What is the effect of coffee on litmus paper?

Coffee is typically acidic, with a pH level around 4.5 to 6, depending on the type and preparation. When litmus paper, which is used to test acidity or alkalinity, is exposed to coffee, blue litmus paper will turn red, indicating acidity. Conversely, red litmus paper will remain red, as it does not indicate a basic solution. Thus, coffee can be identified as an acidic substance using litmus paper.