Detection of exogenous sugars in pineapple juice using compound-specific stable hydrogen isotope analysis

Simon D Kelly,Peter Rinke,Andrew Cannavan,Aiman Abrahim

doi:10.1038/s41538-021-00092-5

Abstract

An improved procedure for determining 2H/1H isotope ratios, using gas chromatography-isotope ratio mass spectrometry, has been used to detect the addition of exogenous C4-plant-derived sugars to pineapple juice. Isotopic techniques are commonly used to identify the addition of low-cost sugars to fruit juices and are difficult to subvert as it is not economically viable to change the isotopic ratios of the sugars. However, the addition of cane sugar to pineapple juice has presented a significant challenge that is only detected by site-specific 13C analysis of the methyl and methylene positions of ethanol derived from pineapple sugars, measured by nuclear magnetic resonance. This new GC-IRMS-based procedure utilises the trifluoroacetate derivative of sucrose to allow direct measurement of the carbon-bound non-exchangeable hydrogen. This provides advantages over alternative isotopic methods in terms of analysis time and sensitivity. This feasibility study has demonstrated the potential to reliably differentiate between authentic pineapple juices and those adulterated with commercial beet and cane sucrose.

Highlights

Stable isotope analysis has been effectively used to detect the economically motivated adulteration of fruit juices for decades. This has involved both the use of gas isotope ratio mass spectrometric analysis, for example, carbon and oxygen isotopes in the soluble solids and water in fruit juice, respectively, and sitespecific deuterium/hydrogen ratios (2H/1H) by quantitative deuterium nuclear magnetic resonance of ethanol fermented from fruit juice sugars[1,2]
The bulk δ13C values of the total sugars generally fall in line with those in the ‘Reference Guideline for Pineapple Juice’ between −13.5 and −11.0‰ reported by AIJN—European Fruit Juice Association[20]
Sugar δ13C values have been measured outside this range and this is attributed to the pineapple plant’s Crassulacean Acid Metabolism (CAM) adapting to its local environment to a greater extent than plants with C3 or C4 metabolism

Summary

INTRODUCTION

Stable isotope analysis has been effectively used to detect the economically motivated adulteration of fruit juices for decades. The addition of C4 sugar syrup to fruit juices, for example, produces a deviation from the normal δ13C value expected for a C3 plant-derived food product, which can be detected using carbon stable isotope ratio analysis. The climatic conditions under which pineapples are cultivated mean that the CO2 fractionation associated with the CAM photosynthetic pathway tends towards that of the C4-plant metabolism producing similar global δ13C values This deficiency was overcome in 2010 by the development of 13C SNIFNMR analysis (site-specific 13C/12C ratio measurements) of the methyl and methylene positions of ethanol derived from pineapple juice sugars with a detection limit of C4 sugars around 15% of the total present[17]. The cheapest common adulterant in pineapple juice is cane sugar sucrose

DISCUSSION

Findings

METHODS