Food fraud in oregano: Pesticide residues as adulteration markers

Importance of the topic

The global spice market is expanding rapidly and oregano is among the most widely used culinary herbs. Its complex supply chains and high commercial value make it prone to adulteration by lower-cost plant materials such as olive, myrtle or sumac leaves. These adulterants can carry distinct pesticide residues not registered for oregano, compromising food safety and authenticity. Accurate screening methods are therefore critical to detect fraud, protect consumers, and ensure regulatory compliance.

Goals and overview of the study

This work aimed to evaluate whether pesticide residue profiling can serve as a marker of oregano adulteration. A total of 76 dried oregano samples (42 genuine, 34 previously flagged as adulterated by FTIR and high-resolution LC-MS) from across Europe were analyzed. Over 400 target pesticides were screened to identify characteristic patterns distinguishing authentic and fraudulent products.

Used methodology and instrumentation

• Sample preparation: Modified QuEChERS extraction with 1% formic acid aqueous hydration, acetonitrile partitioning, salting-out, low-temperature acetonitrile cleanup, and PSA dispersive solid-phase extraction.
• GC-MS/MS: Agilent 7890A gas chromatograph coupled to 7000B triple quadrupole mass spectrometer in EI mode, HP-5MS UI capillary columns, PTV solvent-vent injection, and MRM transitions.
• LC-MS/MS: Waters Acquity UPLC system with HSS T3 column (100×2.1 mm, 1.8 µm), Xevo TQ-S triple quadrupole in ESI+ mode, gradient elution with 5 mM ammonium formate/0.1% formic acid in water and methanol, and MRM acquisition.
• Method validation: Per SANTE/11813/2017 guidelines, recoveries ranged 52–120% (RSD <21%), and LOQs ≤50 µg/kg for most analytes.

Main results and discussion

• A total of 55 different pesticides were detected across all samples; chlorpyrifos, diphenylamine and acetamiprid were most frequent.
• Adulterated oregano showed higher average residue counts (7.5 vs. 5.1 pesticides per sample) and elevated contamination levels.
• Pyrethroid insecticides (cyfluthrin, permethrin, cyhalothrin) and pyriproxyfen appeared predominantly or exclusively in adulterated samples; some residues exceeded EU MRLs.
• Orthogonal partial least squares–discriminant analysis (OPLS-DA) achieved clear separation between genuine and adulterated samples (R2Y=0.71; Q2=0.54) and identified cyfluthrin, pyriproxyfen and cyhalothrin as key adulteration markers.

Benefits and practical applications of the method

• Comprehensive GC-MS/MS and LC-MS/MS screening enables detection of hundreds of pesticides in complex herb matrices.
• Residue profiling provides rapid preliminary screening for suspicious oregano batches prior to confirmatory authenticity tests.
• Implementation in routine control enhances consumer protection, traceability and regulatory enforcement against food fraud.

Future trends and possibilities of application

• Expand residue databases to other herbs and spices to establish universal adulteration markers.
• Integrate non-targeted metabolomics and spectroscopic techniques as a two-tier authenticity protocol.
• Deploy ambient ionization or high-resolution mass spectrometry for in-situ screening at processing and retail points.

Conclusion

Pesticide residue patterns are a powerful tool to distinguish genuine oregano from adulterated products. Markers such as cyfluthrin, cyhalothrin and pyriproxyfen enable reliable screening of fraudulent samples. The validated multiresidue MS methods support enhanced quality control, fraud prevention and consumer safety in the herb and spice market.

References

• Black C. et al. A comprehensive FTIR and LC-HRMS approach to oregano authenticity. Food Chem. 210:551–557 (2016).
• Cajka T. et al. Streamlined QuEChERS and GC-MS/MS analysis of pesticides in tea. Anal. Chim. Acta 743:51–60 (2012).
• EU Regulation (EC) No 396/2005 on pesticide maximum residue levels. Official Journal of the European Union (2005).
• European Spice Association Quality Minima Document (2015).
• SANTE/11813/2017 Guidance on analytical QC and method validation for pesticide residues.