Validation of analytical method and investigation into the effects of brewing temperature on total polyphenol contents in tea infusions prepared from Vietnamese tea products

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 FolinCiocalteu 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 ultrasonicassisted single extraction using methanol:water (7:3 v/v) as an extraction solvent at 70oC with the extraction ratio of 0.200:10.00 (g:mL).