The Organic Acids of Lemon Fruits

The acidity of lemon juice is due chiefly to citric and malic acids. If other acids are present, they exist in very small amounts. The citric acid determined on the pure juice does not differ significantly from the amount precipitated by lead acetate. The total acid content precipitated from lemon juice by lead acetate is equal (within experimental error) to the sum of the citric and malic acids. 2. The difference between the free-acid fraction and the total organic-acid radical (citric and malic) is relatively much greater than the combined acids determined from the alkalinity of the ash. Since this difference is not reflected in the titratable acidity or pH values, it probably represents the quantity of organic acid in the ester form. The alkalinity of the ash represents the organic acid combined in salt form with the excess inorganic cations. 3. The titration curve of lemon juice is very similar to that of a pure citric acid solution, provided a correction is made for the combined acid naturally occurring in the juice. There is a definite relation between the pH and the amount of acid in the salt or combined form. The small fluctuations in pH of mature lemon juice are correlated with the large ratio of free acid/combined acid. 4. The free acids (milligrams per milliliter) increased and the pH of the juice decreased with increase in fruit size. The large reduction in pH (5.20 to 2.60) which occurred in fruits 2.0-4.0 cm. in diameter was due to a correspondingly large increase in free acid. Further increase in fruit size (up to 6.0 cm. in diameter) resulted in a slight decrease of approximately 0.3 of a pH, while the free acids gradually continued to increase in the juice.