From the time of fruit setting early in the spring and throughout the whole summer, samples of fruits were taken directly from the tree. In the fall several bushels of apples were picked and placed in cold storage. These constituted the source of material during the winter. Weekly samples of fruits were analyzed for pH and titratable acidity of their sap, and also for their organic acids content. Various stages in the acid metabolism of fruits were found to correspond chronologically very closely with those of carbohydrate and respiration. During the first two to four weeks of the fruit ontogeny, the pH of the apple juice falls rapidly to a value of about 2.8. Thereafter there is a slow, steady rise which continues until the next spring. This regularity makes the pH of the juice a better indicator of the physiological age of a fruit than either its chemical composition or respiratory rate. Initially, the malic acid content of a fruit is low, and some unknown organic acids predominate. This relation is reversed within the first two to four weeks of growth and during the rest of the ontogeny malic acid forms about 80% of the total organic acids of a fruit.
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Krotkov et al. (1951) studied this question.