Tea was brewed from young leaves of 4 weeks old Thai upland black waxy rice plants of the cultivar ‘Damgatondam’, which has a noticeable fragrant smell from both seeds and leaves. The plants were harvested about 1 cm above soil level, washed, and cut into pieces, about 1 cm long. These pieces were then steamed for 1 min in a steamer rack, withering for 5min by spreading them out in a thin layer, pan-roasted at 70°C for 30 min, and finally dried at 70°C for 90 min. They were either kept as pieces of leaves or ground into a powder, and both were used for brewing tea by infusion in water at 4oC, room temperature (about 25oC), for 1, 3, 5, 7, 9, 11, or 15 h and brewing at 70oC or 98oC for 1, 3, 5, 7, 9, 11 or 15 min. Directly after brewing, the tea was stored at -25oC awaiting analysis. Each sample was analyzed for total phenolic content (TPC) and for antioxidant activity, by ferric reducing antioxidant power (FRAP), 2,2 diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis (3-ethylbenzothiazo line-6-sulphonic acid) (ABTS) assays. Results showed that tea brewed from ground leaves had higher antioxidant activity than tea brewed from ungrounded leaves. The highest level of extraction was at 98oC for 15 min, where the TPC was 62.79 mg GAE/100g, antioxidant activity was 278.04 for DPPH, 195.38 for ABTS and 1150.13 mg TE/100g for FRAP. Samples extracted at 4oC or room temperature also had high levels of TPC and antioxidant activity with the maximum extraction at room temperature for 15 h; where TPC was 46.76 mg GAE/100g, and antioxidant activities were 152.72 for DPPH, 140.42 for ABTS and 950.32 mg TE/100 g for FRAP. Therefore, the level of extraction for tea brewed in all four conditions depended on both temperature and brewing time. Also, extraction of high levels of bioactive compounds depended on large surface areas of the leaves being in contact with water.
Changes of antioxidant activity in honey after heat treatment
