Abstract
The quality control of essential oils (EO) principally aims at revealing the presence of adulterations and at quantifying compounds that are limited by law by evaluating EO chemical compositions, usually in terms of the normalised relative abundance of selected markers, for comparison to reference values reported in pharmacopoeias and/or international norms. Common adulterations of EO consist of the addition of cheaper EO or synthetic materials. This adulteration can be detected by calculating the percent normalised areas of selected markers or the enantiomeric composition of chiral components. The dilution of the EO with vegetable oils is another type of adulteration. This adulteration is quite devious, as it modifies neither the qualitative composition of the resulting EO nor the marker’s normalised percentage abundance, which is no longer diagnostic, and an absolute quantitative analysis is required. This study aims at verifying the application of the two above approaches (i.e., normalised relative abundance and absolute quantitation) to detect EO adulterations, with examples involving selected commercial EO (lavender, bergamot and tea tree) adulterated with synthetic components, EO of different origin and lower economical values and heavy vegetable oils. The results show that absolute quantitation is necessary to highlight adulteration with heavy vegetable oils, providing that a reference quantitative profile is available.
Highlights
Essential oils (EO) are complex mixtures of volatile compounds that are characterised by important biological activities for the plant itself and for humans who have learned to exploit their properties over the centuries
The above genuine EO were spiked on purpose to build a model of adulterations: (1) bergamot and lavender EO were supplemented with synthetic racemic linalool and linalyl acetate (9% and 11%, respectively, for bergamot EO and 27% of both linalool and linalyl acetate for lavender EO), (2) Australian TTO was mixed with 50% Chinese tea tree and (3) all the investigated EO were mixed with different amounts of sunflower vegetable oil
Most of the EO samples analysed in routine quality controls comply with the reference data
Summary
Essential oils (EO) are complex mixtures of volatile compounds that are characterised by important biological activities for the plant itself and for humans who have learned to exploit their properties over the centuries. Product obtained from a natural raw material of plant origin, by steam distillation, by mechanical processes from the epicarp of citrus fruits, or by dry distillation, after separation of the aqueous phase—if any—by physical processes” [1]. The European Pharmacopoeia terms an EO as “an Odorous product, usually of complex composition, obtained from a botanically defined plant raw material by steam distillation, dry distillation, or a suitable mechanical process without heating. Essential oils are usually separated from the aqueous phase by a physical process that does not significantly affect their composition” [2] In both definitions, it is clear that only products obtained by steam/hydrodistillation can be named EO, while products obtained by different extraction procedures involving the use of solvents must be defined as extracts. EO are mainly characterised by the presence of terpenes/terpenoids and phenolic compounds (i.e., phenylpropanoids), that derive from the mevalonate/methyl erithrytol and shikimic acid pathways, respectively
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