Evaluation

Various pharmacopoeial procedures are given for the evaluation of volatile oils. Odour and taste are obviously important in the preliminary examination; however oils should not be tasted neat but only after dilution with a sugar solution in ethanol as prescribed in the BP. Physical measurements including optical rotation, relative density and refractive index are regularly employed for identification and

assessment of purity; similarly thin-layer chromatograms. Capillary gas chromatographic profiles are used to determine the proportions of individual components of certain oils. Advances in gas chromatography have now made possible the ready determination of the chirality of particular components of volatile oils thus detecting adulteration with synthetic material or unwanted other oils; examples of its use are carvone in caraway oil, linalol in coriander oil and linalol and linalyl acetate in neroli oil. The ketone and aldehyde contents of oils such as caraway and lemon respectively are determined by reaction with hydroxylamine hydrochloride (oxime formation) and titration of the liberated acid. Other general tests described in the BP include examination for fixed and resinified oils (residue after evaporation), foreign esters (conversion to a crystalline deposit) and presence of water (turbidity of a carbon disulphide solution).

There have been a number of recent problems (1999) with the occurrence of tetrachloromethane in essential oils, particularly spearmint oil. This is probably not due to deliberate adulteration (the adulterant is present at low ppm levels) but a consequence of cleaning the drums before use with offending solvent and inefficient removal of it before filling the drums. Unfortunately on detection it renders the oil useless, as in food tetrachloromethane is prohibited.

The volatile oil content of crude drugs is commonly determined by distillation (Chapter 16).

Biogenesis

The origin of metabolites with phenylpropane and terpenoid structures has been discussed in Chapter 18. In medicinal essential oils the number of the former is limited but for the monoterpenes which arise at the geranyl pyrophosphate (GPP) level of terpenoid synthesis there are numerous examples. Analyses show that these oils commonly contain 40–80 monoterpenoids, many in relatively small proportion. A major constituent of one oil may be a minor one in another.

Three groups of monoterpenoid structures are involved: (1) acyclic or linear; (2) monocyclic; and (3) bicyclic. In the plant they are sequentially derived from limonene in this order as illustrated in Fig. 22.1. Relatively few enzymes, termed cyclases, appear to determine the skeletal class (e.g. menthanes, pinanes, thujanes etc.) and it is possible that these serve as rate-limiting enzymes. However, regulatory factors for the control of synthesis operate not only at the biosynthetic enzyme level per se but in a hierarchical manner up to the whole-organism level.

Fig. 22.1 Possible biogenetic relationships between some monoterpene types.

Because of the trans geometry of the double bond at the C-2 of GPP, direct cyclization to limonene is not possible and for Mentha spp. it has been shown that (−)-limonene synthase located within the oil glands catalyses the isomerization of GPP to enzyme-bound (+)-3S-linalyl pyrophosphate with subsequent cyclization to (−)-limonene. However, many enzymatic steps are involved in the subsequent modifications and interconversions of these monoterpenes. Some components of volatile oils are sesquiterpenes (C₁₅H₂₄) (q.v.) and they include cadinene, zingiberene (structure, Fig. 18.17) and caryophyllene. The formulae of some of the more common constituents of pharmaceutical volatile oils are given in Fig. 22.2.

Fig. 22.2 Some components of pharmaceutical oils.

It is increasingly evident that some monoterpenes and other components of volatile oils also occur in plants in the glycosidic form. Thus, geraniol, nerol and citronellol occur as glycosides in the petals of Rosa dilecta, thymol and carvacrol as glucosides and galactosides in Thymus vulgaris and eugenol, benzyl alcohol, β-phenylethyl alcohol, nerol, geraniol and geranic acid as glucosides in Melissa officinalis. It is considered that these glucosides of monoterpenols and of 2-phenylethanol are translocated from leaves to flowers as aroma precursors.

