Monoterpene chiral ratios: Chemotype diversity and interspecific commonality in Melaleuca alternifolia and M. linariifolia

This study assessed the impact of chemotype variation upon the reliability of the chiral ratios for key monoterpenes as parameters for tea tree oil standardisation. The chiral ratio for terpinen-4-ol has recently been incorporated into the International Standard Organisation monograph as an additional criterion to aid in distinguishing plantation oil from adulterated oil. Melaleuca alternifolia (the main species used for commercial tea tree oil production) has at least 6 chemotypes, yet only the terpinen-4-ol type has previously been investigated for the enantiomeric ratios of key chiral constituents. If other chemotypes are deliberately or inadvertently blended, and chemotypes differ in chiral ratio, there is the potential for bulk oil to be out of specification. Using chiral column gas chromatography, this study characterized the enantiomeric ratios of the key chiral monoterpenes limonene, terpinen-4-ol and α-terpineol in the 6 most common chemotypes of M. alternifolia and 2 chemotypes of M. linariifolia. The study found that variation among individual trees within a chemotype was negligible, but that there was difference among the average chiral ratios of the 6 chemotypes of Melaleuca alternifolia and between the 2 chemotypes of M. linariifolia. Equivalent chemotypes in each species possessed equivalent chiral ratios for the key constituents tested, suggesting transpecific inheritance of biosynthetic origins in these closely related species. Significant differences in chiral ratios for key constituents confirmed that oil blended from more than one chemotype has the potential to change the chiral ratio from a chemotype average. In practice, however, inclusion of chiral ratio parameter in the International Standard is not additionally restrictive beyond the compositional definition of terpinen-4-ol type oil.