Near-infrared (NIR) spectroscopy and chemometric methods were applied to internal quality control of a Chinese green tea, Longjing, with Protected Geographical Indication (PGI). A total of 2745 authentic Longjing tea samples of three different grades were analyzed by NIR spectroscopy. To remove the influence of abnormal samples, The Stahel-Donoho estimate (SDE) of outlyingness was used for outlier analysis. Partial least squares discriminant analysis (PLSDA) was then used to classify the grades of tea based on NIR spectra. Different data preprocessing methods, including smoothing, taking second-order derivative (D2) spectra, and standard normal variate (SNV) transformation, were performed to reduce unwanted spectral variations in samples of the same grade before classification models were developed. The results demonstrate that smoothing, taking D2 spectra, and SNV can improve the performance of PLSDA models. With SNV spectra, the model sensitivity was 1.000, 0.955, and 0.924, and the model specificity was 0.979, 0.952, and 0.996 for samples of three grades, respectively. FT-NIR spectrometry and chemometrics can provide a robust and effective tool for rapid internal quality control of Longjing green tea. 1. Introduction Tea is one of the most popular beverages around the world and favored for its various healthy benefits [1, 2]. According to the degree of fermentation, teas can be generally classified into three types: unfermented, partially fermented, and fully fermented [3]. In China, although all the above three types of teas are produced and consumed, green tea is the most favorable for its special flavor and taste. Longjing tea, a green tea produced from Hangzhou and its neighboring areas, has been traditionally recognized as a top-grade green tea for its top quality as well as its cultural backgrounds [4, 5]. Longjing tea leaves are roasted soon after picking to cease the natural oxidation process. When steeped, the flat and straight leaves produce a yellow-green color. Its flavor and taste are very gentle and sweet, although it has one of the highest concentrations of catechins among teas [4, 5], which is an important indicator of high-quality green teas. Because Longjing tea has a very high commercial value, the quality control of Longjing tea is urgently demanded against various counterfeit Longjing teas. The internal grading especially among authentic Longjing tea is the foundation for its quality control. As a green tea with Protected Geographical Indication (PGI), the three producing areas of Longjing are explicitly defined as West Lake and
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