Study of the grape glycosidic aroma precursors by crossing SPE-GC/MS, SPME-GC/MS and LC/QTOF methods

Abstract

Depending on the variety, grapes contain several chemical classes of aromatic compounds (i.e., terpenols, norisoprenoids, benzenoids) mainly stored as glycosides in berry skin. These secondary metabolites are the aromatic precursors fraction of grape which is liberated in wine during fermentation. Knowledge of their profile is often required to estimate the aromatic potential transferable to the wine and for chemotaxonomic aims (Nasi et al., 2008; Ferreira and Lopez, 2019).

In general, the methods used to study glycosidic aroma profile involve sample extraction and concentration by passage of large volumes of must or grape extract through a SPE cartridge (the stationary phases commonly used are 1-10 g of C18 or polystyrene-divinylbenzene), then the methanolic fraction eluted containing the glycoside compounds is carried to dryness, resolubilized using a citrate pH 5 buffer, and an enzymatic hydrolysis is carried out overnight to liberate the aglycones which are then analyzed by GC/MS. Main advantage of SPE is until 1000-fold concentration of sample which allows to detect also compounds present at low level but which can play important role in determining the organoleptic characteristics of wine. Usually, the selectivity of SPE towards the compounds studied is low, so performing quantitation by expressing the compounds as mg internal standard/kg grape provides accuracy acceptable for the aim of the study. On the other hand, SPE is laborious, needs long time and is hardly applicable in quality control laboratories. SPME is faster but the selectivity of fiber towards the analytes is often very different and to perform acceptable quantitative analysis it is essential the calculation of calibration curves. Unfortunately, just few standards of the grape aroma compounds are commercially available (Panighel et al., 2014).

In this study SPE-GC/MS and SPME-GC/MS methods are compared by performing analysis of a set of model standard solutions and grape must samples. The use of several internal standards allows to estimate recoveries of the analytes and calculation of corrective coefficients between the two methods. To have also information free of enzymatic artifacts, GC/MS results are crossed with profile of glycosidic aroma precursors determined by LC/QTOF analysis (Flamini et al., 2014).

The study is finalized to develop a quick SPME-GC/MS method which provides exhaustive and reliable qualitative and semi-quantitative information on the grape glycosidic aroma precursors

References

Nasi A., Ferranti P., Amato S., Chianese L. (2008). Food Chem. 110: 762-768
Ferreira V., Lopez R. (2019). Biomolecules 9(12): 818- doi:10.3390/biom9120818
Panighel A., Flamini R. (2014). Molecules 19: 21291-21309 doi:10.3390/molecules191221291
Flamini R., De Rosso M., Panighel A., Dalla Vedova A., De Marchi F., Bavaresco L. (2014). J. Mass Spec. 49(12): 1214-1222 doi:10.1002/jms.34411214