What an interesting question! I've never really thought about the pH content of foods - not since high school biology, anyway. Basically every food except Camembert cheese and egg white seems to be acidic to some degree, according to the US Food and Drug Administration
The unfortunately-named Dr Mirkin
from Georgetown University School of Medicine thinks that it's a nonsensical idea to worry about eating acid or alkaline foods - "You should not believe that it matters whether foods are acidic or alkaline, because no foods change the acidity of anything in your body except your urine."