds can be isolated by transformation into water-soluble hydrazonium salts. Esterification of alcohols is another option. However, chemical separation methods have several drawbacks. They require relatively large amounts of product, they may cause formation of artefacts and they can only separate compounds on the basis of their chemical functionality.
Physical separation methods exploit differences in physical properties of the components of a mixture: density, vapour pressure and solubility (see e.g. the Table on standard physical properties of some essential oils). One commonly used method of separation is fractional distillation. Recent developments allow very small quantities of product to be separated into large numbers of components. Distillation works best with components of low molecular weight and high vapour pressure. Heavy components require higher temperatures, which bring a risk of thermally-induced modification. The commonest technique for separating compounds of a mixture when analysing essential oils is capillary gas chromatography. It entails vaporizing a minute sample of the product and transporting the vapour through a long and fine tube by means of a non-reactive gas, such as nitrogen, under conditions of a controlled increase in temperature. The tube or capillary chromatography column is made of an inert material such as silica and is coated with a thin film of a special absorbent in which the components of the mixture will dissolve. Separation of the components depends on differences in their solubility in the absorbent, which result in differences in the speed with which they pass through the column. A detector situated at the end of the column records the characteristics of each component when it leaves the column. The results are represented in a chromatogram from which components can be qualitatively and quantitatively identified.
Methods of analysis
Capillary gas chromatography is usually coupled with mass spectrometry and infrared spectrometry. The separated components of the mixture enter the spectrometer one by one, so that each of them can be analysed separately.
In gas-phase infrared spectrometry a beam of infrared light is directed at the vapourized sample and the absorption of the light is measured. The degree of absorption depends on the nature of the compound analysed and the wavelength of the light. The analysis results in an absorption spectrum that is unique to the compound tested. The identity of the compound is established by comparison with spectra of reference compounds.
In the mass spectrometer the compound to be analysed is bombarded with high-energy electrons. This results in a partial break-up of the molecules and the ionization of the whole molecules and the fragments. The mass and electric charge of the resulting ions are recorded and the identity of the original molecule can be established by comparing the information with reference data stored in a computer. If the identity of the compound cannot be established directly from the reference data, it can be inferred by piecing together the various fragments.
More detailed information on compounds is obtained by NMR spectroscopy. Nuclear magnetic resonance is the interference between an external magnetic field and the magnetic field generated by the nucleus of certain elements. The interference is influenced by position of the atom in a molecule. In e.g. ethanol, 3 differently bonded H atoms occur, each having a characteristic magnetic resonance. In NMR spectroscopy the resonance spectrum of the compound is recorded. In this spectrum all different H bonds are represented by specific peaks. To identify the compound tested, this pattern of peaks can be compared with reference data. If no matching reference data are available, the information combined with the results of mass spectrometer analysis can be used to infer the chemical nature of the compound.
Systems of quality standards have been developed to facilitate marketing and to guarantee the safety and quality of products. Such standards regulate not only the quality of individual products but also methods of analysis and quality management systems. The most important systems of standards for essential oils are those of the International Organization for Standardization (ISO), Geneva, Switzerland, the Essential Oil Association of the United States (EOA), and the International Fragrance Association (IFRA), Geneva, Switzerland.
The ISO issues 3 types of standards. The first type (ISO 9000) sets quality requirements for management and systems. The second type defines protocols and methods of analysis to be used in the establishement of particular parameters: for example, ISO 356 deals with the preparation of samples, ISO 279 with the determination of relative density, ISO 280 with the determination of the refractive index, ISO 592 with the determination of optical rotation and ISO 875 with the miscibility in ethanol. The third type of standard defines the limits for several characteristics an essential oil must comply with. Traditionally, these have been physical determinations, such as density, optical rotation, refractive index, miscibility with aqueous alcohol, and chemical determinations of groups of components of major interest. Older ISO standards indicated acceptable ranges for alcohol, carbonyl, acid and ester number. The latest ISO standards incorporate a chromatographic profile and concentration ranges for the most characteristic components. However, the variety of methods and protocols of analysis that are used makes it difficult to compare published profiles with the standards. A tabulated overview of ISO standards for physical characteristics of essential oils from plants dealt with in this volume is presented. When no ISO standard was available, the information was supplemented with standards from the EOA.
