ANALYSIS OF THE ORGANIC ACIDS OF ORANGE JUICE

Abstract

The total acidity of orange juice is ordinarily determined by titrating a known volume of juice with a standard solution of NaOH, with phenolphthalein as indicator, the result being expressed as citric acid. This value includes all the substances of an acidic nature in the juice that react with NaOH. It so happens, however, that in most fruits the chief substances reacting with NaOH are the organic acids; for this reason, the titratable acidity represents fairly well the organic acid content of a given juice. The predominating organic acid in orange juice is citric acid ; but other organic acids, namely, tartaric, malic, benzoic, and succinic, have been reported present (2). According to Scurti and De Plato (11), the acidity of the orange is due to citric and malic acids. Menchikovsky and Popper (?) have reported that in the grapefruit of Palestine, 98.72 per cent, of the total acid is citric, 0.05 per cent, tartaric, 1.0 per cent, malic, and 0.23 per cent, oxalic. Nelson (7) found that the nonvolatile fruit acids of California lemons were chiefly citric, with a trace of 1-malic acid. For Florida orange juice, Hartmann and Hillig (5) reported a citric acid concentration of 0.92 per cent, and a malic acid concentration of 0.18 per cent., fresh weight. Hall and Wilson (3) identified formic acid in the volatile fraction of orange juice. All of these acids, if present in the juice, would have an additive influence on the titratable acidity. It was therefore decided to determine the amounts of citric and malic acids in orange juice and correlate these with the free-acid content as determined by titration with standard NaOH. Undoubtedly, other organic acids are present in the juice in minute amounts, but in the present work it has been assumed that they are negligible in comparison with the concentrations of citric and malic acids. Furthermore, the concentrations of combined and free acids have been determined on various juice samples, and the relation between the combined acid and the inorganic cations in the ash of orange juice has. been ascertained.