Analysis of 78 Commercial Oregano Spice Products Finds 24% Adulteration Rate

 

Reviewed: Black C, Haughey SA, Chevallier OP, Galvin-King P, Elliott CT. A comprehensive strategy to detect the fraudulent adulteration of herbs: The oregano approach. Food Chem. 2016;210:551-557.

 

Keywords: Origanum vulgare ssp. hirtum, Origanum onites, oregano, cistus, myrtle, olive, sumac, adulteration, FT-IR, HPLC-MS

 

Oregano (Origanum vulgare ssp. hirtum, Lamiaceae) is widely used as a spice in food, such as Italian and Greek cuisine, but is also a popular ingredient in dietary supplements, mainly in the natural channel where it ranked at #12 in the United States with regard to dollar sales in 2015. What oregano consists of has been a matter of debate – the American Herbal Products Association’s Herbs of Commerce, 2nd edition,¹ allows only O. vulgare ssp. hirtum to be labeled as oregano, while the European Pharmacopoeia and the European Spice Association allow an additional species, O. onites, to be marketed as oregano. Materials containing Lippia graveolens (Verbenaceae) have to be labeled as Mexican oregano. Extraneous materials of up to 2% are considered acceptable in Europe, while the American Spice Trade Association has limited allowable foreign matter to 1%.

 

The aim of this study was to develop and validate a Fourier-transform infrared spectroscopy (FT-IR) assay and a high-performance liquid chromatography-high resolution mass spectrometry (HPLC-HRMS) method for the detection of oregano adulteration. Samples included 53 commercial samples labeled to contain oregano purchased at various retailers in the United Kingdom and Ireland, and 25 samples originating from other European Union (EU) and non-EU countries purchased via the Internet. Also included in the analysis were reference materials of oregano and leaf material from the potential adulterating species – olive (Olea europaea, Oleaceae), myrtle (Myrtus communis, Myrtaceae), sumac (Rhus spp., possibly R. coriaria, Anacardiaceae), cistus (Cistus spp., possibly C. incanus, Cistaceae), and European hazel (Corylus avellana, Betulaceae). Not included were known adulterants of the family Lamiaceae, e.g., winter savory (Satureja montana) or conehead thyme (Thymus capitatus), which is also known as “Spanish oregano.”² For FT-IR analysis, powdered samples were directly analyzed using an attenuated total reflectance (ATR, a device allowing direct analysis without further sample preparation) sampling device, while for HPLC-HRMS, solutions of 50 mg/2 mL solvent (methanol-water 1:1) were sonicated, filtered, dried, re-suspended in 1.5 mL water and filtered again prior to injection. Results from both methods were statistically evaluated by principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA), and were in complete agreement. Overall, 19 (24%) of the 78 commercial samples were adulterated, with levels of adulteration generally between 30% and 70%, although two samples did not contain any oregano at all. The most commonly found adulterant was olive leaf, followed by myrtle leaf. The HPLC-MS and subsequent PCA allowed the distinction between the two oregano species permitted in Europe, Origanum vulgare ssp. hirtum and O. onites, which was not the case for the FT-IR analysis or the HPLC-MS followed by OPLS-DA evaluation.

 

Comment: This paper highlights the difficulties of oregano authentication in the spice trade. In addition to various oregano species sold in local markets and internationally, various chemotypes of Origanum vulgare ssp. hirtum have been reported.³ Both fingerprinting methods presented by the authors have been able to readily distinguish oregano from the targeted adulterants (olive, cistus, sumac, myrtle, and hazel leaves) and can be considered suitable to detect adulteration with these materials.

 

While the presented methods seem to be robust (although the re-suspension of the 50% aqueous methanol extract in water likely leads to a loss of certain components), it is not clear why the authors did not specify how many authentic samples were used to build the statistical models. Other shortcomings include the lack of details on the validation – apparently the term validation was used to establish the predictability of the statistical model rather than a validation of the analytical method – and the omission of scientific names and the means of authentication of the potential adulterant reference materials used in the experiments.

 

References

1.     McGuffin M, Kartesz JT, Leung AY, Tucker AO. American Herbal Products Association’s Herbs of Commerce. 2nd ed. Silver Spring, MD: American Herbal Products Association; 2000.

2.     Marieschi M, Torelli A, Poli F, Sacchetti G, Bruni R. RAPD-based method for the quality control of Mediterranean oregano and its contribution to pharmacognostic techniques. J Agric Food Chem. 2009;57(5):1835-1840.

3.     Stahl-Biskup E, Loew D. Origani herba. In: Blaschek W, ed. Wichtl – Teedrogen und Phytopharmaka. 6th ed. Stuttgart, Germany: Wissenschaftliche Verlagsgesellschaft mbH; 2016:463-464.