Certain foods can increase and decrease the pH of your body, if your body becomes too acidic you might suffer from acidosis which can be severely damaging. Likewise blood serum that is too alkaline can also be damaging too, blood serum pH of approx. 7.3 is about right. Meats, eggs and particularly dairy can have acid forming effects on the body, but one of the most alkaline of all foods are the humble raisin. A raisins potential renal acid load (PRAL) is in the negatives at approx. -21 milliequivalents, whereas cheese can increase the bodies acidity with a milliequivalent of 34.2.
Collison, D. (2010). PRAL - Potential Renal Acid Load. A Measure of the Effect of Foods on the pH of the Body* Retrieved 13th August, 2014, from http://www.huntlycentre.com.au/updates/posts/view/118