VALIDATION OF ANALYTICAL METHOD AND INVESTIGATION INTO THE EFFECTS OF BREWING TEMPERATURE ON TOTAL POLYPHENOL CONTENTS IN TEA INFUSIONS PREPARED FROM VIETNAMESE TEA PRODUCTS

Tóm tắt

Produced from leaves of Camellia sinensis L. (Kuntz), tea is among the most common beverages worldwide. Tea plants have been grown in nearly 30 countries. In this study, the Folin-Ciocalteu method was applied to determine the total polyphenol contents (referred to milligrams gallic acid mass equivalence or GAE) in tea products (one oolong and three ancient teas, including red, green, and white) based on ISO 14502-1:2005 with some modifications, typically ultrasonic-assisted single extraction using methanol:water (7:3 v/v) as an extraction solvent at 70^oC with the extraction ratio of 0.200:10.00 (g:mL). The analytical method was validated using Shimadzu 1800 UV-Vis instrument with favorable linearity of R² > 0.995, linear range of 10-70 mg GAE L^–1, acceptable repeatability, reproducibility (% RSDs were 0.79 and 1.2 for intra-day and inter-day, respectively), and high recoveries (higher than 98% for spiked samples). The total polyphenol contents (mean values, mg GAE g^–1 dried weight, in brackets) performed a descending order of white (206.62) green (201.33) > red (167.42) > oolong (139.18) teas due to the variation in the oxidation levels during the fermentation, particularly for red and oolong teas. Higher polyphenol contents released in tea infusions were observed regarding the increasing brewing temperature, except for oolong tea with its specific fluctuation, maybe due to its "wrap-curled" structure, leading to longer brewing durations for polyphenol to be extracted stably and completely. This study contributes to enriching data for Vietnamese tea products in the context of high production and export.