Table 22.1 may be used to compare the compositions of important volatile oils. The classification is arbitrary, since an oil may contain a number of compounds all about equally important but belonging to different chemical classes. The substance used for classification is not necessarily the one present in greatest amount. Thus, nutmeg is classified on its myristicin and lemon on citral, although these constituents form only a small percentage of these oils.

Table 22.1 Composition of volatile oils.

C₂₀ or diterpenoid compounds include such resin acids as (+)- and (−)-pimaric acid and their isomer, the abietic acid of pine resin. The abietane acids have antimicrobial, antiulcer and cardiovascular properties (for a review covering some 56 acids over the period 1967–92 see A. S. Feliciano et al., Planta Med., 1993, 59, 485). Many diterpenoids (e.g. vitamin A and gibberellic acid) do not belong to the volatile oil–resin group. Similarly, only a small proportion of triterpenoid compounds (C₃₀) are found as resin constituents (e.g. in Euphorbia resinifera).

Extraction of oils used in perfumery

Certain oils used in perfumery, such as oil of rose, are prepared by steam distillation as described above but many of the flower perfumes require other treatment. An important centre for the extraction of flower oils is Grasse, in the south of France. Here the oils are extracted by enfleurage, by digestion in melted fats, by pneumatic methods or by means of solvents. In the enfleurage process glass plates are covered with a thin layer of fixed oil or fat upon which the fresh flowers are spread. The volatile oil gradually passes into the fat and the exhausted flowers are removed and replaced by a fresh supply. Formerly the flowers had to be picked off by hand but this is now done mechanically. Only a small percentage of the flowers, which resist the action of the machine, require removal by the fingers or by means of a vacuum cleaner. The pneumatic method, which is similar in principles to the enfleurage process, involves the passage of a current of warm air through the flowers. The air, laden with suspended volatile oil, is then passed through a spray of melted fat in which the volatile oil is absorbed. In the digestion process the flowers are gently heated in melted fat until exhausted, when they are strained out and the perfume-containing fat is allowed to cool. It will be seen that in each of the above processes the volatile oil has now been obtained in a fatty base. The volatile oil is obtained from this by three successive extractions with alcohol. The alcoholic solutions may be put on the market as flower perfumes or the oil may be obtained in a pure form by recovery of the alcohol. Solvent extraction is based on the Soxhlet principle (see Chapter 16).

Macroscopical characters

All the mints have square stems and creeping rhizomes. The flowers are arranged in verticillasters and have the floral formula K(5),C(5),A4,G(2). The black mint, which is the one most commonly cultivated in England, has purple stems and dark green petiolate leaves which are tinged with purple. The leaf blades are 3–9 cm long and have a grooved petiole up to 1 cm long. They have a pinnate venation with lateral veins leaving the midrib at about a 45° angle, acuminate apex and sharply dentate margin. Glandular trichomes can be seen as bright yellowish points when the lower surface is examined with a hand lens. The leaves are broader than those of M. spicata (spearmint), but narrower than those of M. aquatica (water mint). The small, purple flowers appear in late summer.

Microscopical characters

The microscopy of peppermint leaves is typical of the family, showing numerous diacytic stomata on the lower surface (Fig. 42.2G), three- to eight-celled clothing trichomes with a striated cuticle (Fig. 42.4C), and two types of glandular trichome, one with a unicellular base and small single-celled head and the other with a multicellular head characteristic of the family (Fig. 42.5E). Calcium oxalate is absent.

There is a 5% limit of stems over 1 mm in diameter for the official leaves, and as mints are very susceptible to most diseases, there is a 10% limit of leaves infected by Puccinia menthae.

Of commercial importance has been the development by mutation breeding at the A. M. Todd Co. of a strain of Mitcham peppermint resistant to the wilt disease Verticillium albo-duram var. menthae. The strain retains the Mitcham cultivar organoleptic characteristics and gives a good oil production in verticillium-prone soils where cultivation with the ordinary varieties is impossible.

