Coffee taste and its accompanying odor can build up in a thermos or travel mug. The taste becomes more pronounced if the buildup has occurred over a long time, especially if you leave the thermos in a warm place, such as a car, every day. The lingering taste of stale coffee can ruin the flavor of your freshly brewed coffee or other items you put in the thermos, such as tea or juice. Because you drink out of the thermos, avoid cleaning out the taste and odor with bleach or other harmful chemicals.
Combine three parts baking soda with one part water to make a paste. Scrub the inside of the thermos with this paste using a nylon brush. This removes coffee stains and the source of the lingering coffee taste.
Mix one part baking soda and one part boiling water. Pour it into the thermos. Let the thermos sit overnight. Wash and rinse the thermos well the next day.
Fill the thermos with white vinegar and let it sit overnight. Rinse well and let it dry.
Place the thermos in the sink. Put 2 tbsp. baking soda in the empty thermos. Add 2 to 3 oz. white vinegar. The mixture will foam. After the foaming stops, put the lid on the thermos and shake it for a minute or two. Hold the thermos away from your face and remove the lid; some additional foaming may have occurred and built up under pressure. Rinse the thermos well and let dry.