Coffee stains in a Thermos bottle tend to add a stale coffee taste to any other liquids that you put in your Thermos bottle. The only way to rid the Thermos bottle of this "tainted" taste is to get rid of the stains. This will ensure fresh tasting coffee every day and no stale coffee taste when other liquids are put in the Thermos bottle. Below are some cleaning methods.
Fill the Thermos bottle with hot water to within an inch and a half of the lip. Drop a denture cleansing tablet in to effervesce. Leave the inside of the bottle to soak in the denture cleanser for four hours--longer if the stain is really difficult. Rinse the Thermos bottle out with warm soapy water and rinse well.
Fill the Thermos bottle up with hot water adding 2 tsp. of powdered dishwasher detergent. Stir up well to dissolve completely. Leave to soak for 30 minutes to an hour depending on how severe the coffee stains. When the appropriate time has elapsed, wash the Thermos bottle with warm soapy water and rinse out under warm running water.
Fill the Thermos bottle with hot water and add 2 tbsp. of white vinegar. Stir the mixture and leave overnight. Rinse out the Thermos bottle in the morning and wash in warm soapy water to rid the bottle of the vinegar smell. Rinse well under warm running water.
Fill the Thermos bottle with warm white vinegar (warm the vinegar in a separate microwave-safe container and pour the warm liquid into your Thermos bottle). Add 1 tbsp. of baking soda and stir. Leave this mixture for four hours and rinse out your Thermos bottle. Take a bottle brush and brush out the remnants of the coffee stains. Wash the bottle in warm soapy water and rinse under warm running water to eliminate the vinegar smell.
Fill the Thermos bottle with crushed ice. Add the juice of one fresh lemon and a tbsp. of coarse salt. Mix thoroughly to distribute the ingredients. Leave this mixture to sit overnight. Empty the ingredients out and scrub the inside of your Thermos bottle with a bottle brush allowing the residual salt and lemon juice to do the work by leaving it to sit for another hour. Wash the Thermos bottle in warm soapy water and rinse thoroughly under warm running water.