Acid-Base issues: Alkalosis, Alkalemia, Acidosis, Acidemia
There is an entire science related to acid/base issues and the acidifying and/or alkalising effects of various foods upon human metabolism. For full definitions of the relevant terms and full discussion of these topics, the reader is referred to the vast amount of material easily available on the internet -- from the extremely superficial and frequently misleading, to the seriously dense, underlying science, which is a specialised area of expertise and frequently a bane even of medical graduates.
For the purposes of this website, the important things to note are that the various food items of every person’s diet each metabolise within the body to give rise to either an alkaline or an acid final result, the overall sum of which then substantially determines the body’s overall acid/alkaline (“acid-base”) status. However, reliable results for individual foods can only be obtained with detailed, rigorous feeding of those foods for extended periods to individual human subjects who are isolated and accommodated in a locked research environment. These are very expensive experiments, and since the information is not, in the main, incredibly useful, actual experiments for relatively few foods have been done. Instead, calculated estimates of the acid or alkaline effect of many individual foods have been made from the known (but simplified) chemical make-up of the foods and from the (partially) known effects of these main chemical components. The main components (of each separate food) which have the relevant effects, and which can be easily analysed, are the protein, phosphorus, potassium, sodium, magnesium and calcium contents (some give an acid effect, some alkaline). The amounts of these easily-measured components, in each food, are used in a simple arithmetic formula to estimate the food’s probable overall acid-base effect, and it is only these estimates, either numerical or in word form, which are available for most foods and which are easily found in many places on the internet. Such a calculated estimate is known as the Potential Renal Acid Load, or PRAL, for that food . The PRAL method basically calculates all fruits and vegetables to be alkalising. Not much joy looking for acidifying fruits or vegetables there.
However, there are also organic compounds which have significant or even strong effects but which are not so easily measured when measuring the “easy” components listed above, and which are often not present, in numerous foods, at levels which would change the overall result a great deal. So the effects of these organic compounds are simply left out of the overall PRAL estimates. Organic acids are probably the main such ingredients. Most of these metabolise within the body to produce an overall alkaline effect. Citric acid is one example, and that is why lemons, which are very acid in a crude, "high-school chemistry" sense, mainly due to their citric acid content, give an overall alkaline result once metabolised, as do most fruits and vegetables.
Digging deeper however, one can find numerous completely unsubstantiated reports that cranberries, and perhaps one or two other fruits or vegetables, are “acidifying”. Where did these opinions come from? Well, that is not clear in the case of the claim for every such fruit or vegetable. But for cranberries and prunes (and therefore plums) I eventually tracked the claims back to two very old scientific studies [2,3].
In these studies, the upshot is that cranberries and prunes were given in actual feeding experiments to humans, and clearly demonstrated overall acidifying effects. The effect is mainly due to their quinic acid content. This is a natural fruit acid found in many other fruits, but more abundantly in cranberries, prunes and plums (at least compared to modern table fruits). Some organic acids, such as quinic acid and benzoic acid, metabolise to an overall acid result within the body. Most (eg citric acid, malic acid, etc) do not: they have the opposite effect.
Because comprehensive levels of all these organic acids are not known for many foods, and also for other reasons, PRAL-estimated acid-base effects of particular foods can vary considerably from their actual effects, and this is further exacerbated by seasonal, regional and ripeness variations in all foods.
As the raw, undried fruit, cranberries have a much stronger acidifying effect than plums. Prunes (dried plums) contain many times the sugar content of cranberries and thus seemed relatively less advisable to consume in the quantities which might be required.
 Remer, T., & Manz, F. (1995). Potential renal acid load of foods and its influence on urine pH. Journal of the American Dietetic Association, 95(7), 791-797.
 Blatherwick, N. R., & Long, M. L. (1923). Studies of urinary acidity II. The increased acidity produced by eating prunes and cranberries. Journal of biological chemistry, 57(3), 815-818.
 Fellers, C. R., Redmon, B. C., & Parrott, E. M. (1933). Effect of cranberries on urinary acidity and blood alkali reserve. The Journal of Nutrition, 6(5), 455-463.