The Research Institute for Fragrance Materials (RIFM) of the IFRA publishes comprehensive papers on fragrance compounds. They present many biological data, such as metabolism in mammals, toxicity, carcinogenicity, sensitization and pharmacology.
The Food and Drug Administration of the United States (FDA) and the Flavor and Extracts Manufacturers' Association (FEMA) deal specifically with the safety of products, including the essential oils used in foods. Products it deems safe are issued with a "GRAS" or "generally recognized as safe" statement, which may specify restrictions in relation to their use in certain products.
Substitutes for natural essential oils are synthetic compositions of aroma chemicals that mimic the character of the oil in question. Adulteration is the fraudulous modification of a product. Adulteration of natural essential oils covers a range of actions: standardization, reinforcement, liquidization, reconstitution, and commercialization.
Standardization involves improving the quality of a product to meet the standard requirements. One can standardize the content of characteristic substances by adding such products that have been isolated from another natural source or produced synthetically. Common examples are the addition of citral isolated from Litsea cubeba oil to lemon oil, and of 1,8-cineole from Eucalyptus globulus Labill. to rosemary oil.
Reinforcement (also called extension) is an extension of standardization. When the quality of an essential oil can be improved there is always the temptation to add exaggerated amounts of the characteristic compounds to improve the quality and to make a product with "more olfactive value for money".
In liquidization, the aim is to change not the olfactive quality of a product, but its appearance. Some absolutes may be semi-solid or liquid. If the liquid form is preferred, solvents can be added to the absolute. Various solvents are used for this purpose.
Reconstitution is the compounding of an aroma product using natural or synthetic compounds to obtain a product that is similar to the original natural oil. However, it is quite impossible to reconstitute complete natural oils, because they consist of hundreds of compounds, many of which are unknown. Reconstituted essential oils are used especially in functional perfumery. When a natural essential oil in a perfume is prohibitively expensive it can be replaced by a reconstituted oil. Rose oil, jasmine oil and orange flower oil are often reconstituted.
Commercialization of a product involves expanding its volume and lowering its quality, to make it more profitable. It may involve the use of reinforced, liquidized or reconstituted products. If properly declared, commercialization is an accepted practice. Some buyers cannot afford to pay the cost of a natural product and are willing to buy a commercialized product with similar though inferior characteristics. However, a buyer has the right to know what he or she is buying.
Classification of perfumes
The classification developed for perfumes differs somewhat from the classification of odours, but uses the same terminology. It is based on the assumption that each perfume has a basic, more or less overriding concept, within which variations are possible. Perfumes can be classified into two broad groups, feminine and masculine, but these overlap greatly. Feminine concepts are: floral, oriental and "chypre"; masculine ones: "fougère", oriental and "chypre", whith citrus notes sometimes incorporated. The floral fragrance concept accounts for half of the international fragrance landscape. The concept is very broad and covers most of the spectrum of flower odours. The notes of the oriental concept are reminiscent of the legendary fragrances of the Orient, as typified by the sweet balms and resins of Arabia and precious spices of India and Indonesia. The term "chypre" refers to a famous perfume named Chypre after the island of Cyprus, from which most of its fragrance materials originated. Bergamot, oakmoss and patchouli are typical notes in perfumes of this concept. The "fougère" concept is based on the interplay of lavender, oakmoss and coumarin. Originally intended as a contribution to feminine perfumery, it became such an acknowledged masculine fragrance over time that it now plays virtually no role in feminine fragrances. When a single variation on a concept plays a dominant role it is sometimes used instead of the concept to describe a perfume. Table 3 gives an overview of the variations of the concepts and a few famous examples of each. Each perfume has a top note, a body or middle note and base note or dry-out. The top note is the odour of the most volatile components and is the first odour impression that a perfume gives. The top note gradually gives way to the body note, which represents the main sensory impression of a perfume. The dry-out remains for several to many hours after the application of a perfume. The three notes are described in the same terms as the variations. Perfume descriptions often also name the individual essential oils that can be recognized in the different stages of development of the odour of a perfume.
