The acid level in lemon juice makes it ideal for cleaning pennies, according to About.com. The juice should be diluted with water, with salt added to the mixture. The pennies need to be left to soak in the mixture for five to 15 minutes and then removed and dried.
Pennies are partly comprised of copper, which oxidizes when it comes into contact with air. The oxidation forms a dull-colored coating over the outside of the penny. An acid dissolves the coating, but too much acid can actually begin to break down the coin. Lemon juice contains a relatively low level of acid. About.com also recommends using vinegar for cleaning pennies.