Optimization of a stir bar sorptive extraction method for the determination of varietal aroma compounds in grapes

The current tendency in the extraction procedures is to develop accurate, easy-to-automate, and sensitive methodologies which can reduce sample handling. Stir bar sorpive extraction (SBSE) is a recently developed technique in which a stir bar coated of polydimethylsiloxane (PDMS) is employed to extract analytes directly from minimally treated matrices. A magnetic stirring bar is added to the sample to promote the transfer of analytes to the polymer coating and, after a predetermined extraction period, the analytes are thermally desorbed in the GC injection port. To date, just a few applications use this technique for detecting volatiles directly in grape berries. Therefore, the aim of this study was the optimization of SBSE method for the determination of free varietal aroma compounds in grape using the response surface methodology. Grape extract from Muscat was obtained using a tartaric extraction buffer at pH 3.2 for the pulp and methanol for the skin. The extraction analytical conditions were optimized using a two-level fraction factorial design (25-1 FFD) expanded further to a central composite design (CCD). The optimal conditions obtained were: 40 mL of sample extracted at 750 rpm during 80 min, employing 20 mm stirring bars and with an addition of 25% of NaCl. The optimized method was further successfully validated. Calibration curve, linearity, precision (inter and intra assay), detection and quantification limits, and recovery were studied. Terpenoids, C13-norisoprenoids, benzenoids, C6 compounds were the main compounds identified. The results achieved suggest the effectiveness of SBSE for the analysis of these aroma compounds in grapes.