Estimation of Ascorbic Acid in Orange Juice by a Chronometric Method

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 condensa tion product. Ascorbic acid interrupts formation of the colored product by coupling its oxidation to reduction of the semiquinoid. After all the ascor bic 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 propor tional 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 label ing compliance and a simple method for quantitating ascorbic acid levels in citrus juices were de