Abstract: Water quality was measured on Beijing tap water with and without reverse osmosis membrane filtration and deionized water. In addition, the effect of selected ions showing a considerable difference on color parameters and DPPH free radical scavenging activity of tea infusion was discovered. Our results indicated that Beijing tap water was slightly alkaline, remarkably darkened the color of tea infusion compared with deionized water, and resulted in a distinct decrease in DPPH free radical scavenging activity. Calcium (70 mg/L), magnesium (30 mg/L) and sodium (18 mg/L) ions in Beijing tap water could reduce the color of tea infusion and its DPPH free radical scavenging activity. This suggests that the main reason for the darker color and decreased DPPH free radical scavenging activity of tea infusion brewed with tap water is that tea infusion is kept in a weakly alkaline environment during brewing due to the presence of HCO3－ in tap water, thus causing oxidative degradation of tea phenols.

Key words: Beijing tap water green tea infusion, color value, DPPH free radical, factor, bicarbonate