Aromatic Plants for the Flavor and Fragrance Industries
INTRODUCTION
Essential oils—also known as volatile or ethereal oils or essences—are the odoriferous principles found in aromatic plants that are the raw materials of the flavor and fragrance industries. The term essential, which has alchemical roots, results from the coined phrase ‘‘Quinta essential,’’ or ‘‘quintessence,’’ the fifth element (the other four being land, fire, wind, and water). Alchemists believed that within the plant kingdom there was a single extractive principle that could prolong life indefinitely, hence the importance of Quinta essential.
The term ‘‘oil’’ probably originated from the observation that certain plants contain glands or intercellular spaces filled with oily droplets found to be non-miscible with water. Unlike fatty or fixed oils, which are lipids, essential oils are complex mixtures of volatile compounds that are biosynthesized by living organisms. For an essential oil to be genuine, it must be isolated by physical means only from a whole plant or plant part of known taxonomic origin. The physical methods used to isolate an essential oil are water, water/steam or steam distillation, or expression (also known as cold pressing, a process unique to the production of citrus peel oils).
A limited number of plants (onion, garlic, wintergreen, sweet birch, and bitter almond) yield their oils during processing. In this process, warm water is mixed with the macerated plant material, causing the release of the enzyme-bound volatiles into the water from which the oil is subsequently removed by distillation. Although the practice of distillation was first discovered in the Indus Valley in Pakistan around 3000 B.C., it was not until around 1300 A.D. that distilled aromatic waters were extensively used as medicaments.
Essential oils did not appear as items of commerce until sometime in the 1400s with cedarwood oil, cinnamon oil, rose oil, and sage oil, among the earliest. It is estimated that of the 17,500 aromatic plant species found in the vegetable kingdom, approximately 270 are used to produce essential oils of commerce. Of these 270 oils, ca. 40% are made from cultivated plants; the other 60% are delivered from by-products of primary industry or from readily accessible wild growing plants.
The plant families that possess species that yield a majority of the most economically important essential oils are Apiaceae or Umbelliferae (fennel, coriander, and other aromatic seed/root oils), Asteraceae or Compositae (chamomile, Artemisia sp. oils, etc.), Cupressaceae (cedarwood, cedar leaf, juniper oils, etc.), Geraniaceae (geranium oil), Illiciaceae (star anise oil), Lamiaceae or Labiatae (mint, patchouli, Lavandula sp., and many herb oils), Lauraceae (litsea, camphor, cinnamon, sassafras oils, etc.), Oleaceae (jasmine oil), Pinaceae (pine and fir oils, etc.), Poaceae or Graminae (vetiver and aromatic grass oils), Rosaceae (rose oil), and Santalaceae (sandalwood oil).
WHERE ARE ESSENTIAL OILS FOUND?
Essential oils are contained in special glands or secretory tissues, the type being important characters of the plant family within which the aromatic plant is found. In the Asteraceae (Compositae), schizogenous oil ducts are frequently associated with vascular bundles in the leaves and stems. In the Lamiaceae (Labiatae), there are characteristic multicellular-headed glandular trichomes present on the leaves and calyx of the plant.
The secretion produced by a multicellular head accumulates under a common cuticle that is raised like a blister. In the Lauraceae, there are large oil cells in the stem barks, whereas in the leaf, oil cells tend to be mesophyllic (i.e., found in the interior of the leaf). In the Myrtaceae and Rutaceae, the secretory tissues arise from a special mother cell. This cell divides and the daughter cells separate from one another schizogenous (i.e., they split apart from each other) to leave a central cavity.
The cells surrounding the cavity produce essential oil and the cavity continues to enlarge as the lining walls undergo lysis. Thus, large rounded oil cavities or glands are produced schizolysigenously and are visible to the naked eye (as seen in the oil glands on the peel of an orange, e.g.). Essential oils in the Apiaceae (Umbelliferae) are present in long secretory ducts called vittae that arise schizogenously on the fruits and roots.
In the Zingiberaceae, a continuous layer of large rectangular oil cells occurs in the testa (the seed coat); in fact, the oil cells are just within the thin aril and epidermis of the seed and on the rhizome. In addition, isolated oil cells are scattered throughout a scraped rhizome.
