ALKYL PYRAZINES DETERMINATION BY GAS CHROMATOGRAPHY – ION MOBILITY SPECTROMETRY. THE ROASTED HAZELNUT CASE STUDY

Gas chromatography coupled with ion mobility spectrometry (GC-IMS) is an emerging analytical technique which is rapidly gaining popularity in food flavour analysis due to its robustness, high sensitivity and the second-dimension separation provided by IMS. Most of the current studies are based on untargeted fingerprinting and qualitative approaches. However little research is focused on quantitative studies targeting specific classes of aroma compounds. This is due to two peculiarities of the IMS working principle that makes the quantification challenging: (i) the formation of multiple ionized species (monomer and dimer) from a single analyte, and (ii) the non-linear detector response. In this study, we focused on alkyl pyrazines and their content determination in roasted hazelnuts. Pyrazines, which are Maillard reaction products generated during thermal treatment, are responsible for roasty and earthy notes characteristic of roasted food matrices. Several alkyl pyrazines have been reported as roasted hazelnut key-odorants. Due to their low odour-thresholds, they are crucial to determine the aromatic profile even though their concentrations in kernels is low (ng/kg), thus providing an interesting case study for a GC-IMS targeted and quantitative approach. The concentration-response curves of 8 alkyl pyrazines over several orders of magnitude of concentrations (0.1-100 μg/g) have been studied in two different model matrices (a mix of medium chain triglycerides – MCT – and a hazelnut paste physically treated to remove the majority of volatiles components). The results showed a non-negligible matrix effect, explained by the different fat percentage in MCT and hazelnut paste, and a relevant impact of the pyrazine ring substitution pattern on the concentration-response curve trends, highlighting the need of an external standardization approach to perform a reliable quantification. Five alkyl pyrazines have then been identified in hazelnut paste samples obtained by roasting kernels from different geographical regions (Italy and Turkey). The implementation of a quantitative approach extends the GC-IMS applicability for targeting specific aroma compound classes. This methodology could be successfully applied for food flavour characterization in the agro-industrial field.