Acidic foods tend to break down the enamel which can expose the layer of tooth inside the enamel layer called dentin. 

If an apple or other acidic food is eaten slowly without rinsing your teeth it can be 3.7 times more likely to damage the enamel on the tooth.  It is best to eat acidic foods and drinks quickly and then rinse with water after eating or drinking.  Brushing immediately after eating acidic foods is not recommended as this will remove some of the teeth’s surface.   It is best to rinse with water and then brush an hour later.  This allows the saliva to re-mineralize the enamel on your teeth and makes the enamel more resistant to acid attack.