A colleague of mine just got a new thermos (inox), this model, and actually read all instruction, and we were surprised to read that it is not recommended to pour hot milk in the thermos.
My opinion was that:
- Unlike tea of coffee, a lot of micro-organisms live in milk, will end up being a favorable place for those micro-organisms to generate a culture (which will be more and more difficult to get rid off, and will contaminate any other fluid poured in the thermos)
- If you don't wash it straight away, it will really stink.
- The milk being full of water-insoluble proteins/fat, it will "coat" the internal thermos walls (gross!) - I've already seeing that "coat" forming in plastic containers, and I guess it won't be any different in inox containers.
I've actually not found an answer online, so I thought that here I could find some experts. I realize this could be a cross-question for Biology SE, but I felt it more food-related.