Fig12.19 tannins are responsible for the brown stains in coffee mugs or on a coffee drinker's teeth Because tannins are acidic they can be readily removed with an alkaline cleanser. Use a little laundry bleach on the mug, and brush your teeth with baking sodaFig12.19 tannins are responsible for the brown stains in coffee mugs or on a coffee drinker’s teeth. Because tannins are acidic, they can be readily removed with an alkaline cleanser. Use a little laundry bleach on the mug, and brush your teeth with baking soda