Cranberries are members of the same plant family that includes blueberries and Rhododendron flowers, but they are similar to citrus fruits in their sour taste. The tartness of cranberries may be due to the amount of natural fruit acids they contain.
Cranberries do contain citric acid, but a very minute amount compared to true citrus fruits such as lemons and grapefruit. According to biochemical company Sigma-Aldrich, a 6-ounce serving of a commercially prepared cranberry juice cocktail contains 0.28 grams of citric acid. Cranberries also contain ascorbic acid, which is another name for vitamin C. A cup of whole cranberries provides you with 13.3 milligrams of ascorbic acid.
The citric acid content of cranberry juice helps make it acidic. A study published in the "European Journal of Clinical Nutrition" in October 2002 reported that drinking cranberry juice made urine more acidic -- a property that might be useful in preventing urinary tract infections and treating bladder and kidney stones. Cranberries also contain compounds called proanthocyanidins, which may help prevent infection-causing bacteria from sticking to the walls of the urinary tract.