EFFECTS OF INDUCED SUNBURN DAMAGES ON WHITE WINE PROPERTIES

Abstract

Climate change is a great challenge for the environment and affects the wine industry as well. Sunburn damage of sensitive grapes increase with severe heat periods. Besides significant loss of yield sunburn, modifies sensory properties of the wines and may cause climate-related off-flavours. To initiate sun-burn in a controlled way, in 2021 sunburn was directly induced in the vineyard with the GrapeBurner device, exposing grapes of the varieties Riesling and Pinot Blanc with UV and IR radiation. This device was first assembled by Kai Müller of the university in Geisenheim and consists of a carriage with 6 UV/IR lamps. A 15 min irradiation was applied in early September at 60°Oe. Due to the colder season in 2021 the grapes were not harmed by previous sunburn damage. Nevertheless, we used non-defoliated grapes facing north of the vines to ensure that putative damage was only due to applied irradiation and not previous sun exposure. Three weeks after the treatment, a control and the irradiated grapes were harvested and directed to small scale winemaking following a standardised protocol. Sensory evaluation using descriptive analysis with a trained panel was complemented by aroma analysis using an established head space solid phase micro extraction GC-MS method.

For Riesling, the control had a sweeter and fruitier taste, and was reminiscent of ripe fruit, which correlated with a high content of ethyl esters. In contrast, wines from irradiated Riesling grapes had a smokier flavour and correlated with plant derived odorants such as vitispirane, linalool, β-damascenone, and 4-vinylguaiacol. Also the sensory panel described the wines from treated grapes as more astringent and bitter, as well as with a more intense yellow colour. These results were backed by Pinot Blanc trials, as the control was perceived more fruity due to enhanced ethyl esters. Vice versa, wines from the irradiated grapes were more smoky and astringent. In addition, 2-aminoacetophenone was enhanced, causing the atypical aging off-flavour.

These promising results stimulated further trials, in which the length of exposure was varied gradually, to determine a threshold from which irradiation is causing negative sensory and compositional changes. In parallel, mitigation strategies such as applying reflecting particles or adaptation to sun exposure by early defoliation will be examined in lieu of varying UV/IR radiation.