Food can be classified as either alkaline, neutral or acidic. What classifies food as either acidic or alkaline is a urine pH test. A food will either increase the acidity of urine after digestion or turn the urine alkaline. Generally meats, grains and other high protein foods are acidic while fruits and vegetables are alkaline foods. Of course these are guidelines, and there are exceptions.
While the majority of fruits have an alkaline positive balance on the pH scale, a few fruits are acidic. These include plums (also prunes) and cranberries. However, some fruits which are very acidic before digestion, such as limes and lemons, actually become alkaline forming after being metabolized. Stephen Cherniske, M.Sc. author of "The Acid/Alkaline Mystery Solved" says "the metabolism of most all types of fruit results in an alkaline ash, with the exception of cranberries and some varieties of plums."
According to "The Acid Alkaline Food Guide: A Quick Reference to Foods" all grains are acid forming foods. These include brown and white rice, pasta, oats and wheat bread.
Like grains, meats and other protein-based foods are nearly all acid forming. However some are less so than others. Beef is the strongest acid forming meat, while oily fishes can be be alkaline forming. Eggs are a strongly acid forming protein-based food.
Almost all dairy products are acid forming foods. However The Mayo Clinic Diet Manual, seventh edition classifies butter as neutral and whole milk as alkaline. The most acidic dairy foods tend to be those which require fermentation and contain active cultures or bacteria. These include yogurt with live cultures, fermented cheeses, buttermilk and sour cream.
Soft and distilled water is slightly acidic. In theory distilled water should test neutral, but once oxygen and carbon dioxide mixes after a bottle is opened it would test acidic on a pH scale. In contrast hard water, which contains minerals, is alkaline forming.