- Top of page
- Results and discussion
- Supporting Information
Melaleuca alternifolia (Myrtaceae), native to Australia, is regarded as the most popular among the 150 species of tea tree. Traditionally, Australians inhale the vapours released from the crushed leaves to treat various ailments such as coughs and colds. An infusion is used to treat sore throats or skin ailments (Shemesh and Mayo, 1991). The oil is included as an ingredient in many consumer products, such as soaps, deodorants, shampoos, ointments, cosmetics and herbal remedies, and as an active ingredient in many topical formulations to treat skin infections (Carson et al., 2006). In Australia, Europe, North America and South Africa, the oil is widely available over-the-counter and marketed as an immune system booster and a remedy for coughs, colds and a sore throat (Carr, 1998; RIRDC, 2006, 2007; Ratel Farming, 2011).
Tea tree oil (TTO) has been shown to possess a wide range of biological activities. The oil showed potent anti-fungal and anti-bacterial activities against various pathogens, including Candida albicans, Escherichia coli, Klebsiella pneumoniae, Prevotella intermedia, Proteus vulgaris, Staphylococcus aureus and Trichophyton equinum. Data suggest that the antibacterial and antifungal activities of TTO are mainly attributed to the content of terpinen-4-ol and α-terpineol (Carson and Riley, 1995; Carson et al., 1995; Christoph et al., 2000; Banes-Marshall et al., 2001; Cox et al., 2001; Hammer et al., 2003). Other biological properties of TTO include anti-inflammatory (Golab et al., 2005), antiprotozoal (Mikus et al., 2000) and antitumour activities (Calcabrini et al., 2004).
Tea tree oil (TTO) is regulated by an international standard (Carson and Riley, 2001). Although the standard does not emphasise the species name ‘Melaceuca’ from which the oil originates, there is a set of chemical criteria for the desired chemotype. Monocyclic terpenes (γ-terpinene, α-terpinene, limonene, terpinolene), monocyclic terpene alcohols (terpinen-4-ol, α-terpineol) and the terpene ether/oxide (1,8-cineole) are the major constituents. The chemical composition of the oil is exclusively correlated to the quality of the raw plant material used. Commercially, the terpinen-4-ol chemotype is generally preferred (Carson et al., 2006) and good quality TTO should have a minimum terpinen-4-ol content of 30% and a maximum 1,8-cineole content of 15% (Nardoni et al., 2010). It was advised by Raman et al. (1995) that TTO with lower levels of 1,8-cineole be produced to prevent skin irritation and Carson and Riley (1995) suggested that the percentage of 1,8-cineole should be low as it can negatively influence the therapeutic potential of the oil.
Gas chromatography coupled to mass spectrometry (GC–MS) is the accepted analytical tool used for quality control analysis of essential oils. However, because this analytical technique is expensive and requires skilled personnel, alternative fast and efficient methods are therefore desirable for the rapid assessment of TTO. Vibrational spectroscopy has been proposed as an alternative rapid quality control method. Spectroscopy methods such as mid- infrared (MIR) and near-infrared (NIR) in combination with chemometric data analysis have proven to be suitable methods for the quality assessment of buchu and geranium oil amongst others (Sandasi et al., 2010, 2011) and also to detect adulteration of African essential oils (Juliani et al., 2006; Tan et al., 2012). As the quality and chemical composition of the commercial essential oils of Melaleuca alternifolia can differ from one producer to another, this study proposes a rapid, efficient, non-destructive method that requires little or no sample preparation for the qualitative and quantitative assessment of TTO using MIR and NIR spectroscopy. Chemometric tools were used to develop calibration models to enable the rapid prediction of major TTO compounds in commercial essential oil brands for routine analysis.