Determination of selected phenolics, carotenoids and norisoprenoids in Riesling grapes after treatment against sunburn damage

Abstract

Riesling represents the most widely cultivated grape variety in Germany and is therefore of particular economic interest. During recent years an increase in the petrol-note as well as in undesirable bitter and adstringent notes has been reported. These changes are most likely linked to increasing temperature and sunlight exposure of grapes due to climate changes.
The “petrol note” is caused by the formation of the C13-norisoprenoid 1,1,6-trimethyl-1,2-dihydronaphthalin (TDN), which originates from acid-labile precursors formed by the carotenoid degradation in the grape. The negative orosensory changes are thougt to be related to phenolic components in wine since some polyphenols have already been described as astringent and/or bitter. The grape responds to increased sunlight exposure by storing polyphenols, especially flavonoids, in the berry skin. The question whether viticultural treatments such as applications of particle-film forming products like kaolin and calcium carbonate preparations to reflect sun light and to mitigate sunburn damage on grapes and thus minimize organoleptic defects as well as off-flavors in resulting wines has not yet been sufficiently answered. 
In this study, we investigated the influence of defoliation at different degree in conjunction with the application of particle suspension to protect against sunburn damage in respect to selected carotenoids, C13-norisoprenoids and polyphenols in grapes. For comparability and possible correlation of results, qualitative and quantitative determination of C13-norisoprenoid and polyphenols was performed from the same sample extract. The carotenoid profile was analyzed by UHPLC-DAD and HPLC-APCI-MSn. Quantification was performed by UHPLC-DAD as lutein equivalents using an internal standard (β-apo-8-‘carotenal). Quantification of C13-norisoprenoids was conducted via SIVA with deuterated standards by HS-SPME-GC-MS/MS. The qualitative and quantitative analysis of polyphenols was done by HPLC-ESI-MSn and UHPLC-DAD by means of external calibration with representative substances for respective substance classes. The applied treatments showed effects on the qualitative and quantitative profiles of the analyzed constituents in grapes. While increased sunlight exposure induced the degradation of carotenoids, the mean content of C13-norisoprenoids and polyphenols increased.