1. Coffee cannot be brewed in a thermos. The stainless steel in the thermos cup contains metal elements, which is relatively stable under normal circumstances. However, in acidic environment, some substances that will react with metals and be harmful to the body may be dissolved out, which will affect the life and health of the cup.

2. The material of the thermos cup is basically stainless steel, and there is an acidic substance called tannic acid in coffee, which will cause slight damage to the thermos cup, resulting in a greatly shortened life of the thermos cup and poor heat preservation performance. It is easy to produce chemical reactions, which will have an impact on health after a long time. Moreover, after long-term insulation and heating, coffee will destroy the taste and easily deteriorate.