Detection of exogenous sugars or organic acids addition in pineapple juices and concentrates by C-13 IRMS analysis

When pineapple concentrate is adulterated, organic acids are often added to ''naturalize'' the chemical composition. To improve isotopic methods devoted to the study of pineapple product authenticity, three major components (sugars, malic acid, and citric acid) have been used as individual probes for the determination of carbon-13 isotope ratios. After a cleanup step, sugars and organic acids were separated from each other by an anion exchange process, and pure malic and citric acids were isolated by preparative reversed-phase HPLC. This method has been applied to the stable isotope analysis of pineapple juice samples from different locations and production years. A correlation between the carbon isotope ratios of sugars and organic acids has been observed, and cutoff points concerning the difference of delta(13)C values between those metabolites have been defined. In the case of the addition of sugar from C3 plants (such as beet); it has been demonstrated that the detection limit can be as low as 10% or even 5%, whereas it is often >20% when using the conventional carbon-13 method on the whole juice. Malic acid and citric acid addition also becomes detectable using these individual measurements.