Vitamin C in orange juice

Importance of the Topic

Ascorbic acid (vitamin C) plays a vital role in tissue repair, enzymatic reactions and immune defense, while serving as a key antioxidant. Accurate quantification of this nutrient in fruit juices is critical for nutritional labeling, quality control and regulatory compliance in the food and beverage industry.

Objectives and Overview

This application note describes a photometric titration method in accordance with ISO 6557-2 to determine ascorbic acid content in orange and blood orange juices. By employing an automated titrator equipped with an optical sensor (Optrode), the procedure aims to enhance endpoint objectivity and reproducibility.

Methodology

Samples of orange and blood orange juice were spiked with oxalic acid to stabilize ascorbic acid, then centrifuged to remove pulp. Aliquots of the clarified supernatant were titrated with standardized 2,6-dichlorophenol-indophenol (DPIP) until the photometric sensor signaled the equivalence point.

Instrumentation

The analysis utilized a Metrohm 907 Titrando system with 800 Dosino dosing units and an 801 magnetic stirrer, together with the Optrode optical sensor offering multiple wavelength selection for precise endpoint detection.

Main Results and Discussion

Triplicate determinations yielded mean vitamin C concentrations of 363.5 mg/L in orange juice and 570.8 mg/L in blood orange juice, with relative standard deviations of 1.28% and 1.29%, respectively. The titration curves exhibited well-defined inflection points, confirming reliable and color-independent endpoint recognition.

Benefits and Practical Applications

This photometric titration approach delivers operator-neutral endpoint detection, is fully automatable and meets ISO 6557-2 requirements. It is particularly suitable for routine quality assurance in industrial laboratories, offering high throughput and consistent results.

Future Trends and Potential Applications

Integration of inline optical sensors with real-time data processing and machine learning algorithms could further refine endpoint detection in complex matrices. Additionally, miniaturized photometric titration devices and adaptation to other antioxidant assays may expand the method's scope in both research and industrial settings.

Conclusion

The ISO 6557-2-based photometric titration using DPIP and an optical sensor constitutes a robust, reproducible and automated technique for vitamin C determination in orange juices, offering objective endpoint identification and increased analytical throughput.

Reference

1. ISO 6557-2:2003. Fruit and vegetable juices — Determination of vitamin C content — Part 2: Photometric method.
2. Metrohm AG. AN-T-086: Vitamin C in orange juice — Photometric titration according to ISO 6557-2.