Biogenesis of peppermint monoterpenoids

The biosynthetic isoprenoid origin of monoterpenes was mentioned at the beginning of this chapter. As the essential oils of the Labiatae are synthesized in the cells of the glandular trichomes, techniques such as cell and root culture are of little value as experimental tools. However, new procedures, using gentle abrasion of leaf surfaces with glass beads, have been developed for isolating in high purity and excellent yield, peltate glandular trichomes of peppermint which retain their biosynthetic activity.

Developmental changes in the oil composition of the leaves include the disappearance of limonene, the accumulation of 1,8-cineole, the conversion of menthone to menthol and the acetylation of menthol. All these processes begin at the distal extremity of the leaf and shift progressively to the leaf base (B. Voirin and C. Bayer, Phytochemistry, 1996, 43, 573).

A proposed pathway for the formation of monoterpenes in peppermint is given in Fig. 22.3. A number of enzymes involved in the reactions have been characterized. The hydrolase system involving (−)-limonene-3-hydroxylase in the formation of the alcohol (−)-trans-isopiperitenol is cytochrome-P450-dependent and is associated with the oil gland microsomal fraction. The remaining steps are catalysed by operationally soluble enzymes of the oil cells. It will be noted that (+)-pulegone is a branching point for the formation of menthol stereoisomers.

Fig. 22.3 A possible biogenetic scheme for some monoterpenoids of Mentha × piperita.

Japanese peppermint oil is derived from Mentha canadenis var. piperascens; it contains 70–90% menthol, for the extraction of which it is largely used. The commercial dementholized Japanese oil contains approximately the same amount of menthol and its esters as the American oil.

Characters

Mint has more or less crumpled, opposite, ovate-lanceolate leaves, 3–7 cm long. The apex is acute or acuminate, and the margin unequally serrate. The leaves differ from those of peppermint in that they are almost sessile and have a bright green colour free from purple.

Constituents

Oil of spearmint contains (−)-carvone, (−)-limonene, phellandrene and esters. As with M. × piperita limonene is the precursor of the monoterpenoids and in this case the action of a (−)-limonene-6-hydroxylase predominates to give the alcohol (−)-trans-carveol which is oxidized to carvone (Fig. 22.4). Dihydrocarvone is formed later in the season and is absent from plantlets produced by shoot-tip culture. Again like peppermint, oil production is influenced by the age of plant, time of collection, chemical varieties and hybridization.

Fig. 22.4 Biosynthetic pathway for major constituents of spearmint oil.

Macroscopical characters

The petiolate oblong-lanceolate leaves are up to 10 cm length and 2 cm in breadth, greenish-grey on the upper surface and tomentose on the lower with a markedly reticulate venation. The leaf apex is rounded, the base rounded or cordate and the margin crenulate. The odour and taste are characteristically pungent.

Microscopical characters

The upper epidermal cells have beaded anticlinal walls, the lower ones are thin-walled and sinuous; both epidermi possess diacytic stomata. Glandular trichomes of the typical labiate type occur on both surfaces with rarer uniseriate glandular trichomes having a double- or single-celled head. Clothing trichomes are numerous, particularly on the lower surface, composed of a short thickened basal cell with articulated and bent terminal cells. A few single-celled warty-walled trichomes are present. The long protective trichomes serve to distinguish S. officinalis from S. sclarea and S. pratensis (M. Then et al., Acta Pharm. Hung., 1998, 63, 163).

Constituents

The volatile oil of sage contains about 50% of α- and β-thujone together with cineole, borneol and other constituents (Fig. 22.2). Varieties and other species of sage contain differing amounts of thujone.

Non-volatile components of the leaf include diterpenes, phenolic glycosides based on caffeic and p-hydroxybenzoic acids (for recent isolations see M. Wang et al., J. Nat. Prod., 1999, 62, 454), and tannins.