Production and international trade
Few comprehensive data have been published on the economics of essential oils. This reflects the great complexity of the global network of the production and trading of essential oil. The species involved are numerous (over 400), the economic significance of most individual oils is small, systematic compilation of data is rare and market knowledge is often jealously guarded e.g. by wholesale traders. Most import and export statistics tend to lump together several essential oils and even natural and synthetic materials. The blurred distinction between essential-oil plants and spices and, to a lesser extent, medicinal plants, further complicates the situation. The most recent comprehensive review of essential-oil production and marketing dates from 1993 (Lawrence, 1993). Only for a limited number of products are world market prices published regularly. An estimate of the production and value of the 20 most important essential oils is given in Table 4 (Lawrence, 1993). Over half of the world's exports comes from developing countries (Table 5). Among them, 4 countries predominate: China, Brazil, Indonesia and India. China and India show the greatest capacity to adapt to marketing changes and opportunities. All 4 countries are characterized by large and strong local markets for the essential oils produced, which dampens fluctuations in world demand and prices. They all have a strong position for a limited number of traditional products and have made long-term investments in research, training and supporting services.
Many constituents of essential oils are derived from the isoprene molecule, which also plays a role in the synthesis of important biological compounds such as chlorophylls, gibberellins, carotenoids and steroids. It is not surprising therefore that aroma compounds are found throughout the plant kingdom and to a lesser extent in the animal kingdom. There are 108 families of higher plants known that yield over 2000 essential oils (Gildemeister & Hoffmann, 1956-1966). Many families are represented by one species only, but of the Myrtaceae, 330 essential oils derived from over 300 species and varieties are mentioned, including 213 essential oils from Eucalyptus (sensu lato). Economically, the most important families yielding essential oils are: Gramineae (Cymbopogon spp., Vetiveria spp.), Labiatae (Mentha spp.), Lauraceae (Litsea spp.), Myrtaceae (Corymbia spp.), Oleaceae (Jasminum spp.), Pinaceae (Cedrus spp., Picea spp., Pinus spp.), Rosaceae (Rosa spp.), Rutaceae (Citrus spp.) and Santalaceae (Santalum spp.), while many spices that also yield essential oil are Umbelliferae. A few cryptogams yield essential oil; the most important ones are the lichen Evernia prunastri (L.) Ach. and the seaweed Fucus vesiculosus L. One family famous for its scented flowers, but hardly mentioned here, is the Orchidaceae. Modern analytical equipment is sufficiently sensitive to analyse orchid odour, but the amounts present in the flowers is too small to extract the oil commercially. Perfumes with an odour approaching that of orchid flowers can be composed using natural or synthetic products and may soon be important in perfumery. However, it is still impossible to compose a perfume that captures the true odour in all its depth and complexity.
The animal kingdom contributes only a few, but nonetheless famous, fragrance materials, most notably musk from the musk deer, ambergris from the sperm whale and civet from the civet cat.
Essential oils may be found in any part of a plant and are commercially extracted from roots, wood, bark, leaves, flowers, fruits and seeds. To some extent this reflects the many functions of the essential oil in the plant: in flowers the fragrance may attract insect pollinators, in fruits animals that distribute the seed, while in leaves the essential oil may function as insect repellant and in wood as preservative.
Aroma compounds are sometimes toxic materials formed or stored in special organs in plants. The variability of these organs reflects the varied taxonomy of essential oil plants; only a few examples of the many structures found can be given here. Oil glands may be simple or compound hairs on the leaves, as in Pelargonium and several Labiatae, where the gland consists of a multicellular hair with the oil concentrated in the enlarged apical cell or in the space between the cell-wall and the outer cuticula. Pogostemon Desf. has 2 types of oil glands: the common epidermal hair glands and mesophyll glands. The mesophyll glands are complex structures located in the palisade tissue and consist of a large secretory cell located near a small vascular bundle. The secretory cell has very dense cytoplasm and a large nucleus and is surrounded by a cuticula. The essential oil produced is contained in the space between the cell wall and the cuticula. In the Rutaceae and Myrtaceae the essential oil is concentrated in large subepidermal glands arising from a specialized mother cell. The mother cell divides into daughter cells that separate from one another and disintegrate to leave a central cavity. The cells surrounding the cavity produce essential oil and the cavity enlarges by the breakdown of the walls of the surrounding cells. In Santalum spp. the compounds that are distilled as essential oil are deposited in the xylem as part of the formation of the heartwood. The formation of fragrant wood is much more complex in Aquilaria spp., where it is only found in old, diseased trees. It is assumed that trees weakened by fungal attack are infected by a secondary fungal parasite that is involved in the formation and deposition of the essential compounds.
