How to make my thermos flask not ruin my coffee?
This is a serious question, and I don’t think there is really a proper travel mug out there that doesn’t alter the taste. The Thermos one is a bad offender, but it becomes tolerable. I had to stop drinking from my stainless steel one because it imparted a slightly metallic taste. Often, if it’s not the mug, it’s the plastic lid that alters the coffee taste. You can try thoroughly rinsing your mug (and its lid) in a part vinegar / warm-hot water solution. Make sure you rinse out all of the vinegar with soapy water. Right now I commute with a porcelain mug with a silicone lid, and it’s about the best solution I could find to the bad-tasting-travel-mug problem.
