Excitation-Emission Matrix Fluorescence Spectroscopy Coupled With Chemometric Methods For Authentication Of Dark Tea

In recent years,the phenomenon of counterfeit has increased with the rapid development of the dark tea market.At present,the authentication of dark tea mainly relies on sensory evaluation,supplemented by gas chromatography,liquid chromatography and other instrumental analysis.The former requires highly trained professionals and the results are affected by the subjective judgment of the reviewers,while the latter often requires complex sample preparation processes and long analysis times,which are difficult to meet the needs of large-scale sample analysis.Based on this background,considering that the excitationemission fluorescence matrix(EEM)spectroscopy technology can obtain rich chemical information of tea infusion in less than 30 seconds,the EEM of dark tea were coupled with chemometric methods to establish a fast and reliable authentication method for dark tea products,which can provide technical support for strengthening the supervision of the dark tea market.In the paper,a total of 320 tea samples from 23 kinds of dark tea products and 15 kinds of other domestic tea products were investigated.Four primary fluorophores were identified in the dark tea for the first time,including humic-like substance,heme-like substance,aromatic amino acids,and theabrownin through the parallel factor analysis,database comparison,and theabrownin-spiked tea infusion analysis.Among them,heme-like substance and theabrownin are associated with the plie fermentation and anaerobic fermentation process in the production of dark tea,and their fluorescence signals constitute the main feature of the dark tea spectrum that is different from other domestic tea spectrums.It is found that compared to dark brick tea,the content of heme-like substance in Fu brick tea was significantly higher,and the content of theabrownin was significantly lower by comparing the EEM of different brick types of dark tea.In this paper,the classification algrorithms for the brick types and product types of dark tea were developed based on the full fluorescent data,principal component scores and parallel factor scores.Linear discriminant analysis,k-nearest neighbor analysis,partial least squares discriminant analysis and N-way Partial least squares discriminant analysis(NPLS-DA)were compared,and the influence of removing the low excitation wavelength region and the spectral correction on the classification accuracy were studied.Finally,the NPLS-DA model based on full fluorescent data showed the highest correct classification rate,which is96.5% for the classification of brick types and 98.3% for the classification of dark tea products.