Volatile Oils from Eucalypts

Ken Stuckey

The following article is based on one which was published in the November 1985 issue of the newsletter of the Eucalyptus Study Group. Some minor editorial amendments have been made where necessary to bring details up to date.

Essential oils are the aromatic 'essences' found in various plant parts. Because they evaporate when exposed to the air at ordinary temperatures they are called volatile or essential oils. These oils possess characteristic odours, are colourless, insoluble in water and are separated from plant material by steam distillation or solvent extraction.

Essential oils from the leaves of eucalypts have attracted interest since the earliest days of settlement in Australia. One of the first articles of export from the newly established colony of New South Wales in 1788 was a quarter of a gallon of an essential oil steam-distilled from the leaves of a eucalypt (Eucalyptus piperita) growing on the shores of Port Jackson.

The therapeutic properties of the Australian flora so fascinated the botanist Baron Ferdinand von Mueller that he prompted a Victorian pharmacist colleague, Joseph Bosisto, to investigate the commercial production of eucalyptus oil. Bosisto responded and, in 1852, commenced operations at Dandenong, Victoria. This was the beginning of the Australian essential oil industry. Between 1854 and 1891 Bosisto displayed commercial eucalyptus oils at no less than seventeen exhibitions around the world. Through his pioneering efforts the eucalyptus industry spread to other states as the demand grew and its products became known in world trading centres. By 1900 the industry was firmly established, and for the next fifty years Australia remained the world's largest supplier of eucalyptus oil.

One problem confronting the industry from the outset was the variation in the oils marketed. At the turn of the century, new light was thrown on the problem by two early curators of the Museum of Applied Arts and Sciences, Sydney, in a series of investigations into the botanical classification of the eucalypts and their oils. Further progress was made by successive curators at the Museum with the discovery of chemical variation within species, a phenomenon which had been troublesome to distillers. The curators found that populations of E.dives at Braidwood yielded piperitone and phellandrene, whereas cineole-rich oils were obtained from populations of the same species at Tumbarumba.

"There are over 700 species and varieties of the eucalypt but only about 20 have been exploited commercially....."

There are over 700 species and varieties of the eucalypt but only about 20 have been exploited commercially and less than a dozen are presently of economic importance. The commercial eucalyptus oils are broadly divided into three classes; medicinal, industrial and perfumery oils, according to their specific uses.

• Medicinal oils -- the active therapeutic agent of medicinal oils is cineole, of which a minimum of 70% is required to meet commercial specifications. Medicinal oils are graded on their cineole content, and it is common practice to improve them by blending and purification. These oils are used for inhalants, embrocations, soaps, gargles, sprays, lozenges and dentrifices. Eucalypt species which contain commercial quantities of cineole are E.polybractea, E.radiata, E.dives, E.sideroxylon, E.leucoxylon, E.goniocalyx, E.viridis, E.smithii, E.cneorifolia, E.globulus, E.dumosa and E.oleosa.
• lndustrial oils -- these contain phellandrene and piperitone as the principal constituents. Phellandrene oils are used in the manufacture of inexpensive disinfectants, household and industrial liquid soaps, and germicidal preparations. Piperitone oils provide the base material for the manufacture of synthetic menthol and thymol. Species used to provide these oils are E.dives, E.elata and E.radiata.
• Perfumery oils -- two species of eucalypt have been exploited to supply oils for the perfumery industry: E.citriodora (now Corymbia citriodora) for citronellal and E.macarthurii for geranyl acetate and eudesmol. Extraction of these oils in Australia is no longer economically viable.