NIR spectroscopic determination of polyphenol content in teas and tea extract at 2142 nm

Abstract

Various teas, a tea extract, and a catechin mixture reagent were measured in transmittancereflectance mode using visible and NIR (Vis-NIR) spectroscopy. Total polyphenol (TPP) content in the tea samples was determined using Vis-NIR spectra. Then, reference TPP content was determined by the colorimetric ferrous tartrate method. Partial least squares (PLS) regression, a method of multivariate analysis, was performed in combination with various wavelength ranges and spectral pretreatments. Usually, multivariate analyses in which many independent variables are applied have been used for NIR data to obtain high accuracy for the determination. Nevertheless, surprisingly, only 2142 nm, which has high sensitivity and is in the NIR region, was identified for the determination of TPP content in various teas and tea extracts based on regression coefficient values, selectivity ratio (SR) values of the best PLS model [correlation coefficient (R = 0.98, n = 69)], correlation coefficients (a correlation spectrum) between the reference TPP content and Savitzky-Golay smoothing with the second derivatives (SG2D) with standard normal variate (SNV) preprocessing of raw spectra, and the peaks of the raw spectra and their SG2D with SNV. The R value between SG2D with SNV at 2142 nm and reference TPP content was 0.96 (n = 69).