Action and uses

Sage as an infusion is used as a mouthwash and gargle for its antiseptic and astringent action. Recent attention has focused on the cholinergic activity of the drug and its possible role in the treatment of Alzheimer’s disease and memory loss. It is interesting to note that long before recent advances in the understanding of the neurobiology of Alzheimer’s disease, plant materials including sage and balm (Melissa officinalis) were recommended in old reference books as possessing memory-improving properties (see E. K. Perry et al., J. Pharm. Pharmacol., 1999, 51, 527). The phenolic glycosides of sage together with those of Melissa officinalis and Lavandula angustifolia possess antioxidant properties (J. Hohmann et al., Planta Medica, 1999, 65, 576).

Constituents

The compositon of the essential oil is considered under ‘Rosemary Oil’, below. Hydroxycinnamic acids include caffeic acid and a dimer rosmarinic acid (a characteristic metabolite of the subfamily Saturejoidae to which Rosmarinus belongs). For the dried leaf, the BP sets a minimum requirement of 3.0% for total hydroxycinnamic acids expressed as rosmarinic acid.

In recent years, a large number of phenolic abietane diterpenoids have been recorded for the leaves including the potent antioxidant carnosic acid together with its degradation products carnosol, rosmanol, epirosmanol and 7-methylepirosmanol. For new recently isolated diterpenes, see A. A. Mahmoud et al., Phytochemistry, 2005, 66, 1685; C. L. Cantrell et al., J. Nat. Prod., 2005, 68, 98. Triterpenes include the alcohols α- and β-amyrin, ursolic acid and oleanolic acid.

In vitro cell cultures of rosemary have been produced which biosynthesise carnosic acid, carnosol and rosmarinic acid (A. Kuhlmann and C. Rohl, Pharm. Biol., 2006, 44, 401).

Uses

Rosemary leaves have many traditional uses based on their antibacterial, carminative and spasmolytic actions.

Constituents

The volatile oil composition of thyme can vary enormously and various chemotypes have been recorded, particularly regarding thymol and carvacrol. The official drug is required to contain not less than 1.2% volatile oil, of which not less than 40% consists of thymol and carvacrol. It is these phenols that are largely responsible for the antiseptic, antitussive and expectorant properties of the drug. Other common variables of the oil are cymene (10–24%) and γ-terpinene (4–18%). Two extreme variations recorded are an almost complete lack of thymol and carvacrol in T. vulgaris and an oil containing 74% thymol from T. zygis.

A number of monoterpenoid glycosides occur in the leaves, particularly glucosides and galactosides of thymol and terpineol; seven new such constituents have recently been described (J. Katajima et al., Phytochemistry, 2004, 65, 3279; Chem. Pharm. Bull., 2004, 52, 1013). Flavones, highly oxygenated flavones, flavanones and dihydroflavonals may be responsible for the spasmolytic effect of the leaves, and the biphenyls reported in 1989 may have deodorant properties. Other constituents include rosmarinic acid (see ‘Rosemary’) up to 2.6%, various acids, tannins and resins. Rosmarinic acid and 3′-O-(8″-Z-caffeoyl) rosmarinic acid have been reported as the most important radical scavengers of the leaves (A. Dapkevicins et al., J. Nat. Prod., 2002, 65(6), 892).

Uses

Although in Britain oregano is not used medicinally to any extent, its thymol content gives it strong antiseptic properties. Traditionally its uses include the treatment of digestive disorders, pharnygeal infections and mild fevers.

Uses

Although it is the volatile oil of lavender that is principally used for medicinal purposes the BHP 1983 cites flatulent dyspepsia, colic and depressive headache as indications for use of the flowers. It may be used in combination with other drugs such as rosemary, valerian, meadow-sweet and others.

