摘要：

Tea is second only to water as the most highly consumed beverage worldwide and its medicinal and health properties have been widely explored. It is prepared from the young, tender leaves of Camellia sinensis (L.) (family Theaceae) which undergo different manufacture procedures to give various types of teas: black, green, white, and oolong. In addition, the growing season, geographical region, processing of the leaves and whether teas are blended or unblended, create the vast selection available commercially and contribute to each tea's uniqueness. Considering the increasing interest in the health properties of tea, unrelated in part to its antioxidant activity, and the different methods of making a cup of tea in different countries and cultures, the present study aimed to discover, whether the antioxidant activity of different tea infusions could be affected by steeping time, temperature and manufacture. At first, different tea infusions (white, black, green, oolong) coming from different regions of the world were prepared by hot infusion at 90°C for 7 min or by a cold one at room temperature for 2 h, and analyzed. Analysis of the antioxidant activity of these infusions showed that the white tea, unlike black, oolong and green tea, exhibited a greater activity when steeped for 2 h in water at room temperature. We therefore wanted to verify if this feature could be common to most white teas, whose beneficial effects are often attributed to their antioxidant activity and which has been barely studied to date. Accordingly, a batch of eight white teas was analysed: five from China and three from Malawi (Africa). The teas were prepared according to the typical hot brewing method for white tea (70°C for 5-7 min) and at room temperature for 2 h, and antioxidant activity, chelating activity and overall content of polyphenols, flavanols, and catechins (HPLC) were measured. In addition, to verify if the extraction of antioxidant compounds was affected by steeping time and temperature, some of the studied white teas and their corresponding green teas were extracted at different times and temperature and antioxidant activity, polyphenols content and flavanols of the infusions were determined. We also aimed to determine whether the manufacturing process of tea affects its antioxidant activity, metal chelating activity and total phenol content (TPC) when variables which influence tea components (geographical region, environmental conditions, cultivar type, plucking techniques) are kept to a minimum. For this purpose, a set of five Malawian teas were analyzed, all originating from the same tea estate and same superior cultivar but whose leaves (grown and plucked under the same conditions) were processed differently in the same factory to give black (orthodox and CTC), white and green (decaffeinated and non-decaffeinated) teas. The results of these studies contribute to gaining further knowledge on how the potential health benefits of this popular beverage may be maximised by the different methods of preparation and manufacture.

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