From what I understand different foods leave a residue or ash in the body that can be acid or alkaline. Tomatoes, oranges etc are acidic in the stomach but the ash they leave in your cells is alkaline (I think true for all fruit and veg). Stomach acid is required to absorb some nutrients like calcium that neutralize acids in the cells so if you don't have sufficient stomach acid then you become acidic on a cellular level. I use 'Arm & Hammer' baking soda from the drugstore. There is a reasonably good article (below) that may help explain or do a search on google to get additional info although beware as there is quite a bit of conflicting info out there. The baking soda works and if you change your diet then it should be a long term change. http://www.price-pottenger.org/Articles/Acid_alk_bal.htm
Keep us posted.