PEPPERMINT AND NATIVE AND SCOTCH SPEARMINT OILS
As noted in Table 3, approximately 3500 tonnes of peppermint oil, 980 tonnes of Native spearmint oil, and 760 tonnes of Scotch spearmint oil are produced annually. The largest share (more than 90%) of peppermint oil production is in the United States. A breakdown of the regions of production. Of ca. 980 tonnes of Native spearmint oil produced, 55% was produced in the United States, with 52.5% produced in the West (Washington, Montana, Idaho, Oregon) and 2.5% in the Midwest (Michigan, Indiana, Wisconsin). China and India produced 23% and 19%, respectively. In contrast, of 760 tonnes of Scotch spearmint oil produced worldwide, the United States produced 72% (59% in the West and 13% in the Midwest).
The other principal countries of production were Canada (17%) and India (10%).[10] Although peppermint and both spearmints are produced in the U.S. Midwest and West, the yield of oil is much higher in the West because of its longer, hotter growing season. All U.S. mints are produced clonally from stolons. Because of the widespread incidence of a peppermint wilt fungus (Verticillium dahliae), the disease-free rootstock is used to replenish or commence a new area of planting. Similarly, both spearmints are susceptible to mint rust (Puccinia methane), so disease-free stolons are used to also commence or replenish new spearmint plantings.
Unlike spearmints, peppermint is considered a long-day plant because it needs a minimum midsummer day length of 15 hours for flowering and for the production of oil of commercially acceptable aroma and quality. This day length can be achieved north of the 40th parallel. To yield commercially acceptable oil quality, the plants are harvested just as the first flowers start to form. This is done to ensure that the undesirable oil component menthofuran is minimized. Day length and flowering have little effect on the oil composition of either Native and Scotch spearmint. In fact, to produce the highest oil yield, both spearmints are harvested in full flower. Even though Scotch and Native spearmints are produced from different Mentha species, they are both rich in L-carvone; however, the subtle differences in oil composition make them discernibly different. This in turn affects their end-use.
Unlike peppermint oil production in the United States, a marketing order is in place for both the Scotch and Native spearmint oils covering an area of the Pacific Northwest due north of the west Wyoming border to the Canadian border, south to the southern border of Utah, and west from the Utah border to the Pacific coast. The administration of the marketing order controls the quantity of oil produced by each grower annually and sets a price for the oil.
ROSE OIL
Although only a small quantity of rose oil is produced annually, it is an extremely high-priced oil, selling for as much as $3500 per kg. The main countries where damask rose (Rosa damascena Mill.) is grown are Bulgaria (around Kazanlik), Turkey (in Isparta and Budur provinces), Ukraine, Morocco, and Moldova, whereas R. centifolia L. is grown mainly in France and Morocco. The world production of both rose oils is estimated at ca. 5 tonnes (4 tonnes damask and 1-tonne centifolia).
In Turkey, shallow holes 50 cm50 cm are prepared in which bare root cuttings are planted and trimmed so that the cutting just shows above the soil level. The cuttings are watered regularly so that 3 years after planting, the starts have reached maturity and the roses are ready for harvest. A mature planting yields ca. 5 tonnes of flowers per hectare and can continue producing flowers for 20–30 years. It takes ca. 3.5–4.0 tonnes of flowers to yield 1 kg of oil. The flowers are harvested early in the morning, the time when their oil yield is maximized. At one time, all rose oil was produced by steam and water distillation in which rose flowers were placed on a grid above the water in the bottom of a still.
The water was boiled by direct fire; the steam generated released the oil that was condensed with a standard condenser. Today, more than 95% of exported oil is produced with steam distillation in which the steam is generated in a satellite boiler and passed through a still containing evenly spaced, stacked, multiple grids on which the flowers are placed. This allows the oil to be readily removed without the flowers becoming a waterlogged mass, as happens in steam and water stills. More rose flowers are used to produce rose concrete and rose absolute that is used to produce rose oil. Concrete is a hydrocarbon extract of the fresh flowers generally produced by percolation of the flowers with hexane, with the spent flowers and residual hexane removed. The concrete is rich in hydrocarbon-soluble materials and devoid of water-soluble materials. It is generally a waxy, semisolid, dark-colored mass.
