A new study has found that the coating of a ceramic tea set affects the health benefits of tea

Tea was introduced as a medicinal beverage around 2700 BC, and has become one of the most popular beverages worldwide. One of the main reasons for its popularity is its rich content of flavonoids and polyphenols, which contribute to the antioxidant properties and flavor and aroma of the tea, providing many potential health benefits. These compounds are extracted from tea leaves during the brewing process and can be affected by several factors, such as water temperature, brewing time and materials used in the tea vessels.

Recently, researchers from the Nagoya Institute of Technology (NITech) in Japan revealed, for the first time, that the choice of glaze on ceramic tea sets used for tea preparation plays a key role in retaining catechin flavonoids.

In their study published online in the journal Scientific reportsAssociate Professor Takashi Shirai, along with Dr. Yunzei Shin, Mr. Sota Shido, and Dr. Kunihiko Kato from the Center for Advanced Ceramics Research at NITech, examined the effect of four different types of typical Japanese commercial glass – Oribi, Namako, Irabu, and Tomei – on catechin content. It is the most abundant flavonoid found in green tea.

While the glaze is primarily composed of feldspar minerals, such as silicon, aluminum, sodium, and calcium oxides, it also contains distinct types of oxide minerals that give a unique look and texture to the ceramic vessel. Oribe paint contains mostly copper oxides (Cu) and imparts a vibrant green color, while Namako paint contains cobalt oxides (Co) for a dark blue appearance. The Irabo paint contains iron oxides (Fe) that impart orange tones, while the Toumei paint contains a high percentage of titanium (Ti), providing a translucent finish.

To examine the effect of vitrification on tea catechins, the researchers fermented a green tea solution using ionic exchange water at 80°C for three minutes. The tea leaves were separated, and the supernatant (liquid above the solid residue) was mixed with the glaze powders coated on the ceramic tiles. The glaze and tea mixture was then allowed to react for six hours, followed by removal of the glaze powder through centrifugation and filtration.

The researchers noted that the pure tea solution had a clear bright yellow color, but after six hours of decomposition, it turned yellowish-brown. In contrast, tea solutions degraded by different glass materials showed black or dark brown colors. In other words, the extent of the color change depends greatly on the type of glaze.

In addition, the researchers also observed a selective reduction in the amount of altered catechins in tea. Tea solutions mixed with Oribe, Namako, and Irabu glazes showed significantly lower concentrations of epicatechin, epicatechin gallate, epigallocatechin, and epigallocatechin gallate, while tome glaze selectively degraded epigallocatechin gallate. The decrease in catechin concentration and the resulting color change can be attributed to the process of catechin oxidation, which forms brown thearubigins and reddish-orange theaflavins and their oxide pigments.

“During the decomposition process, copper, cobalt, iron and T-oxides in the glazing powders can act as a Lewis acid catalyst and promote the oxidation of catechin molecules to orthoquinones, followed by further reaction to form thearubigins and/or theaflavins and their oxides. Another oxidation route is through the polymerization of free radical catechins,” explains Dr. Shirai. “Interestingly, thearubigins and theaflavins are the main components of fermented teas such as black tea. In other words, green tea brewed with certain ceramic tea sets can be converted into black tea.”

In summary, this study highlights that the choice of glaze materials used in ceramic tea sets can significantly influence the concentration of beneficial compounds such as catechins in tea. “The specific function of glass in catechin degradation not only provides essential information for the design and development of functional materials, but can also impact daily tea drinking and long-term human health issues,” Dr. Shirai concludes.