New insight into scale inhibition during tea brewing: Ca²⁺/Mg²⁺ complexing and alkalinity consumption

Scale not only affects the taste and color of water, but also increases the risks of osteoporosis and cardiovascular diseases associated with drinking it. As a popular beverage, tea is rich many substances that have considerable potential for scale inhibition, including protein, tea polyphenols and organic acids. In this study, the effect of tea brewing on scale formation was explored. It was found that the proteins, catechins and organic acids in tea leaves could be released when the green tea was brewed in water with sufficient hardness and alkalinity. The tea-released protein was able to provide carboxyl groups to chelate with calcium ions (Ca²⁺), preventing the Ca²⁺ from reacting with the carbonate ions (CO₃^2-). The B rings of catechins were another important structure in the complexation of Ca²⁺ and magnesium ions (Mg²⁺). The carboxyl and hydroxyl groups on the organic acids was able to form five-membered chelating rings with Ca²⁺ and Mg²⁺, resulting in a significant decrease in Ca²⁺ from 100.0 to 60.0 mg/L. Additionally, the hydrogen ions (H⁺) provided by the organic acids consumed and decreased the alkalinity of the water from 250.0 to 131.4 mg/L, leading to a remarkable reduction in pH from 8.93 to 7.73. It further prevented the bicarbonate (HCO₃^-) from producing CO₃^2- when the water was heated. The reaction of the tea constituents with the hardness and alkalinity inhibited the formation of scale, leading to a significant decrease in turbidity from 10.6 to 1.4 NTU. Overall, this study provides information to help build towards an understanding of the scale inhibition properties of tea and the prospects of tea for anti-scaling in industrial applications.