Rapidly Inspecting Of Tea Leaf By Near-infrared Spectroscopy Analysis

In combination with the chemometrical methods,the near-infrared(NIR) spectroscopy analysis technology was used for tea quantitative analysis,in which the content of tea polyphenol and caffeine were inspected.In order to provide new reference for the nondestructive examination for tea quality detection,several essential issues on near-infrared spectroscopy technology in building the NIR forecast model were investigated in terms of the spectral preprocessing methods choice,the modeling method optimization,the modeling sample numbers optimization,correlation analysis technology,the comparison of WQF and MPA spectral,and the optimal wavelength selection.180 tea samples of different production were collected for this study.Chemical values of two nutrition ingredient consisting of tea polyphenol and caffeine in tea samples both were detected by spectrophotometer.And the near-infrared spectra were synchronously detected by means of near-infrared instruments of two different types made by different enterprises:one was MPA Fourier Near-infrared Spectroscopy and the other was WQF-400N Fourier Near-infrared Spectroscopy.In order to find the best method of pretreatment,various methods were used to pretreat the original spectra.Meanwhile,this research compared four-wavelength optimization methods.The moving windows partial least squares(MWPLS) could have the most superior model with the best width of the windows.When the width of the windows of tea polyphenol and caffeine were 71 and 91,the NIR model veracity were maximal which reached 2.066 5 and 0.288 1.The methods of MWPLS were used to select wavelength and then established the tea polyphenol and caffeine NIR model respectively in the different numbers of calibration sets by the best method of pretreatment.The results showed that the number of calibration sets had an obvious influence on tea polyphenol and caffeine model.With the number of calibration sets increased,the root mean square error of prediction(RMSEP) reduced and became steady in the end.When the number of tea polyphenol and caffeine calibration sets were 40 and 55, the NIR model veracity became steady.And when the numbers became 85 and 100 respectively,the NIR model veracity were maximal which reached 1.797 0 and 0.257 8.The conclusion in this study had a significant referenced value for selecting proper sample numbers in estimating NIR model.As natural samples,tea contained a large number of complex background substances.All of the original spectra by WQF after correlation analysis,were used to build calibration models on caffeine using PLS regression analysis,which prediction results were compared with the models not processed by the correlation analysis.After correlation analysis,the prediction correlation coefficient increased by 2.96%,the root mean square error of prediction decreased by 1.27%.The purification and strengthening ability for available signal of correlation analysis technology was proved.Compared with the imported equipment,domestic instruments did not have an obvious peak,and the spectra shifted significantly and had more noise.This paper analysed the spectra after different pretreatments.The results showed that domestic instrument could not reach the accuracy as that of imported equipment but the signal to noise ratio could be enhanced after the correlation analysis was applied.