This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
The acids in fruits are chiefly acetic, malic, citric, tartaric, oxalic, and in some instances salicylic and boric. Malic acid and malates occur in apples, pears, currants, blackberries, raspberries, quince, pineapple, cherries, and rhubarb. Citric acid and citrates occur in large quantities in lemons, oranges, grapefruit, and lime, and slightly in quince, gooseberry, strawberry, raspberry, currant, and cranberry. Tartaric acid occurs in grapes. Bertrand and Agulhon have found traces of boric acid in many fresh fruits and vegetables.
According to Blyth, the percentage of free acid present in the various fruits is as follows: Pear, 0.2; grape, 0.79; apple, 0.84; plum, 0.85; cherry, 0.91; peach, 0.92; strawberry, 0.93; apricot, 1.16; blackberry, 1.19; raspberry, 1.38; gooseberry, 1.42; prune, 1.5; mulberry, 1.86; currant, 2.15. Lemon-juice contains about 6 per cent. of citric acid.
It must be remembered that the relative acidity cannot be determined by taste, as the proportions of sugar differ in the different fruits. For example, while strawberries, currants, gooseberries, huckleberries, apples, pears, and prunes contain between 5 and 8 per cent of sugar, raspberries, blackberries, apricots, plums, and peaches contain less than 5 per cent.; cherries contain 10 per cent., and grapes, from 15 to 24 per cent. (Blyth, Fresenius). The amount of sugar also regularly increases with the ripeness of the fruit.