Stable isotope analysis of amino acids: a new tool for food authentication

Stable isotope ratios have been used for thirty years for food authentication and in the last few years emerging methods aimed at individual chemical compounds have provided a means of obtaining a more in-depth understanding. Here, in particular, the feasibility of using compound-specific nitrogen and carbon isotope analysis of plant-derived amino acids was investigated in two different studies. Initially, d15N and d13C determination of amino acids was used to discriminate between conventional and organic wheat grown using synthetic nitrogen fertilizers, animal manure, or green manure from nitrogen-fixing legumes. After protein hydrolysis and derivatization, determination of amino acid d15N and d13C values was carried out by gas chromatography combustion isotope ratio mass spectrometry (GC-C-IRMS). The results demonstrated that d13C of glutamic acid and glutamine in particular, but also the combination of d15N and d13C of 10 amino acids, can significantly improve the discrimination between conventional and organic wheat, as compared to stable isotope analysis of bulk tissue. Subsequently, the variability of d15N values along the wine production chain in different vineyards and the influence of the fermentation process (e.g. different yeasts, white and red vinification, etc.) were investigated for the first time. Furthermore, compound-specific d15N analysis of proline, extracted using a resin from grape juice and wine, was performed by GC-C-IRMS for the first time. Despite nitrogen isotope fractionation along the oenological chain, the d15N values of leaves, grapes, wine and proline reflected the nitrogen isotopic signature of the soil. In particular, proline values in both grape must and wine were very similar to the d15N values of the soil used for plant growth. It was thereby demonstrated that d15N could be used as an additional and new isotopic marker to trace the geographical origin of wine.