ESTIMATION OF ASCORBIC ACID IN
ORANGE JUICE BY A CHRONOMETRIC METHOD
Abstract. The redox couple between ascorbic acid (vitamin C) oxidation and reduction of the
semiquinoid form of p-phenylenediamine (PPDA) is the basis of a new chronometric assay of
vitamin C. Commercial horseradish peroxidase or citrus peroxidase isolated from seeds catalyzes
the hydroperoxidation of PPDA through a re-versible semiquinoid form to a colored condensation product. Ascorbic acid interrupts formation of the colored product by coupling its oxidation to
reduction of the semiquinoid. After all the ascorbic acid is oxidized, oxidation of the semiquinoid
proceeds to the dye form. By using a standard peroxidase preparation, ascorbic acid content of
orange juice was shown to be inversely proportional to the time lag from initiation of the re
action to first appearance of colored product. A rapid "yes-no" method for testing juices for labeling compliance and a simple method for quantitating ascorbic acid levels in citrus juices were developed