Lavender oil types

The taxonomy of the lavenders is confusing and Continental oils differ among themselves owing to the fact that a number of different species, varieties and hybrids are distilled. The true lavender, L. officinalis, yields the best oil when grown at a fairly high altitude, the variety growing under these conditions being known as ‘petite lavande’. At a lower altitude the ‘lavande moyenne’ yields a somewhat less esteemed oil. ‘Grande lavande’, L. latifolia Villers (L. spica DC), yields a much coarser oil, which is sold as oil of spike. The above plant readily hybridizes with L. officinalis yielding a plant known as ‘grosse lavande’ or ‘lavandin’, the oil of which is intermediate in character between that of the parent forms. According to Tucker (Baileya, 1981, 21, 131), of the many names applied to this hybrid species, the correct one is L. × intermedia Emeric ex Loiseleur. As hybrids the plants do not breed true and are normally propagated vegetatively; a simple efficient method for the in vitro shoot regeneration from the leaves has offered possibilities for future breeding (S. Dronne et al., Plant Cell Reports, 1999, 18, 429). The lavandin oil market is controlled by the French with Spain the principal producer.

The evergreen plant flowers from July to September and the fresh flowering spikes yield about 0.5% of volatile oil. The amount varies according to variety, season and method of distillation; modern steam stills give a rather larger yield than those in which the flowers are boiled with water. Genuine Continental lavender oil normally contains over 35% of esters. Oil of spike, which is largely used in cheap perfumery, contains little ester but a high proportion of free alcohols (about 23–41% calculated as borneol); 30 components have been identified. The nature of the alcohols also varies from a mixture of linalol and geraniol in the best lavender oil to borneol in oil of spike. Hybrids are of intermediate character (e.g. ‘lavandin oil’) and contain about 6–9% of esters and about 35% of alcohols. An analysis of the Spanish oil (J. de Pascual Teresa, Planta Medica, 1989, 55, 398) enabled the identification of 50 compounds, the principal ones being 1:8-cineole, linalol and camphor; in contrast to the oil of L. angustifolia, linalyl acetate was not present.

A GC profile together with prescribed percentage ranges of 10 components of the pharmaceutical oil is given in the BP/EP; linalol (20–45%) and linalyl acetate (25–46%) are the principal constituents with a maximum limit for camphor of 1.2%. Chiral chromatography is used to determine the chiral purity of the linalol and linalyl acetate contents.

As with other Labiatae, Lavandula cell cultures do not produce essential oil and for L. vera rosmarinic acid is the principal phenolic component together with caffeic acid and traces of others. An enol ester of caffeic acid is a blue pigment also found in cell cultures (see E. Kovatcheva et al., Phytochemistry, 1996, 43, 1243).

Species of Lavandula other than the above are also cultivated. L. stoechas has a markedly different odour and of its 51 volatile components, fenchone, pinocaryl acetate, camphor, eucalyptol and myrthenol predominate. Large producers are Spain and France. Oil from wild plants growing in the Algiers region of Algeria contained as significant constituents fenchone (31.6%), camphor (22.4%), p-cymene (6.5%) lavandulyl acetate (3.0%) and α-pinene (1.0%). Fifty-four components amounting to ca 73% of the oil were identified (T. Dob et al., Pharm Biol., 2006; 44, 60).

Uses

Lavender oil is principally used in the toiletry and perfumery industries and occasionally in ointments, etc., to mask disagreeable odours. It is employed pharmaceutically in the antiarthropod preparation Gamma Benzene Hexachloride Application. Lavender flowers are included in the BHP and are indicated for the treatment of flatulent dyspepsia and topically, as the oil, for rheumatic pain. The oil is extensively used in aromatherapy (q.v.).

History

Caraway fruits were known to the Arabian physicians and probably came into use in Europe in the thirteenth century.

Macroscopical characters

The commercial drug (Fig. 22.5) usually consists of mericarps separated from the pedicels. The fruits are slightly curved, brown and glabrous, about 4–7 mm long, 1–2.3 mm wide and tapered at both ends; they are crowned with a stylopod often with style and stigma attached. Each mericarp shows five almost equal sides, five narrow primary ridges, and, when cut transversely, four dorsal and two commissural vittae. They have a characteristic aromatic odour and taste.

