Effects of water hardness on the flavor and antioxidant activity of Ishizuchi dark tea

•Sensory testing show a higher degree of liking for tea brewed with hard water.•An increase in pH from 4 to 5 for tea with increasing water hardness.•The antioxidative activity of tea infusion is independent of water hardness. Ishizuchi dark tea, a traditional Japanese post-fermented tea, has attracted considerable attention for its bioactivity. In this study, the effect of water hardness on Ishizuchi dark tea infusions was analyzed. Sensory testing revealed that the teas brewed in hard water were less sour in taste than those brewed in soft water. The most abundant organic acid found in the tea infusions was lactic acid, with a constant concentration of 4 mmol L−1, regardless of water hardness. The pH of the tea infusions increased from 4 to 5 with increasing water hardness, and a similar increase in pH was observed with increasing concentrations of Ca2+ (or Mg2+) and HCO32− in an aqueous solution of 4 mmol L−1 lactic acid. The antioxidative activity of the tea infusion was clearly independent of water hardness. Therefore, hard water is suitable for brewing Ishizuchi dark tea because tea brewed in hard water has a less sour taste, which is more favored, while maintaining the same degree of antioxidative ability.