Evaluation of a mathematical approach to detect fraudulent substitution of Darjeeling tea with other types of tea using the elemental profiles obtained by Energy Dispersive X-ray Fluorescence

The willingness of consumers to pay higher prices for high quality specialties, such as Darjeeling tea, goes hand in hand with an increase of fraudulent practices in which Darjeeling tea is substituted totally or partially by cheaper teas. Currently, to evaluate the percentage of substitution that a method can detect, Darjeeling tea is mixed in different proportions with non-Darjeeling teas, and after homogenisation the mixture is analysed. This time-consuming approach implies the use of valuable amounts of sample and, therefore an alternative approach is needed. Here a method is described to calculate the minimum detectable substitution percentage of Darjeeling tea by other teas without needing to prepare real mixtures. The approach is based on the use of virtual mixtures made with the results obtained for commercially available Darjeeling and non-Darjeeling teas. The method used for authentication purposes, made use of the elemental profiles of tea obtained by Energy Dispersive X-ray Fluorescence, combined with chemometrics and modelling by Partial Least Square-Discriminant Analysis. The false positives percentage at different substitution levels, was evaluated and compared with the results obtained with real mixtures of Darjeeling and non-Darjeeling teas. Comparable results were obtained with both approaches. Twenty percent was the lowest substitution level that could be detected with an acceptable sensitivity (94 %) and specificity (86 %). A fast, easy to implement approach has been developed and validated, to calculate the minimum substitution percentage that can be detected by an authentication analytical method, without the need to carry out additional laboratory experiments. • EDXRF is a suitable technique for the elemental characterisation of Darjeeling tea. • Darjeeling tea can be authenticated using EDXRF combined with modelling tools. • PLS-DA is a suitable modelling tool to authenticate Darjeeling tea. • Virtual samples can be used to validate the method to detect partial substitution. • Use of virtual samples speeds up the validation process and reduces its cost.