There is a scientific basis for the stainless steel vacuum flask not to be used to make tea

Tea leaves contain a lot of tannins, theophylline, aromatic oils and a variety of vitamins, so it should only be brewed with water at about 80 degrees. If you use a thermos cup to make tea, soak the tea leaves in high-temperature, constant-temperature water for a long time, just like decocting on a warm fire. A large number of vitamins in tea are destroyed, aromatic oils are volatilized, and a large amount of tannins and theophylline are leached, which not only reduces the nutritional value of tea, but also makes tea juice unscented, tea tastes bitter, and harmful substances increase. If the tea leaves are soaked for too long, the tea may release heavy metals, which is harmful to health. If you like to drink tea, you can brew the tea first, separate the tea leaves and pour them into a thermos, which is safer and more convenient.