Black tea stain formed on the surface of teacups and pots. Part 2 – Study of the structure change caused by aging and calcium addition

The black tea stain formed on the surface of teacups consists of oxidized tea polyphenols together with several percent calcium silicates, as shown in our preceding paper. The previous study introduced the hypothesis that the calcium bridging in the polyphenols and the oxidative polymerization promote the stain formation and development. The present infrared spectroscopy (IR) study showed that the increase in the stain formation following the addition of calcium ions is caused by the calcium bridging between the polyphenol molecules. Mass spectroscopy (MALDI-TOF-MS) study provided sufficient evidence that exposing the stain to air also enhances the polymerization of polyphenols. Those structure changes made the stain much less soluble in water. The effectiveness of calcium-chelating agents on the removal of the stain was in accordance with the order of chelating formation constants.