Fig. 22.5 Caraway. A, mericarps showing attachments to carpophore; A₁, mericarp sectioned longitudinally to show position of embryo; A₂, mericarp side view (×8). B, transverse section of mericarp (×50); C, portion of vitta isolated by alkali maceration (×25); D, sclereids of mesocarp; E, endosperm cells with micro-rosette crystals of calcium oxalate; F, endocarp layer in surface view (all ×200). c.m, commissural meristeles; em, embryo; en, endosperm; end, endocarp; mc, mesocarp; r, three of five primary ridges; ra, position of raphe; s, stylopod; s.c, secretory canal; t, testa; v, vitta; v.b, vascular bundle with associated finely pitted sclerenchyma.

Microscopical characters

A transverse section of a caraway mericarp (Fig. 22.5) shows five primary ridges, in each of which is a vascular strand with associated pitted sclerenchyma and having a single secretory canal at the outer margin of each. The six vittae which appear somewhat flattened and elliptical in transverse section may attain a width of 350 μm; they extend from the base of the fruit to the base of the stylopod. They are lined with small, dark reddish-brown cells and contain a pale yellow or colourless oleoresin (Fig. 22.5B, C). The raphe lies on the inner side of the endosperm, which is non-grooved. Occupying the majority of the transverse section is the endosperm, with thickened cellulose walls (having also deposits of a β-(1,4)-mannan as a reserve polysaccharide) and containing fixed oil and aleurone grains having one or two microrosettes of calcium oxalate. The embryo, which is situated near the apex of the mericarp, will only be seen in sections passing through that region.

More detailed examination shows that the outer epidermis of the pericarp is glabrous (cf. aniseed) and has a striated cuticle (cf. fennel). The mesocarp consists of more or less collapsed parenchyma and lacks the reticulated cells of fennel. The endodermis (or inner epidermis of the pericarp) (Fig. 22.5F) consists of a single layer of elongated cells, arranged more or less parallel to one another and not showing the ‘parquetry’ arrangement of coriander.

Constituents

Caraway contains 3–7% of volatile oils (BP not less than 3.0%), 8–20% of fixed oil, proteins, calcium oxalate, colouring matter and resin.

Uses

Large quantities of caraway fruits are used for culinary purposes. The fruits and oil are used in medicine for flavouring and as carminatives. The carminative and antispasmodic properties have been experimentally verified.

Macroscopical characters

The drug usually consists of separate, broadly oval mericarps, about 4 mm long and 2–3 mm broad. The fruits are very much compressed dorsally, the two central ridges being prolonged into membranous wings, while the dorsal ones are inconspicuous. The fruits have an aromatic odour and taste similar to those of caraway.

Microscopical characters

Each mericarp has four vittae on the dorsal surface and two on the commissural. The outer epidermis has a striated cuticle (distinction from fennel), and the mesocarp contains lignified, reticulate parenchyma (distinction from caraway). The endosperm is much flattened but otherwise resembles that of caraway.

Constituents

The volatile oil (Dill Oil BP/EP) resembles oil of caraway in containing carvone and limonene. The European fruits yield about 3–4% of volatile oil, which should contain from 43 to 63% of carvone; the carvone content is determined by reaction with hydroxylamine hydrochloride (oxime formation) and titration of the liberated acid. Other constituents reported for the oil include trans– and cis-dihydrocarvone, trans– and cis-carveol, limonene, D– and L-dihydrocarveol, α- and γ-terpinene, α-phellandrene and others. Chemical types based on the proportion of carvone present, and the presence or absence of dillapiole and myristicin have been distinguished.

Monoterpene glycosides have been isolated from the water-soluble fraction of the fruits.

For further details on constituents, see T. Ishikawa et al., Chem. Pharm. Bull., 2002, 50, 501; M. Kosar et al., Pharm. Biol., 2005, 41, 491.