Few general statements can be made about the ecological requirements of essential-oil plants. Some species are grown in widely differing habitats. Citrus bergamia is grown in the hot and dry subtropical region of Calabria (Italy) but also in more humid, tropical parts of Guinea and Ivory Coast. Polianthes tuberosa, originally from Mexico, was formerly widely grown in southern France and is now cultivated mainly in Karnataka (India). Pelargonium spp. originating from South Africa were introduced into France, where natural hybrids developed that are now cultivated from temperate England to equatorial Kenya. Although a location should meet the basic ecological requirements of the essential-oil crops grown, it seems much more important that very high standards of cultivation and processing are maintained and that they are accompanied by a well organized trading network. This may explain the former role of Grasse (France), the very large production of several essential oils in the small islands of Nosy Bé and Mauritius near Madagascar and the recent developments in Hainan (China). However, ecology plays an important, but subtle role in the quality of essential oils. In Bulgaria, rose oils can be distinguished, like wine, by the year in which they were produced. Many oils are therefore traded with their origin attached to their name: vetiver oil Bourbon from Mauritius or otto of roses Turkey.
In only a few cases can differences in composition of essential oil be attributed unequivocally to ecological factors, e.g. the effect of temperature on the quality of geranium oil.
Crop husbandry measures in essential-oil crops differ little from those of other annual and perennial crops. Weed control needs special attention in crops that are harvested in bulk as weeds may reduce the quality of the essential oil if they are distilled with the crop. The use of herbicides and chemicals for disease and pest control often needs special care, as pesticide residues may also affect the quality of the oil.
The harvesting of most essential-oil crops too is similar to harvesting comparable crops. Cymbopogon spp. are harvested in bulk and require little special care. The wood of tree crops such as Cinnamomum camphora is chipped before distillation in a way similar to chipping wood for pulping. Crops grown for their flowers are a special case. Picking the flowers of crops such as Rosa and Jasminum requires an enormous labour input. It takes about 2.5 hours to harvest 1 kg of Jasminum flowers. Moreover, the flowers have to be picked early in the morning before temperatures become too high. To obtain 1 kg concrete or 0.5 kg absolute, 1000 kg of flowers are required. This accounts for the very high cost of some aroma materials and the gradual shift of production to low-income countries. Production of crops such as Pelargonium first moved to Mauritius, but when wages there increased, China became the main country of production. The production of many crops has followed this trend; only the very labour-intensive crop Rosa is still hardly grown outside its traditional areas of production: Bulgaria, France, India, Morocco and Turkey. Established trade lines and guaranteed quality apparently compensate for increasing costs.
Genetic resources and breeding
The essential-oil industry is relatively small and comprises a large number of crops. In the market for high quality essential oils used in luxury perfumery, suppliers try to meet the demands of buyers for products of well-established qualifications. In such a market it is difficult to introduce a new variety with almost unavoidably somewhat different characteristics. In the production of lavender oil, specifications have been laid down for the oil of several clones. Even for lavandin oil, which is a less exclusive product than lavender oil, there are detailed specifications for the oil from the main cultivars. The small size of the market for individual essential-oil crops also hampers the establishment of breeding programmes and gene banks. At the same time, however, the perfume industry is a creative one, always searching for new fragrances and experimenting to compound new attractive products. This would suggest that there is potential for introducing new products, provided that a stable supply of a high and constant quality can be guaranteed.
The situation is different for crops yielding essential oils used mainly in cosmetics and functional perfumery. The oils are selected more for their main aroma components than for the delicacy of their fragrance, on the basis of a large number of compounds and on the balance between them. For Cymbopogon spp., which yield essential oils used primarily for their main components, there are breeding programmes and a considerable number of improved cultivars have been released e.g. in India. Programmes to select superior trees have also been established, e.g. in Australia and Brazil, for Corymbia citriodora, which is grown mainly for its citronellal. Most breeding work is conducted in India, at the Central Institute of Medicinal and Aromatic Plants, Lucknow and by the regional institutes associated with it.
The current revival of interest in "natural" i.e. animal-based or plant-based materials applies strongly to aromatherapy and to flavour and fragrance products. Whereas the use of essential oils in general medicine is likely to remain limited to cough syrups and antiseptic preparations, aromatherapy has found a niche that is increasing in importance. Its objective of invigorating a person through the use of natural aroma materials combined with massages and baths appeals to a growing group of enthusiasts. Consequently, it represents an expanding market for essential oils.
