HPLC/EA-IRMS: Identifying adulterated coconut juice using isotope fingerprints

Importance of the topic

The authenticity of coconut water has gained considerable attention due to its classification as a fruit juice in Europe and its perceived health benefits as a low-carbohydrate beverage. Recent trends in adding sugar to enhance flavor present a risk of fraudulent labeling, undermining consumer confidence and regulatory compliance.

Study Objectives and Overview

This study aimed to evaluate carbon isotope fingerprints for detecting the addition of C4-plant sugars to commercial coconut juices. By comparing isotope signatures of bulk sugars and individual sugar components (sucrose, glucose, fructose) from authentic and commercial samples, the work assesses the sensitivity and reliability of these approaches.

Methodology and Instrumentation

Authentic coconut samples (pulp, total sugars, individual sugars) were prepared in-house, while 24 commercial bottled coconut waters were purchased. Pulp δ13C values were measured by Elemental Analysis Isotope Ratio Mass Spectrometry (EA-IRMS) following centrifugation and solvent washes. Total sugars were precipitated with Ca(OH)2, neutralized, and isolated. Individual sugars were filtered and analyzed by coupling High-Performance Liquid Chromatography with EA-IRMS (HPLC/EA-IRMS). All samples were processed according to ENV12140 guidelines and bracketed with calibrated reference materials against VPDB.

Used Instrumentation

• Thermo Scientific EA IsoLink IRMS System
• Thermo Scientific LC IsoLink IRMS System

Main Results and Discussion

Authentic coconut juices exhibited δ13C values in the C3 plant range (–33‰ to –22‰), consistent with guideline limits. Bulk sugar analysis flagged 5 of 24 commercial samples (21%) as adulterated using a –21‰ cutoff. Analysis of individual sugars improved detection, identifying 9 of 24 samples (38%) with added C4 sugar and achieving a detection limit below 10% addition.

Benefits and Practical Applications

Carbon isotope profiling of specific sugars offers enhanced sensitivity for authenticity screening. This approach can be implemented in food quality control, regulatory testing, and supply-chain verification to ensure accurate labeling and prevent economic fraud.

Future Trends and Potential Applications

Further developments may include multi-isotope analyses, automated sample preparation, and integration with non-targeted screening tools. Expansion to other high-value juices and development of rapid field-deployable devices could broaden the impact of isotope fingerprinting in food authentication.

Conclusion

Individual sugar isotope analysis by HPLC/EA-IRMS significantly improves the detection of C4 sugar adulteration in coconut water compared to bulk measurements, offering a robust method for safeguarding product integrity.

References

• Psomiadis D, Zisi N, Koger C, Horvath B, Bodiselitsch B. Journal of Food Science and Technology 2018, 55, 2994.
• Farquhar GD, Ehleringer JR, Hubick KT. Annual Review of Plant Physiology and Plant Molecular Biology 1989, 40, 503–537.
• O’Leary M. Bioscience 1988, 38, 328–336.
• AIJN. Provisional Reference Guideline for Coconut Water/Juice 6.27; European Fruit Juice Association: Brussels, Belgium, February 2017.