Uses

Like caraway, dill is used as a carminative and flavour; it is much used in infant’s gripe water.

Allied drug

Indian Dill, derived from a variety of A. graveolens consists of whole cremocarps which bear pedicels and are narrower and less compressed than the European drug. Indian dill oil contains dillapiole and less carvone.

Macroscopical characters

The drug (Fig. 22.6A) usually consists of the whole cremocarps, which, when ripe, are about 2.3–4.3 mm diameter and straw-yellow. Each consists of two hemispherical mericarps united by their margins. Considerable variation exists in coriander. The Indian variety is oval, but the more widely distributed spherical varieties vary in size from the Ukrainian 2.3–3.7 mm to the Moroccan 4.0–4.3 mm. The apex bears two divergent styles. The 10 primary ridges are wavy and inconspicuous; alternating with these are eight more prominent, straight, secondary ridges. The fruits have an aromatic odour and a spicy taste. They are somewhat liable to insect attacks.

Fig. 22.6 Coriander. A, Whole fruit (×8); B, transverse section of fruit (×16); C, fragment of epicarp in surface view with stoma and small prismatic crystals of calcium oxalate; D, endosperm cells with microrosette crystals of calcium oxalate; E, layers of sclerenchyma from the mesocarp; F, lignified parenchyma of the mesocarp and underlying endodermis showing ‘parquetry’ arrangement (all ×200). a, line of attachment of mericarps; b, sepal; c, carpophore, c.s, commissural surfaces; c.v, commissural vitta; en, endosperm; end, endodermis; par, lignified parenchyma of mesocarp; p.r, primary ridge; ra, raphe; r.v, remains of dorsal vittae; s, stylopod; scl, sclerenchyma; s.r, secondary ridge; t, testa.

Microscopical characters

A transverse section of a fully ripe fruit shows only two mature vittae in each mericarp, both on the commissural surface (Fig. 22.6B). The numerous vittae present in the immature fruit on the dorsal surface of each mericarp gradually join and are eventually compressed into slits. The outer part of the pericarp, which possesses stomata and prisms of calcium oxalate, is more or less completely thrown off. Within the vittae-bearing region of the mesocarp a thick layer of sclerenchyma is formed, which consists of pitted, fusiform cells. These sclerenchymatous fibres tend in the outer layers to be longitudinally directed and in the inner layers to be tangentially directed. In the region of the primary ridges more of the fibres are longitudinally directed; in the secondary ridges nearly all are tangentially directed. Traversing the band of sclerenchyma and corresponding in position to the primary ridges are small vascular strands composed of a small group of spiral vessels. The mesocarp within the sclerenchymatous band is composed of irregular polygonal cells with lignified walls. The inner epidermis of the pericarp is composed of ‘parquetry’ cells, which in the powder are often seen united to the cells of the inner mesocarp. The testa is composed of brown flattened cells. The endosperm is curved and consists of parenchymatous cells containing fixed oil and aleurone grains. The latter contain rosettes of calcium oxalate 3–10 mm diameter (see Fig. 22.6 C–F).

Constituents

Coriander fruits contain up to 1.8% of volatile oil according to origin (BP/EP standard not less than 0.2%). The distilled oil (Coriander Oil BP/EP) contains 65–70% of (+)-linalool (coriandrol), depending on the source, and smaller amounts of α-pinene, γ-terpinene, limonene and p-cymene together with various non-linalool alcohols and esters. Some 40 constituents have been identified. The BP/EP uses GC for the evaluation of the oil with linalool and geraniol as internal standards; there is also a test for chiral purity ((R)-linalool, maximum 14%). Other constituents isolated from the fruits include flavonoids, coumarins, isocoumarins, phthalides and phenolic acids. T. Ishikawa et al. (Chem. Pharm. Bull., 2003, 51, 32) obtained 33 compounds from the water-soluble fraction of a methanolic extract of the fruits; new constituents included monoterpenoids, monoterpenoid glycosides, monoterpenoid glucoside sulphates and aromatic compound glycosides. The high content of fats (16–28%) and protein (11–17%) in the fruits make distillation residues suitable for animal feed. The fruits yield 5–7% of ash.

