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The Quality Control of Tea by Near-Infrared Reflectance (NIR) Spectroscopy and Chemometrics

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The Quality Control of Tea by Near-Infrared Reflectance (NIR) Spectroscopy and Chemometrics Hindawi Journal of Spectroscopy Volume 2019, Article ID 8129648, 11 pages https://doi.org/10.1155/2019/8129648 Review Article The Quality Control of Tea by Near-Infrared Reflectance (NIR) Spectroscopy and Chemometrics Ming-Zhi Zhu ,1,2,3,4 Beibei Wen,1,2,3,4 Hao Wu,5 Juan Li,1 Haiyan Lin,2 Qin Li,3 Yinhua Li,2 Jianan Huang ,1,2,3 and Zhonghua Liu 2,3,4 1Key Laboratory of Tea Science of Ministry of Education, Hunan Agricultural University, Changsha 410128, China 2Hunan Provincial Key Laboratory of Crop Germplasm Innovation and Utilization, Hunan Agricultural University, Changsha 410128, China 3National Research Center of Engineering Technology for Utilization of Functional Ingredients from Botanicals, Hunan Agricultural University, Changsha 410128, China 4Collaborative Innovation Centre of Utilization of Functional Ingredients from Botanicals, Hunan Agricultural University, Changsha 410128, China 5Key Laboratory of Tea Plant Biology of Henan Province, College of Life Science, Xinyang Normal University, Xinyang 464000, China Correspondence should be addressed to Jianan Huang; [email protected] and Zhonghua Liu; [email protected] Received 29 June 2018; Revised 24 October 2018; Accepted 12 November 2018; Published 2 January 2019 Guest Editor: Sarfaraz Ahmed Mahesar Copyright © 2019 Ming-Zhi Zhu et al. 5is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Tea is known to be one of the most popular beverages enjoyed by two-thirds of the world’s population. Concern of variability in tea quality is increasing among consumers. It is of great significance to control quality for commercialized tea products. As a rapid, noninvasive, and nondestructive instrumental technique with simplicity in sample preparation, near-infrared reflectance (NIR) spectroscopy has been proved to be one of the most advanced and efficient tools for the control quality of tea products in recent years. In this article, we review the most recent advances and applications of NIR spectroscopy and chemometrics for the quality control of tea, including the measurement of chemical compositions, the evaluation of sensory attributes, the identification of categories and varieties, and the discrimination of geographical origins. Besides, challenges and future trends of tea quality control by NIR spectroscopy are also presented. 1. Introduction unique qualities of color, aroma, and taste but also con- tribute various health benefits for the human body [3]. Tea Tea is known to be one of the most popular beverages polyphenols account for 18∼36% of dry weight in tea leaves, enjoyed by two-thirds of the world’s population [1]. In 2017, and the astringent and bitter taste of tea is mainly con- 5.68 million tons of tea were produced all over the world, in tributed by tea polyphenols. Tea catechins (flavan-3-ols) are which 2.55 million tons were produced in China. 5e tea the major ingredients in tea polyphenols. Tea catechins in- quality is influenced by various factors, such as cultivars, clude (−)-epigallocatechin gallate (EGCG), (−)-epicatechin picking standard, tea processing technology, storage con- gallate (ECG), (−)-epigallocatechin (EGC), (−)-epicatechin dition, and time. Concern of variability in tea quality is (EC), (−)-gallocatechin gallate (GCG), (−)-gallocatechin increasing among consumers. It is of great significance to (GC), and (+)-catechin (C), among which EGCG is the most control quality for commercialized tea products [2]. abundant component [4]. 5e consumption of EGCG has 5e tea quality is determined by its major active been proved to have therapeutic effects for multiple diseases, components, including polyphenols, caffeine, and free such as cancer, metabolic syndrome, obesity, and cardio- amino acids. 5ese compounds not only endow tea with vascular and neurodegenerative diseases [5, 6]. 5e anticancer 2 Journal of Spectroscopy property of EGCG appears to involve the suppression of recognition methods for accurate analysis by removing the angiogenesis, induction of apoptosis, altered expression of disturbance of the noise, variability, uncertainties, and cell-cycle regulatory proteins, and activation of killer caspases unrecognized features. Nevertheless, NIR spectroscopy is [7, 8]. 5e suppression of angiogenesis by EGCG is associated characterized by high penetration depth. 5is property al- with the change in various miRNA expressions, the inhibition lows direct analysis of strongly absorbing or even highly of the VEGF (vascular endothelial growth factor) family, etc. scattering samples, such as turbid liquids or solids, without [9]. 