When a thermos flask is left unclean for some time or it is shallowly cleaned, there is a tendency of dirt building up in the interior of the thermos flask. The smell grows so deep that when you put in your beverage, for instance, it contains an unpleasant smell upon drinking. Automatically, you’ll start thinking that your thermos flask has lost its ‘cool’. Problem is that the built up dirt tends to come out when it comes into contact with hot fluids. When it comes out, it mixes with the liquid which explains the ‘going bad’ smell. All you are required to do before you get rid of the flask is to deep clean it.
This way you will have little to no complaints about the smell produced over time that the thermos flask hasn’t been cleaned. You will use your thermos flask for a long time with great service if you do this. There, the smelling problem is fixed.