Effects of different steeping methods and storage on caffeine, catechins and gallic acid in bag tea infusions

Abstract

Bag teas, packed 3 g of ground black, green, oolong, paochoung and pu-erh tea leaves (the particle size used was 1–2 mm), were steeped in 150 mL of 70, 85 or 100 °C hot water to study the effects of the number of steeping (the same bag tea was steeped repeatedly eight times, 30 s each time, as done in China for making ceremonial tea) and varied steeping durations (0.5–4 min) on caffeine, catechins and gallic acid in tea infusions. The changes in tea infusions during storage at 4 or 25 °C for 0–48 h and the variations in these compounds of bag tea infused with 150 mL of 4 or 25 °C cold water for 0.5–16 h were also investigated. A HPLC method with a C18 column and a step gradient solvent system consisting of acetonitrile and 0.9% acetic acid in deionized water was used for analysis. Results for all kinds of tea samples showed that the second tea infusion contained the highest contents of caffeine, catechins and gallic acid when bag teas were steeped in 70 °C water. It was different from that steeped at 85 and 100 °C, the highest contents existed in the first infusion. These compounds decreased gradually in later infusions. Higher amounts of caffeine, catechins and gallic acid could be released from bag teas as hotter water was used. As steeping duration prolonged, these ingredients increased progressively, however, their levels were lower than that cumulated from the infusions with the identical bag tea prepared recurrently at the same temperature and time points. (−)-Gallocatechin gallate and (+)-catechin existed in these tea infusions rarely and could not be detected until a certain amount of them infusing. Except gallic acid that showed a significant increase and caffeine that exhibited no significant change, all kinds of catechins decreased appreciably after tea infusions were stored at 25 °C for 36 h; nevertheless, all of them showed no evident changes at 4 °C storage. The caffeine, catechins and gallic acid in tea infused with cold water also increased with increasing duration. Their contents in 25 °C steeped tea were higher than that made at 4 °C; moreover, their infusion rates from bag teas to cold water were markedly lower than that steeped in hot water. Infusing efficiencies of non-gallated catechins were higher than gallated catechins under cold water steeping.