| Colour plays an important role in tea testing. Lead chrome green illegally added in tea affects the tea quality and safety seriously. This paper is mainly focused on tea colour. Based on spectroscopy, the internal chemical formation mechanism of green tea color was studied and the effective detection method of lead chrome green in tea was developed. The main results were achieved as follows:(1) The color differences of different varieties and grades of green tea were studied, and the correlation analysis between lipid-soluble pigment contents and tea color was carried out. Finding that the colour changed along with the changes of tea grades, and certain correlations existed between tea colour and pigment contents, among which the influence of chlorophyll contents on the " green color " of tea was the most significant.(2) Detection methods of lipid-soluble pigment contents in tea powder and tea were respectively built based on near-infrared spectroscopy. In tea powder, the detection models for 9 pigments all obtained Rp2 above 0.935; Successive projections algorithm (SPA) selected 6-9 characteristic wavelengths for these pigments, which greatly simplified and optimized these detection models; In tea, SPA selected 6-10 characteristic wavelengths for these pigments and the combination of LS-SVM greatly improved the performances of these detection models, all Rp2 were improved to above 0.960. Compared with previous studies, both the prediction ability and model simplification degree were enhanced, and it made the nondestructive detection realized.(3) The illegally added lead chrome green in tea was detected by infrared spectroscopy. Infrared spectroscopy could well distinguish the tea with or without lead chrome green, and the accuracy was up to 100%; The combination of iPLS and SPA selected 19 characteristic wavenumbers for lead chrome green; Based on these 19 characteristic wavenumbers, LS-SVM model obtained good results, with RP2 of 0.913 and RMSEP of 0.227.(4) The illegally added lead chrome green in tea soup was detected by Raman spectroscopy. By the analysis of Raman spectra, characteristic fingerprint peaks of lead chrome green were found; Based on the correction of integrated intensity from 2804 cm-1 to 230 cm-1, good detection model was obtained; The combination of wavelet transform and SPA selected 8 characteristic wavenumbers for lead chrome green and the belonging of each characteristic wavenumber was also found. |
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