The unripe plant has an unpleasant, mousy odour, which is also present in oil distilled from unripe fruits (mainly aldehydes such as n-decanal contained in peripheral vittae). Marked changes occur in volatile oil composition during ontogenesis; the peripheral vittae flatten and lose their oil, all of which is then produced by the commissural vittae. During ordinary storage of the fruits, the oil composition undergoes considerable alteration.

Uses

Very large quantities of the spice are produced in many countries for domestic purposes, such as for use in curries. In the former USSR linalool is isolated from the oil as starting material for other derivatives. Pharmaceutically coriander and its oils are used as a flavouring agent and carminative.

Macroscopical characters

The drug (Fig. 22.7A) consists of greyish-brown, pear-shaped, somewhat compressed cremocarps, which are usually attached to pedicels 2–12 mm in length. The cremocarps are 3–6 mm long and 2–3 mm broad. The Spanish (Alicante) and Italian are distinguished by their large size and light colour, while the German and ‘Russian’ are smaller, more ovoid and darker. Each mericarp has five somewhat wavy ridges and is slightly pubescent on the dorsal surface. They have an aromatic odour and a sweet, aromatic taste.

Fig. 22.7 Aniseed. A, Side view of cremocarp showing line of attachment to the two mericarps (×8); B, transverse section of mericarp (×25); C, covering trichomes of epicarp (×200); D, branched and unbranched vittae isolated by alkali maceration (×25). a, Line of attachment of mericarps; c, carpophore; c.s, commissural surfaces; c.v, commissural vitta; en, endosperm; e.t, epicarp bearing trichomes; m, meristele; mc, mesocarp; r, three of five primary ridges of one mericarp; ra, raphe; s, stylopod; v, vittae.

Microscopical characters

Microscopical examination shows that the epidermis bears numerous papillae and unicellular hairs. On the dorsal surface of each mericarp are from 15 to 45 branched vittae. A small amount of vascular tissue and reticulated parenchyma is present. The endosperm is slightly concave on the commissural surface and contains protein and fixed oil (see Fig. 22.7B–D).

Constituents

Anise fruits yield 2–3% of volatile oil (BP/EP2.0%), which is almost identical with that obtained from star-anise fruits. A number of water-soluble constituents have been isolated from the fruits including various glucosides, see E. Fujimatu et al., Phytochemistry, 2003, 63 (5).

Aniseed Oil

BP/EP tests and standards for the oil include: TLC identification of anethole and anisaldehyde; maximum limit of 0.1% for fenchone and 0.01% for foeniculin; trans-anethole 87–94%; estragole 0.5–5.0%, anisaldehyde 1.0–1.4% and linalol <1.5% determined by gas chromatography.

Star anise oil

The essential oil should contain 87–94% trans-anethole, 0.5–5.0% of estragole and smaller amounts of anisaldehyde (0.1–0.5%) and foeniculin (0.1–3.0%); other minor components include chavicol methyl ether (an isomeride of anethole), p-methoxyphenylacetone, safrole and other minor components. The oil is, for all ordinary purposes, indistinguishable from that of P. anisum but differences in the gas chromatographic profiles can be seen. The oil is liable to atmospheric oxidation and both anisic aldehyde and anisic acid are normally present. This change is said to diminish the tendency of the oil to solidify, which it normally does on cooling to about 15°C. In the past, the oil was imported in lead containers and some pharmacopoeias give a limit test for heavy metals.

Both aniseed oil and star anise oil are used as flavouring agents and as carminatives. Anethole (a colourless crystalline solid m.p. 21°C) may be prepared from the oil or manufactured synthetically.