5e beneficial health effects of EGCG are presumed to be further pretreatments [32]. related with its antioxidative property. Another possible Generally, the whole procedures of NIR spectroscopy mechanism is through the direct binding of EGCG to target include spectral data acquisition, data preprocessing, spec- proteins, leading to the regulation of signal transduction tral data preprocessing, calibration models building with a pathways, transcription factors, DNA methylation, mito- set of samples, and models validating using a set of in- chondrial function, and autophagy [10]. Caffeine is another dependent samples [33]. A typical NIR spectrum of tea is major functional component in tea and provides the bitter shown in Figure 1 [34]. 5e preprocessing of spectral data taste for tea [11, 12]. Caffeine has the therapeutic effects for should be used for eliminating the noise and baseline shift various diseases, including metabolic syndrome, type 2 di- from the background and instrument [33]. 5e commonly abetes, liver diseases, and cardiovascular and cerebrovascular used preprocessing methods in tea analysis include standard diseases [13, 14]. Free amino acids provide umami taste for normal variate (SNV), multiplicative scatter correction the tea infusion. Among free amino acids, theanine accounts (MSC), and Savitzky–Golay (SG) smoothing [35, 36]. for approximately 50% of the total free amino acids in tea Various variables selection methods, such as synergy interval leaves [15]. 5eanine not only offers a brisk flavor and an partial least squares (Si-PLS) and successive projections attractive aroma but also alleviates the astringency and bit- algorithm (SPA), are used for the screen of useful variables terness caused by polyphenols and caffeine. Several studies [37]. Multiple unsupervised and supervised pattern recog- have proved that theanine has significant health and cognitive nition methods have been used for the qualitative analysis benefits by influencing stress levels and learning efficiency (the discrimination of tea categories, varieties, and geo- [16]. graphical origins) and quantitative analysis (the de- Besides the chemical components, tea quality is influ- termination of chemical components in tea and enced by various factors, including the sensory attributes, optimization of processing conditions). 5ese pattern rec- classification, and geographical origins [17–19]. Multiple ognition methods include principal component analysis analytical approaches have been used for the quality control, (PCA), artificial neural network (ANN), linear discriminant such as colorimetric measurements, high-performance liq- analysis (LDA), support vector machine (SVM), soft in- uid chromatography (HPLC), high-performance liquid dependent modeling of class analogy (SIMCA), partial least chromatography coupled with mass spectrometry (HPLC- squares (PLS), and backpropagation artificial neural network MS), gas chromatography (GC), and gas chromatography (BP-ANN) (Table 1) [33]. coupled with mass spectrometry (GC-MS) [20–30]. How- In this article, we review the most recent advances and ever, these methods not only are expensive, time- applications of NIR spectroscopy and chemometrics for the consuming, and destructive but also need specialists for quality control of tea, including the measurement of the operation and cannot be applied for online applications. chemical compositions, the evaluation of sensory attributes, 5erefore, near-infrared reflectance (NIR) spectroscopy, a the identification of categories and varieties, and the dis- rapid, noninvasive, and nondestructive instrumental tech- crimination of geographical origins. nique with simplicity in sample preparation, has been de- veloped and applied for the quality control of tea in recent 2. The Application of NIR Spectroscopy in Tea years [19]. NIR spectroscopy is a spectroscopic method using the near-infrared region of the electromagnetic spectrum 2.1. Chemical Composition. 5e major compositions in tea ranging from 750 nm to 2500 nm (14,300∼4000 cm−1). An include polyphenols, catechins, caffeine, free amino acids, NIR spectrometer is usually composed of a light source, a and moisture. 5ese compositions are closely relevant to the monochromator, a sample presentation interface, and a overall quality of tea, and they thus are the key indexes of tea detector. 5e NIR radiation can be absorbed, transmitted, or quality. 5e monitoring of these compositions contents in reflected after interaction with samples. 5e feedback of tea is critical for the quality control [54]. NIR spectroscopy spectral stretching and bending of the chemical bonds has been successfully used for the prediction of major (O–H, N–H, and C–H) can be captured by utilizing different compositions contents in tea in recent years. Nonetheless, measurement modes of NIR equipment. 5erefore, the only
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