Many people like to keep a thermos in their office, with tea leaves always ready in a drawer, and start brewing tea in the thermos as soon as they get to work, drinking it until the end of the day when the tea is still warm. They believe this makes the tea taste better and keeps it warm for a long time, but this practice is actually incorrect.
Tea is rich in nutrients such as proteins, fats, sugars, vitamins, and minerals, making it a natural health beverage. The tea polyphenols, caffeine, tannins, and tea pigments it contains also have various pharmacological effects. If you brew tea in a thermos, the tea leaves are steeped in high-temperature water for a long time, much like simmering over a low flame. This causes a large amount of substances like tea polyphenols and tannins to leach out, making the tea color dark and giving it a bitter, astringent taste. At the same time, because the water temperature remains high, the aromatic oils in the tea will volatilize quickly, and a large amount of tannic acid and theophylline will seep out. This not only reduces the nutritional value of the tea and diminishes its aroma but also increases the amount of harmful substances. Furthermore, nutrients like vitamin C are destroyed when the water temperature exceeds 80°C, and prolonged high-temperature steeping causes excessive loss, thereby reducing the tea's health benefits.
In addition, some people also use enamel teaware to brew tea, which is also not advisable. Enamel teaware can easily wear out over time, exposing the iron layer underneath. Metal components can then dissolve into the tea, causing the tea's color to turn yellow and lose its original flavor. Moreover, enamel teaware dissipates and transfers heat quickly, which also makes the tea's aroma dissipate easily. Therefore, it is not recommended to use thermos flasks or enamel teaware for brewing tea. If you want to drink hot tea, you can brew it in a Zisha clay pot or ceramic teaware and then pour it into a thermos flask.