Studies On Mechanism Of Tea Scum

STUDIES ON MECHANISM OF TEA SCUMABSTRACTIt is a common hygienic practice that teapots and teacups are cleaned washed thoroughly with clean water after every brew.Improper washing of these items,after repeated brewing,will result in a collection of tea scum on the surface of teacups.To study the mechanism of tea scum,this study used deionized（DI）water,tap water,deionized water containing Ca²⁺(DI-Ca²⁺),deionized water containing Ca²⁺and HCO₃^-(DI-Ca²⁺-HCO₃^-),deionized water containing Ca²⁺and SO₄^2-(DI-Ca²⁺-SO₄^2-),and deionized water containing Ca²⁺and Ac^-(DI-Ca²⁺-Ac^-)to brew green tea（Gt,non-fermented）,oolong tea（Ot,half-fermented）and black tea（Bt,fully fermented）with different degrees of fermentation.The physicochemical properties of tea infusions and the tea scum were analyzed.Factors including tea polyphenols,Ca²⁺,p H,temperature and gas atmospheres that may affect the formation of tea scum were analyzed as well.In addition,time of flight mass spectrometry（TOF-MS）was used to analyze the tea scum collected from Epigallocatechin gallate（EGCG）treated sample to study the formation process and the products of tea scum.The results were as follows:（1）There was no significant difference in the amount of tea scum accumulated from the various tea types used in this study.However,there was variation in the amount of tea scum in relation to water quality.DI-Ca²⁺-HCO₃^-treatment produced more scum than other water treatments.Fourier transform infrared spectroscopy（FTIR）of tea scum showed the characteristics of tea polyphenols in different types of tea.The similarity of FTIR spectra of the scum was high,while the differences between DI-Ca²⁺-HCO₃^-treatment and other treatments were significant.X-ray diffraction（XRD）showed that all the tea scum contained Ca C₂O₄·2H₂O,tea scums from tea brewed with DI-Ca²⁺-HCO₃^-contained significant amount of Ca C₂O₄·2H₂O and Ca CO₃ crystals.The results of scanning electron microscope-energy dispersive X-ray energy spectroscopy（SEM-EDS）showed that the main elements of tea scum are organic carbon（C）,oxygen（O）and inorganic calcium（Ca）,and the content of these elements in different tea scum vary significantly,indicating that tea scum was not likely to be a pure substance with specific chemical formula.These results showed that though HCO₃^-was not necessary for tea scum,its existence will significantly change the amount and related properties of tea scum.（2）The contents of tea polyphenols in Gt infusions were significantly higher than in Ot and Bt infusions.However,there was no significant difference between the Ot and Bt infusions.After 24 h,the contents of tea polyphenols decreased,possibly due to the oxidation and participation of tea polyphenols in the formation of tea scum.There was no significant difference in the reduction of tea polyphenols among different types of tea.However,in terms of water quality,the decrease in tea polyphenols in the DI-Ca²⁺-HCO₃^-treatment was significantly higher,and the decrease amount was 23.98,22.03 and 22.5 mg in Gt,Ot and Bt infusions,respectively.Except tap water and DI-Ca²⁺-HCO₃^-treatments,the p H of other treatments didn’t change in 24 h,but the p H of DI-Ca²⁺-HCO₃^-was significantly higher,probably due to the hydrolysis of HCO₃^-,resulting in the increase in p H in the tea infusions.Moreover,there was no significant difference in the Ultraviolet-visible spectroscopy（UV-VIS）of different tea infusions at 0 and 24 h.（3）With the increase in the tea polyphenols content in tea infusions,the amount of tea scum increased to 3.0 mg at the concentration of 0.5 mg/m L and then decreased.With the increase of Ca²⁺concentration,more tea scum was formed,reaching a maximum of 13.3mg when Ca²⁺concentration was increased by 150 ppm.More tea scum formed when the temperature increased,from 3.0 mg at 25℃to 8.0 mg at 85℃.With the increase in p H in tea infusions,the amount of scum increased to 18.6 mg at p H=8,then decreased rapidly.In oxygen atmosphere,the amount of scum（8.8 mg）was significantly higher than that in the air（5.9 mg）and N₂（3.2 mg）.（4）In TOF-MS of EGCG scum,EGCG oxidation products,complexes of Ca²⁺with EGCG and complexes of Ca²⁺with EGCG oxidation products were found,which indicated that the main compositions of EGCG scum were various complexes of EGCG and its oxidation products with calcium ions.Based on the above results,the mechanism of tea scum was proposed.Tea polyphenols in tea infusions were oxidized under alkaline and oxygen conditions,and eventually formed complexes with Ca²⁺.Subsequently,the complexes adsorbed with Ca C₂O₄·2H₂O and Ca CO₃ crystals to form tea scum.