Identification of several glycosidic aroma precursors in six varieties of winemaking grapes and assessment of their aroma potential by acid hydrolysis

In winemaking grapes, it is known that most aroma compounds are present as non-volatile precursors, such as glycosidic precursors. In fact, there is strong evidence supporting the connection between the content of aroma precursors and the aromatic quality of wine ^[1]. Acid hydrolysis is preferred to reveal the aroma potential of winemaking grapes, as it predicts more accurately the chemical rearrangements occurring during fermentation in acidic environments ^[2]. In this study, a method involving a fast fermentation followed by acid hydrolysis at 75ºC was used to evaluate the accumulation of aroma compounds over time in fractions obtained from six different varieties of winemaking grapes. The aim was to identify the specific aroma glycosidic precursors responsible for this variability by confirming their structures and establishing correlations with the liberated aromas. The variability of aroma compounds with respect to both time and variety was investigated through a two-way ANOVA. A Principal Component Analysis of the volatile aromas confirmed the differences between grape varieties and hydrolysis time. The most notable differences were observed in the hydrolysates at 96 hours, where the accumulation of aromas such as norisoprenoids and phenols was prominent. A method involving Solid Phase Extraction followed by UHPLC-QTOF-MS/MS was employed to isolate and identify the pool of aroma precursors. Most of the glycosides identified based on their fragmentation patterns were indeed disaccharides with the structure of pentose-hexose and hexose-deoxyhexose. However, it should be noted that a few trisaccharides and monosaccharides were also identified in the study. The identification of some of these glycosides was further confirmed through direct correlation with the corresponding accumulated aroma compound. This study provides further evidence of the significant aromatic potential of winemaking grapes through their pool of aroma precursors, and it also suggests the possibility of exploring alternative methods for treating grapes to evaluate their aromatic potential.

Acknowledgements:

This work was funded by the Spanish Ministry of Science and Innovation (MICIN) (project AGL2017-87373-C3-1-R). E. S. A. has received a grant (PRE2018-084968) from the Spanish FPI programs associated to the same project. LAAE acknowledges the continuous support of Gobierno de Aragón (T29) and European Social Fund. 

References:

1. Ferreira, V., & Lopez, R. (2019). The actual and potential aroma of winemaking grapes. Biomolecules, 9(12), 818. DOI 10.3390/biom9120818
2. Loscos, N. et al. (2009). Comparison of the suitability of different hydrolytic strategies to predict aroma potential of different grape varieties. Journal of Agricultural and Food Chemistry, 57(6), 2468–2480. DOI 10.1021/jf803256e