Strawberry: They may stain your shirt, but they could work to whiten teeth, since they contain an enzyme called malic acid. Try mashing up strawberries and rubbing them on your teeth. Leave for five minutes then rinse with water and brush and floss as usual.

Apple, Celery &amp; Carrot: These healthy fruits and veggies act like natural stain removers by increasing saliva production, which serves as the mouth's self-cleaning agent. They also may kill bacteria that causes bad breath, and their crunch factor works to scrub teeth clean.
<blockquote>“ One 2008 study found toothpastes that contain baking soda — long considered a tooth-whitening home remedy — do a better job of removing plaque than toothpastes without the ingredient.”</blockquote>
Orange &amp; Pineapple: Eating tart fruits such as oranges and pineapples may cause the mouth to produce more saliva, which washes teeth clean naturally. Beware of using lemons directly on teeth, because too much acidity can damage teeth.

Source: www.aarp.org