In the food industry, natural products are enjoying a strong consumer preference, while the use of synthetic compounds is regulated by ever stricter laws and regulations. A huge firm such as Nestlé reflects this interest, using as flavour products: 75% naturals, 12.5% enhanced naturals and 12.5% nature-identical and artificial products. The growth of the market for exotic foods also contributes to a steadily increasing demand. At the same time, the growing demand for ready-made, i.e. industrially prepared food increases the use of essential oils, as this use can be controlled more accurately than that of fresh plant materials.
The underlying basis for the role of phytochemicals in perfumery is the ability of plants to synthesize molecules, many of which have complex structures beyond the dreams or economics of the organic chemist. Plants further compound large numbers of fragrance materials into a single essential oil, giving its odour a richness that cannot be copied by a perfumer using synthetic compounds.
The rapid developments in the analysis of essential oils, especially the combined chemical and olfactive characterization of compounds present in trace amounts only, will encourage increased application of synthetic compounds in perfumery. Although the use of synthetic products in industrial and functional perfumery is likely to grow, there is little doubt that essential oils will continue to play a central role in luxury perfumery and cosmetics. The contrasting pattern in production between lavender oil and vetiver oil exemplifies these trends. The production of lavender oil, which is strongly supported by research, is relatively stable in spite of competition from existing synthetics, while the production of vetiver oil is declining. Although vetiver oil seems to be protected by a lack of synthetic alternatives, irregularities in production and quality have depressed world demand and production.
The production of essential-oil crops can expand along 3 routes. The first is for such crops to follow market demand in areas where their production is already well established. To cope with increasing competition in inceasingly open markets and higher wages, productivity and product quality control will have to be improved constantly. Research and extension should support these developments. The second option is for new essential-oil crops to be introduced into regions traditionally producing essential oils. This requires intensive economic research and agronomic testing. South-eastern China, Morocco, Argentina and Brazil are examples of areas where new crops have been introduced successfully. The most challenging option is to establish a new essential-oil industry. This can meet with lasting success, as demonstrated by the varied production in the Indian Ocean islands of the Comoros, Mauritius and Madagascar and the bergamot production in West Africa. The history of production of geranium oil in East Africa, however, shows that disruptions in supply and quality control can cause irrevocable havoc in an industry.
The development of new uses for essential oils forms a special field of research. The use of phytochemicals including essential oils for the control of microorganisms in agriculture is gaining importance. The essential oils from Mentha ×piperita L. and several Cymbopogon spp. have been tested for their fungicidal effect against Helminthosporium oryzae, a devastating leaf spot disease in rice. And on the basis of the traditional use of aromatic plants, essential oils are also being tested to control insects in 3 broad areas: as antifeedants, juvenile hormones and insecticides.
Table 1. Classification of fragrance products. ───────────────────────────────────────────────────────────────────────────
"extrait" perfume or "parfum", containing 15-30% perfume oil in high grade (90%) alcohol; "eau de parfum", consisting of 15-18% perfume oil in 80-90% alcohol; "eau de toilette" or toilet water, having 4-8% perfume oil in 80% alcohol; aftershave perfumes, containing 3-5% perfume oil in 70% alcohol; eau-de-cologne, consisting of 3-5% perfume oil in 70% alcohol, fresher and more volatile than similar "parfums" and "eaux de parfum"; splash colognes are toilet waters containing only 1-3% perfume oil in very dilute alcohol.
body care products, e.g. creams and lotions containing 0.5-2% perfume oil; hair care products, e.g. shampoos and conditioners having 0.5-2% perfume oil; deodorants, containing 0.5-1% perfume oil.
soaps containing 1-2% perfume oil; detergents containing 0.1-0.5% perfume oil; bathing products containing 0.1-0.5% perfume oil; house-cleaning products containing 0.2-0.5% perfume oil.
aroma compounds added to hazardous products, e.g. cooking gas; compounds masking unpleasant odours of products, e.g. plastics.
─────────────────────────────────────────────────────────────────────── Source: Adapted from Curtis & Williams, 1994; Müller & Bräuer, 1992.
