Joseph Jbrophy

School of Chemistry, University of New South Wales, Sydney, Australia

INTRODUCTION

The preceding chapters have discussed the commercial species, M. cajuputi, M. quinquenervia and M. alternifolia (including the closely related M. linariifolia). Now we present data on other species which have potential for commercial production.

According to the latest research there are upwards of 230 species within the genus Melaleuca. Over the last 30 years these have been documented in works by Blake (1968); Carrick and Chorney (1979); Byrnes (1984, 1985, 1986); Barlow (1986, 1988); Barlow and Cowley (1988); Cowley et al. (1990); Craven and Barlow (1997), as well as more popular works by Wrigley and Fagg (1993); and Holliday (1989). Of these approximately 230 species (Craven 1998), only 3 (M. alternifolia., M. cajuputi and M. quinquenervia) are at present commonly used for the commercial production of essential oil.

With so many species to choose from it is obvious that during this present century a significant amount of research has been carried out examining the essential oils of the various Melaleuca species. This has been carried out both to detail the contents of the various essential oils and also to search for potentially commercial oils.

Research on the essential oils of members of the genus Melaleuca at the University of New South Wales has been carried out to both supplement and extend our knowledge of the chemistry of this genus. We have had the advantage of the availability of specimens collected for the current revision of the genus by Craven. To date approximately 180 species have been either examined for the first time or re-examined and the contents of their essential oils documented. During the course of this work some species have shown essential oils that may give promise of commercial use. These species, and the oils obtained from them, are discussed in more detail in this chapter.

The oils were obtained by steam distillation with cohobation in a modified Dean and Stark apparatus. A full description is given in Brophy and Doran (1996). Analysis of the oils was by gas chromatography (GC) and combined gas chromatography-mass spectrometry (GC/MS). Compound identification was, for the most part, by matching of spectra and retention time of various peaks against authentic pure materials or oils of known composition. Usual GC conditions are given in Brophy and Doran (1996).

In choosing species to include in this chapter the three main criteria considered were that the oil, qualitatively, should be interesting and that the oil quantitatively be useful. This latter criterion meant that the oil yield should, all other things being equal, generally be greater than 1% based on fresh foliage. One further restriction which has been placed on the two above criteria is that the species should be a taxonomically defined species. This last

restriction has at present excluded close to 100 collections from being considered for this chapter.

As might be expected from so much study of morphological variation, over time there have been different interpretations placed on the data and different names and associations proposed. The species referred to in this chapter follow Craven, though reference in the individual sections is made to previous treatments.

This chapter highlights 15 species of Melaleuca and 1 species from the related genus Asterornyrtus. The species (M. acadoides, M. alsophila, M. bracteata, M. citrolens, M. dissitiflora, M. ericifolia, M. leucadendra, M. linophylla, M. quinquenervia, M. squamophloia, M. stenostachya, M. stipitata, M. trichostachya, M. uncinata, M. viridiflora and A. symphyocarpa) all met the selection criteria and produce either aromatic oils or cajuput type (high 1,8-cineole) oils. Asteromyrtus symphyocarpa is included with the Melaleuca oils because it was once considered a member of the genus Melaleuca before being placed in the genus Asteromyrtus (Craven 1988) and it produces an essential oil which is a potential substitute for cajuput oil (Brophy et al. 1994).

Of the 15 species of Melaleuca included in this chapter, 2 are newly named (M. stipitata and M. squamophloia) and have had only one report of their chemistry (Brophy and Doran 1996). A further species (M.linophylla) has its oil reported on for the first time.

During the course of our recent work on M. leucadendra a much larger number of specimens have been examined, encompassing the whole of its range across northern Australia. This has shown that there are two sets of chemical forms of this species. One chemical form is terpenoid and this form occurs in the western half of its range, extending from Western Australia to approximately mid Northern Territory. In the eastern half of its range, from mid Northern Territory to the east coast of Australia, the oil is essentially entirely aromatic. At the sampling location where both forms were found (Kapalga) there appeared to be no interbreeding, with the oils obtained being either terpenoid or aromatic but never a mixture of the two.

There must be considerable doubt about which species is being considered when M. viridiflora is mentioned in publications prior to 1968. There was confusion between what is now called M. viridiflora and what is now called M. quinquenervia, two species that overlap considerably in range and are of similar appearance. Without access to botanical voucher specimens for these oils it is not possible to say just which species is producing the oil, though it is thought that the species producing oils rich in nerolidol and/or linalool, especially coming from New South Wales, is M. quinquenervia.

Basically, M. alsophila, M. dissitiflora, M. stipitata and one chemotype of M. uncinata produce oils rich in terpinen-4-ol and low in 1,8-cineole. Some of these are lemon scented. M. bracteata, M. squamophloia, one chemotype of M. leucadendra, and one chemotype of M. viridiflora produce oils which are aromatic (in either the chemical sense or perfumery sense) while M. trichostachya, M. acacioides, and some chemotypes of M. citrolens produce oils which might have perfumery potential. M. linophylla, M. stenostachya, and A. symphyocarpa all produce oils with similar compositions to cajuput oils.

M. laterifolia subsp. laterifolia, M. pustulata and M. radula all produce oils in yields of 1-2% in which 1,8-cineole accounts for 70-87% of the oil. It should also be mentioned that both M. alternifolia and M. linariifolia have chemotypes that are characterised by oils rich in 1,8-cineole. These are usually shunned in favour of the terpinen-4-ol rich oils. M.

nanophylla gives an oil in 1.7% yield in which the major compounds are the twofi-triketones flavesone (24-42%) and leptospermone (10-25%) accompanied by a-pinene (24-40%) (Brophy 1998). There is also a record of a chemical variety of M. dealbata producing an oil in 1.5% yield in which the major compound was leptospermone (70%) (Lassak and Southwell 1977). Oils rich in polyketones such as leptospermone are being used in body care products because of their powerful anti-microbial properties (Joulain 1995).

SPECIES DIGESTS

Melaleuca acacioides

M. acacioides occurs as a shrub or small tree, 4-10m tall, and may develop a multistemmed habit when open-grown. It is found in coastal and sub-coastal (usually saline and seasonally flooded) habitats in far northern Queensland, the north of the Northern Territory and islands of the Torres Strait in Australia and extends to southern Papua. It has potential for production of posts and small poles, fuelwood and windbreaks on difficult sites near the coast.

M. acacioides produced an essential oil, in 0.2-0.4% yield (based on fresh leaves), which was almost entirely composed of sesquiterpenes. The oil has a distinctive pleasant aroma which is associated with the sesquiterpene alcohol fraction. The main components were fi-selinene (21-30%) and a-selinene (53-54%). The next most abundant compounds were selen-11-en-4-ol (6-8%), d-cadinene (0.9-6%), fi-caryophyllene (1-2%), globulol (0.71%) as well as some unidentified oxygenated sesquiterpenes in the range 0.1-3%. Monoterpenes were very poorly represented. Table 1 gives a typical oil analysis.

Table 1 Compounds identified in the essential oil of Melaleuca acacioides

Compound

Compound

ti tr seHna-11 -cn^t-ol r u rj viridiflorol CisHTKO

C,SH,60

unknown, uiw 200

c,,H26o

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