Sensory and aroma impact of mitigation strategies against sunburn in Riesling

Abstract

Climate change is an environmental challenge that impacts the wine industry due to frequent sunburn damage triggered by heat waves, sun radiation and severe water deficits. This leads to severe yield losses and sensory changes in the resulting wines, presumably caused by climate-induced off-flavours. This study aims to develop viticultural and oenological strategies to mitigate sunburn damage in the highly sensitive variety Riesling and its detrimental sensory properties. In 2020, we combined the timing and intensity of defoliation measures with the application of kaolin and calcium hydroxide suspensions, reflecting a portion of the solar radiation. Seven treatments were replicated in three field experiments. Grapes from each field replication were maintained as a fermentation replication and, therefore, separately vinified following a standardised protocol. Replicates, including field and fermentation replicates, were further used as sensory replications. Descriptive analysis (DA) and temporal dominance of sensations (TDS) revealed a significant sensory impact of experimental trials in eight of twelve attributes. Additionally, wines were analysed by gas chromatography-mass spectrometry. The early sun exposition treatment with partial defoliation of the grape zone after flowering, followed by a second defoliation at berry closure, reduced fruity aroma but increased smoky notes due to enhanced 4-vinylguajacol formation as well as the intensity of the atypical ageing note (ATA) reminiscent of acacia blossom, naphthalene and fusel alcohols. Applying kaolin or calcium hydroxide particles on the berry skin slightly mitigated these unpleasant effects and even increased fruitiness and sweetness. Late defoliation at berry closure diminished the green note, which was most prominent in the non-defoliated control. Further smokiness and ATA intensities were lowered, in contrast to their rise due to early defoliation. However, late defoliation increased 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN), causing the petrol off-flavour. TDS analysis revealed a higher fruity and sweet dominance during the first 10 seconds due to early defoliation, while late defoliation fostered dominant and lingering sourness. In conclusion, early defoliation, which lowers sunburn incidence due to an early adaptation towards sun exposure, increases smoky and ATA flavours and diminishes fruitiness. These detrimental sensory effects could be significantly mitigated by applying reflecting particles of kaolin and calcium hydroxide on berry skins.