I agree with philip that alcohol will pick something up. I thought I was about bitten through two of my older MM stems without a puncture, just chomped them flat (serial clencher) so I decided to replace them. So, I took the stem off and decided to sterilize them before I put them in a bag to save in case it happens again (I'd rather have a restricted air flow than no stem if it ever came to that in the future). Dipped my cleaner, ran it through and came out with a deep, dark brown pipe cleaner. Did that until it was clear and then some more until they were dry and then I had air flow.
So now everything gets an alcohol swab once a month and everyone is happy.