Table 2. Classification of odour qualities and similarity coefficients. ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
└── pineapple ┌── apple
└── ethereal ┌─ brandy 0.3┌┤ │└─ wine
└── grape citrus aldehydic wax fat ┌── butter
└── cream ┌── root
│┌─ floral 0.3└┤ └─ green
lily of the valley
┌── orange blossom
│┌─ herbal 0.2└┤ └─ spicy
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── Source: Ohloff, 1990.
Table 3. Perfume classification. ───────────────────────────────────────────────────────────────────────── Concept Variation Examples Feminine fragrances floral green Vent vert fruity Lauren fresh Diorissimo, Anaïs anaïs floral Quelques fleurs, Giorgio aldehydic Arpège, Madame Rochas sweet l'Origan, Poison oriental amber Shalimar, Samsara spicy Youth dew, Opium "chypre" fruity Chypre, Femme floral-animalic Miss Dior, Cabochard floral Coriandre fresh Diorella green Alliage Masculine fragrances "fougère" lavender Silvestre, Cool water fresh Drakkar noir, Jazz woody-amber Paco Rabanne, Zino Davidoff oriental spicy Old spice, Patou pour homme amber Habit rouge "chypre" woody Vétiver, Macassar leathery Aramis fresh Armani, Fahrenheit citrus Eau sauvage ───────────────────────────────────────────────────────────────────────── Source: Glöss, 1995.
Table 4. Production and value of the 20 most important essential oils.
Essential oil Species Production (t) Value 106 US$ orange oil Citrus sinensis (L.) Osbeck 26 000 58.5 cornmint oil Mentha arvensis L. var. piperascens Malinv. ex Holmes 4 300 43.4 eucalyptus oil cineole type Eucalyptus globulus Labill., E. polybractea R.T. Baker and other spp. 3 728 29.8 citronella oil Cymbopogon winterianus Jowitt and C. nardus (L.) Rendle 2 830 10.8 peppermint oil Mentha ×piperita L. 2 367 28.4 lemon oil Citrus limon (L.) Burm.f. 2 158 21.6 eucalyptus oil citronellal type Corymbia citriodora (Hook.) K.D. Hill & L.A.S. Johnson 2 092 7.3 clove leaf oil Syzygium aromaticum (L.) Merrill & Perry 1 915 7.7 cedarwood oil (United States) Juniperus virginiana L. and J. ashei Buchholz 1 640 9.8 litsea cubeba oil Litsea cubeba (Lour.) Persoon 1 005 17.1 sassafras oil (Brazil) Ocotea odorifera (Vell.) Rohwer (syn. O. pretiosa (Nees.) Mez.) 1 000 4.0 lime oil distilled (Brazil) Citrus aurantifolia (Christm. & Panzer) Swingle 973 7.3 native spearmint oil Mentha spicata L. 851 17.0 cedarwood oil (China) Chamaecyparis funebris (Endl.) Franco 800 3.2 lavandin oil Lavandula ×intermedia Emeric ex Loisel. 768 6.1 sassafras oil (China) Cinnamomum micranthum (Hayata) Hayata 750 3.0 camphor oil Cinnamomum camphora (L.) J.S. Presl 725 3.6 coriander oil Coriandrum sativum L. 710 49.7 grapefruit oil Citrus ×paradisi Macf. 694 13.9 patchouli oil Pogostemon cablin (Blanco) Benth. 563 6.8 Source: Lawrence, 1993.
Table 5. Principal essential-oil exporting countries. Country Value (103 US$) Quantity (t) Proportional value (%) China 141 967 14 693 18.6 European Community 124 811 9 656 16.3 United States 122 833 8 435 16.1 Hong Kong 47 250 6 869 6.2 Brazil 36 389 4.8 Indonesia 33 354 2 450 4.4 India 24 643 1 156 3.2 Switzerland 16 850 459 2.2 Argentina 15 743 1 503 2.1 Paraguay 11 478 1 130 1.5 Singapore 10 993 859 1.5 Thailand 10 818 778 1.4 Haiti 9 776 228 1.3 Japan 9 526 582 1.2 Turkey 8 891 15 1.1 Morocco 8 731 542 1.1 Former USSR 7 943 203 1.0 Mexico 7 896 1 493 1.0 Canada 6 893 0.9 Egypt 6 810 70 0.9 Note: Re-exports are included in the data. Data from countries with incomplete export statistics are partly based on statistics from importing countries. Due to discrepancies between import and export statistics, the total value of imports exceeds the value of exports by about 25%. Source: Hay & Waterman, 1993.