Fermentation quality evaluation of tea by estimating total catechins and theanine using near-infrared spectroscopy

Concentrations of total catechins and theanine are important indices for monitoring the degree of fermentation, a key factor in determining the types, qualities, and taste characteristics of tea, during manufacturing process. Until now, there has not been any near-infrared (NIR) study applying theanine as an index for monitoring the degree of fermentation of tea. It is necessary to understand and monitor the degree of fermentation according to the concentrations of total catechins and theanine in tea by using NIR spectroscopy, especially for manufacturers of tea products. NIR spectroscopy integrated with modified partial least-squares regression (MPLSR) method was used for spectral analysis of total catechins and theanine in 161 tea samples in the full visible NIR wavelength range of 400−2498 nm (25000−4003 cm⁻¹). Both the optimal calibration models of total catechins and theanine built by MPLSR showed satisfactory predictability, with coefficient of determination of calibration (R_c²) and residual predictive deviation (RPD) up to 0.94 and 3.38, respectively. High total accuracy rates of 100 % were also achieved for the discrimination of green tea, partially fermented tea, and black tea in the validation sets by the optimal MPLSR calibration model of theanine. The results indicated that NIR spectroscopy has the potential to be adopted as an effective method of rapid and accurate inspection of the degree of fermentation of tea. This technique could contribute substantially to quality assurance during fermentation process of tea.