Novel analysis unveils complex composition of tea tree oil

Characterisation of key compounds offers way to verify claims of authenticity, underpinning unique value proposition.

An investigation characterising the components of essential oil distilled from Australian tea tree will underpin the integrity and value proposition of the local product, and pave the way for access to premium international markets.

The ability to verify the components of Australian tea tree oil is considered pivotal to allaying concerns over products being adulterated with synthetic compounds, or other essential oil by-products, and will ensure the product’s authenticity can be better verified in the future.

Stephen Taylor, Research Manager with the AgriFutures Tea Tree Oil Program, said preserving the reputation of authentic Australian tea tree oil was imperative given the industry’s large export focus. With this in mind, the Program invested in work to understand the chemical profile of the product, against which future adulteration testing results can be compared.

“Tea tree oil is an essential oil distilled from Melaleuca alternifolia tree leaves. It’s been used by Indigenous people for thousands of years and has become popular more broadly because of its various therapeutic properties,” Stephen said.

“The Australian industry has a farm-gate value of $40 million and about 90% of production is exported — the major markets are in North America and Europe.

“But because of its popularity, we’re seeing evidence of commercial products being adulterated, which cannot be detected with current authentication standards.”

Pure tea tree oil may be diluted with less-expensive essential oils, such as eucalyptus oil, or synthetic chemicals or fragrance compounds that imitate the aroma and properties of tea tree oil but are less effective. The adulterated products can cause skin irritation, allergic reaction or other adverse effects, and lack the therapeutic benefits associated with the pure product.

The work characterising the chemical profile of Australian tea tree oil was completed by a research team led by Dr Kathryn Linge, Senior Scientist at ChemCentre in Western Australia.

Kathryn’s team used a novel enantiomeric gas chromatography–mass spectrometry technique to better understand the unique composition of Australian tea tree oil and determine how adulterated products might be identified.

During the project, the research team utilised a library of 100 authentic and known provenance M. alternifolia oil samples to verify the technique. The method was also applied to a selection of commercially sourced tea tree oil samples.

“A key aim of our project was to develop a straightforward diagnostic tool to identify, verify and reduce the incidence of adulterated tea tree oil in the market, and offer Australian producers a way to guarantee the authenticity of locally produced tea tree oil,” Kathryn said.

“This research project provides vital information to the Australian tea tree oil industry, helping to reduce the occurrence of adulterated tea tree oil products in the market, while also supporting the development of new, improved authentication services for growers, manufacturers and resellers.

“Ultimately, the data produced from this research project will provide assurance to consumers the tea tree oil they purchase is as claimed on the label, and therefore safe and effective to use, improving consumer confidence.”

When oils ain’t oils

Using the novel analysis technique, the team’s research confirmed 88 of the 100 samples of known provenance were consistent with the internationally recognised standard, ISO 4730. However, 12 of the samples were found to have oxidised.

This internationally recognised standard specifies the test methods and values for relative density, refractive index, optical rotation and miscibility in ethanol, as well as minimum and maximum values for 15 chemical constituents, and previously published enantiomeric ratios, with p-cymene identified as the major marker of oxidation.

According to Kathryn, detecting adulteration in tea tree oil is difficult because many of the 15 constituents outlined in ISO 4730 are also found in other essential oils.

“Focusing only on the 15 constituents outlined by ISO 4730 ignores the potential for minor constituents to be used to confirm tee tree oil purity,” she said. This is where the novel analysis technique has come into its own.

In addition to the 15 constituents outlined by ISO 4730, using the novel technique the team found 95 additional components that could potentially be used to identify or authenticate Australian tea tree oil and reveal adulterated samples.

“We found the additional 95 components in all 88 authentic, non-oxidised tea tree oil samples we tested,” Kathryn said.

“Yet our analysis of the additional minor components identified two commercial samples that differed from authentic tea tree oil, confirming that the ISO 4730 standard does not always identify adulterated tea tree oil.”

From this the team concluded that analysing the enantiomeric ratio of key terpenes found in tea tree oil — terpinen-4-ol, α-terpineol and limonene — offers the clearest evidence of adulteration.

The discovery that 12 of the 100 authentic samples supplied for the project were oxidised has delivered an unexpected, additional outcome for the research team.

“We now have a comparison of oxidised and unoxidised tea tree oil, and we found more components in the oxidised tea tee oil — an unexpected yet useful outcome,” Kathryn said.

“We found the clearest indicator of oxidation to be a higher p-cymene concentration than that found in fresh tea tree oil.”

These findings could be helpful for future investigations into the impacts on skin of oxidised tea tree oil products.

“Our research indicates that the novel enantiomeric gas chromatography–mass spectrometry technique offers significant potential to rapidly authenticate tea tree oil by analysing the enantiomeric ratios and quantifying the percent contribution of p-cymene in the total ion chromatogram,” Kathryn said.

Learn more about the AgriFutures Tea Tree Oil Program
Read journal article for more detailed information