An absolute is a highly concentrated alcoholic extract of the concrete. It is prepared from the concrete by dissolving the concrete in hot alcohol and then rapidly cooling the mixture to precipitate the waxes that are removed, generally by filtration or by a cooled surface scraping of the slurry. The alcohol is then removed by distillation under a high vacuum. The resultant absolute contains the concentrated aromatic portion of the flowers. The yield of oil from rose flowers is ca. 0.02%; the yield of concrete is 0.22–0.25%, whereas the yield of absolute from the concrete is 50–60%. It is estimated that ca. 15.0 tonnes of rose concrete are produced annually, from which ca. 8.0 tonnes of rose absolute are produced.
QUALITY OF ESSENTIAL OILS
Commercial essential oils are sold in compliance with standards of Physico-chemical characteristics such as odor and color, specific gravity, refractive index, and optical rotation, and chemical characteristics such as gas chromatographic analysis. Standards for oil acceptance can be of national or international origin. Standards are set out as monographs on each oil, such as International Standards Organization (ISO), French Standards (AFNOR), German Standards (DIN), and Food Chemical Codex Standards (FCC).
USES AND TRENDS OF ESSENTIAL OILS
In 1990, it was estimated that 50% of all essential oils produced were used in flavors.[18] This can be readily understood because in the United States the per capita consumption of soft drinks is 166 L annually, whereas the annual per capita consumption for the rest of the world is ca. 6 L. As a result, if soft drink consumption increased 3% worldwide, it would result in a 40% increase in essential oils needs.
Over the past 25 years the production levels of some oils have changed for a number of reasons:
1) synthetics have partially replaced their use (spike lavender oil, camphor oil);
2) replacement oils are richer in desirable components than the original oil (litsea cubeba oil has partially replaced lemongrass oil because it is a better source of citral, lavandin has replaced lavender because it is a cheaper source of a similar odor character);
3) wild collection of plant material has been reduced because of the scarcity of raw material (amyris oil, cascarilla oil); and
4) wild collection has been restricted by legislation, by either regional or federal mandate (sassafras in Brazil).
Over this same period, the use of oils such as orange, lemon, and lime has increased. Although citrus-flavored soft drinks have mainly driven the increase in use of citrus oils, their use in such top-selling fragrances as K One, Cool Water, Drakkar Noir, Escape (male), Eternity (for men), Hugo, Polo Sport (male and female), and Tommy has also helped maintain this increase. The production of comment oil has also increased, not for the value of the oil but for the natural isolate L-menthol obtained from the oil by freeze crystallization. The pharmaceutical, oral hygiene, cosmetic, tobacco, and confectionery industries are responsible for the use of an estimated 23,000 tonnes of L-menthol (both natural and synthetic).
There is a particularly increased use of menthol in India (in chewing tobacco); the use of oral care products in China is also on the increase. The production of tea tree oil [Melaleuca alternifolia (Maiden et Betcher Cheel)] for use as a natural additive and home care treatment for burns and wounds has also increased in recent years. This oil is produced in commercial quantities only in Australia, where most households are thought to keep a vial of it in the medicine cabinet.
Most other oils have relatively stable levels of production, although their production level in one country may be negatively affected by their production in other countries. Since the mid-1980s there has been increased awareness of essential oils due to the popularization of aromatherapy. Although the use of essential oils in aromatherapy does not affect world production by more than 1.0%, it has led to an increase in the number of oils new to commerce. A list of these oils and their countries of origin.
ESSENTIAL OIL MARKET
Although it is difficult to determine the exact size of the essential oil market, it has recently been estimated at $310 million with a projected annual increase of 4%. The $310 million approximation is probably an underestimate, however, because the value of the top twenty oils listed in Table 3 totals ca. $325 million. Consequently, the total worldwide market size for all essential oils is probably closer to $400 million.
CONCLUSION
Aromatic plants are of value because of their aromatic principles, namely their essential oils. The type and methods of essential oil production have not changed much over the centuries. However, the volumes and origins have changed. Over the past century, there has been a shift in essential oil production from Europe and the Americas to Asia primarily because of labor and fuel costs and the urbanization of rural areas. Currently, there is a worldwide reduction in essential oil development funding and the number of scientists familiar with these specialty crops is diminishing. Nevertheless, essential oils continue to be important ingredients of food, cosmetic, and pharmaceutical products. Over the next century, the need for cooperation between the consumer and producer will become mandatory even though there may be further global changes in areas of production.