Capillary gas chromatography (CGC) can be used to check the authenticity of various fruit juices by analysing their oligosaccharide fingerprint profile. Thus, it enables the analyst to detect, for instance, the dilution of apple juice concentrates with cheaper sweeteners such as invert sugar syrup. In order to detect this, two marker peaks (IS1 and IS2) that are present in most invert sugar syrups are monitored in the CGC chromatogram of apple juice. In freshly pressed apple juice, these peaks are normally not found. As a result of apple juice processing and, in particular, the thermal treatment required to concentrate and pasteurise the juice, these peaks may however be present in authentic apple juice concentrate and juice made from it. It is nevertheless possible to distinguish between juice showing this "heat effect" and that to which invert sugar has been added since the IS2/IS1 peak ratio is very different in heated apple juice and in invert sugar syrups. Measuring this ratio and setting a maximum IS2/IS1 cut-off limit enables the analyst to assess whether the sample meets the set norms or has been adulterated. In the past, many laboratories experienced difficulties when comparing the results of this analysis and, in particular, IS2/IS1 peak ratios. We present evidence that this unsatisfactory situation is mainly due to the chromatographic behaviour of the IS1 peak. It appears indeed to be difficult to set up chromatographic conditions in all laboratories in order to produce reproducible results of the IS2/IS1 ratio. We have therefore examined the possibility of standardising results of this analysis and demonstrate that the comparison of any unknown apple juice sample with an external apple juice reference sample enables different laboratories to reach similar conclusions. The IS2/IS1 peak ratio in this reference sample was adjusted to the significant cut-off limit. The assessment (fail or pass) is thus based on a standardised IS2/IS1 ratio that is independent of several important analytical parameters which tend to strongly influence it.
