Effects of brewing temperature on release of caffeine and antioxidants in black tea and short-term effectiveness of black tea in improving cognition, mood, and alertness

Abstract

Background: Black tea is the second most consumed beverage globally and accounts for a significant proportion of caffeine and polyphenol intake in the UK and elsewhere (Yashin et al. 2015). Tea is historically associated with cognitive benefits such as mental clarity, alertness, positive impact on mood, and psychological well-being (Camfield et al. 2014). These distinct attributes are associated with two major constituents of tea: caffeine and antioxidants. Mostly, the research on tea has focussed on the impacts of infusion time and release of caffeine and antioxidants; while there is very limited research available that investigates the effects of infusion temperatures. Also, there is a lack of exploration in comparative human intervention studies on the short-term effects of teas with different antioxidant levels in improving cognition, mood, and alertness. Objective: To investigate the effects of infusion temperature on the release of caffeine and antioxidants in black tea. Further, to examine changes in antioxidant levels of tea after the addition of milk. Finally, the short-term comparative effectiveness of tea on cognitive ability, mood and alertness will be explored. Design: Four tea blends were tested in the laboratory for caffeine and antioxidant levels at a varying range of temperatures. Among the four blends, Tetley and Twinings were selected for a randomised controlled trial. Participants consumed tea-with-milk or tea-without-milk. The study was conducted at QMU. Twenty participants were recruited from the university and the public. Following 12 hour of fasting and abstinence from tea and caffeinated drinks, participants were tested for cognition and alertness by CAF-VAS and Stroop tests in the morning; afterward they were provided with a standard cup of tea with milk or without milk and the same tests were repeated after a gap of 60 minutes. The experiment was conducted on two non-consecutive days. On day 1, tea with the lowest antioxidant levels was tested and on day 2, tea with the highest released antioxidants was tested. Results: On HPLC analysis, all four blends exhibited the highest mean caffeine concentration at the brewing temperature of 100°C (127.3 ± 21.07 mg/235ml). Among the 4 blends, Yorkshire had the highest (134.2 ± 23.1 mg/235ml), while Twinings had the lowest mean values (99.09 ± 21.3 mg/235ml). On Folin assay, Tetley displayed the highest (206.3 ± 29.1 GAE/235ml), while Twinings had the lowest mean value (192.4 ± 39.2 GAE/235ml). Based on Folin assay, Tetley and Twinings were selected for the participant-based study. These two blends exhibited significantly higher antioxidants with milk (P ≤ 0.02) compared to the without-milk analysis. FRAP analysis showed the highest value for Tetley (14.02 ± 1.5 mMFeSO₄), while Yorkshire had the lowest FRAP value among the four blends (11.88 ± 1.9 mMFeSO₄). In FRAP analysis, tea-without-milk had significantly higher values (P ≤ 0.003) compared to tea-with-milk. On the Stroop test, participants showed significant improvement in response time after Twinings (P ≤ 0.03), while Tetley had no significant impact with or without-milk. On CAF-VAS, participants consuming Twinings showed improvement in relaxation (P≤0.005), alertness (P ≤ 0.01), and alleviating tiredness (P ≤ 0.002), Tetley exhibited improvement in the same parameters among the tea-without-milk group but the significant improvement was witnessed in the with-milk group (P ≤ 0.01). Conclusion: In-cup caffeine concentration increases with temperature and maximum concentration is attainable at 100°C; while for the release of antioxidants, the optimum temperature range is 80-100°C, which is a standard brewing temperature in most UK households. Among the four blends, Twinings, with the lowest caffeine and antioxidants, was the most effective in improving cognition, mood, and alertness. Key words: Caffeine, antioxidants, cognition, parameters.