Introduction

Climate change is having a huge influence on humanity, especially in the field of agriculture, including grape growing and, subsequently, on wine quality. According to Droulia et al. and the IPCC Climate Change Reports of 2007 and 2014 (Droulia and Charalampopoulos, 2021; IPCC., 2014) climate change is one of the greatest environmental concerns facing humankind in the 21^st Century. More frequent and severe extreme weather events such as heat waves and drought periods increase UV-B radiation harming leaves and grapes and intensifying evaporation which limits water availability of the vines. The sharp CO₂ rise enhances temperature, leading to growing evaporation rates not only in vines but also diminished soil moisture by 30 to 50 % in Western Europe (Schultz, 2000). At the same time, rising CO₂ concentration stimulates net assimilation rates in grape vines, more efficient water use and higher single berry weights but no differences in the number of bunches per vine as was found for the cultivars Riesling and Cabernet-Sauvignon (Wohlfahrt et al., 2018). According to the studies by Wohlfahrt et al., these effects of the changing CO₂ level could lead to an alteration of grape and wine composition. The effect of climate change on the aromatic potential of grapes and wine quality is described to lower herbaceous and vegetal notes due to rising average temperatures and increased formation of 3-isobutyl-2-methoxypyrazines due to water stress, both in Cabernet-Sauvignon and Sauvignon blanc (Pons et al., 2017). Nevertheless, concentrations of carotenoid derivates, namely C₁₃-norisoprenoids, rise when grapes are more exposed to light and higher temperatures, which can be both positive and negative for the aroma profile of a wine (Pons et al., 2017). An indirect consequence of rising temperature is the resulting water stress for plants, which has an influence on the physiology of the vines and, consequently, on the biosynthesis of aromatic compounds and their precursors as well. For the typicity of a German Riesling wine, climatic conditions play a huge role and are more important than other environmental factors in particular soil (van Leeuwen and Darriet, 2016). It is common practice to protect grapes against excessive sunlight to prevent the formation of off-flavours such as 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN) or the atypical ageing note (ATA) or those masking varietal character such as 4-vinylguajacol (Fischer, 2007). A rise in temperature is also relevant as the grapevine requires a minimal temperature sum during the vegetation period for proper vegetative growth and grape ripening, which are defined by the Huglin Index (HI) or Growing Degree Days (GDD) (Huglin, 1978). In contrast, high temperatures hinder sugar accumulation and favour malate degradation, leading to wines of low acidity (Coombe, 1987, Droulia and Charalampopoulos, 2021). Overall, a higher temperature can benefit the fruity and floral aroma of wines as long as it is not excessively high (Pons et al., 2017). Moreover, the aroma and, therefore, the occurring climate-related off-flavours of wine are not only dependent on a singular aroma compound but on the profile and interactions of the multiple odour-active compounds found in the wine and the release of aroma compounds from their odourless precursors during wine maturation (Robinson et al., 2014).

Global warming actuates warm spring temperatures, which leads to early flowering, faster ripening and, consequently, higher grape maturity with increased sugar accumulation (Fischer, 2007). Secondary metabolites such as anthocyanins and carotenoids are also increased to protect berries against detrimental effects, which results in wines with modified varietal aroma and deeper colour (Droulia and Charalampopoulos, 2021; González-Barreiro et al., 2015; Mori et al., 2007). Temperatures above 35 °C negatively affect the plant's photosynthetic system and trigger severe skin damage, which leads to the oxidation of polyphenols and shrivelling, as well as the higher incidence of fungal infections in grapes (Berry and Björkman, 1980; Steel and Greer, 2008). The phenomenon of sunburn can be differentiated between sunburn necrosis and sunburn browning. The latter is a result of both high sun exposure and high temperature, especially occurring after véraison. Sunburn necrosis can be led back to much higher temperatures than necessary for sunburn browning and is likely to appear yet at pre-véraison (Gambetta et al., 2021). Gambetta et al. mentions different strategies to mitigate sunburn, such as moderate or no defoliation, coverage by shading nets, and application of reflecting particles such as kaolin. Early defoliation after flowering seems to lower the vulnerability of grapes by promoting a higher accumulation of photoprotectants such as catechins, cinnamic acid derivates, anthocyanins and carotenoids. In contrast, removing leaves at a later stage at véraison had no mitigation effect, which was also true for not defoliated grapes (Gambetta et al., 2021, Pastore et al., 2013). Application of kaolin particles on the leaves of cv. Merlot did not affect the volatile composition of the resulting wine, while in the same trial, a water deficit enhanced terpene alcohols and C₁₃-norisoprenoids (Ou et al., 2010). In previous studies, defoliation measures, especially early leaf removal at flowering, induced compositional changes of the flavonoids in white wine where the content and concentration of phenolics such as skin quercetin glycosides and hydroxycinnamic acids significantly increased due to more intense illumination (Friedel et al., 2015). So far, the sensory implication of sunburn has not been studied yet. Data on the combination of defoliation with the application of reflective particle films are still very limited, especially regarding the aroma composition and sensory properties of the resulting wines.

Thus, it is the objective of the present study to investigate the main effects of timing and extent of defoliation and the application of a reflecting particle film, as well as their interactions, examining the composition of volatiles and sensory modification in Riesling wines.

Materials and methods

Results

1. Viticultural impact of mitigation strategies

Sunburn damages on clusters were assessed right before véraison. Late defoliation resulted in significantly higher sunburn frequency and severity than observed for early defoliation or for the non-treated control. The largest but not significant effect in lowering sunburn damage was due to the combination of early defoliation and application of kaolin. However, this putative protecting effect of kaolin was not observed when the application was made in the late defoliation treatment. Calcium hydroxide application did not affect sunburn frequency and severity at the early defoliation stage. However, applied in the late defoliation treatment, calcium hydroxide lowered the frequency of sunburn events but not their severity. These results only display the effects of viticultural treatments conducted in the vegetation period of 2020.

2. Sensory impact of sunburn mitigation treatments

According to the ANOVA (Table 2), eight out of twelve attributes, including colour, varied significantly among treatments. In contrast, replicates combining the field and fermentation variation were insignificant in all attributes except for colour.

First, the data with significant attributes are presented in the form of a spiderweb diagram (Figure 1) to illustrate the differences between the variations with different defoliation and application treatments.

Figure 1. Impact of viticultural treatments on odour and in-mouth intensities obtained from descriptive analysis (17 J × 2 Rep.; * p < 0.1, ** p < 0.01, *** p < 0.001, letters indicate significant differences among treatments according to Fishers Least Significant Difference (LSD) test, ATA = atypical ageing note).

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Wines were best discriminated by their yellow colour. The control without defoliation displayed the weakest yellow colour. Early defoliation yielded the deepest colour and the application of reflecting particles led to significantly weaker colours. The same effect of colour attenuation by the reflecting particles was observed for late defoliation, albeit at a lower level. Early defoliation showed the least intensity of citrus/mineral, while the application of reflecting particles on early defoliated grapes yielded significantly higher intensities. Parallel to the colour enhancement, the early defoliation showed the strongest smoky/honey notes and atypical ageing off-flavour. Early defoliation treatment alone and with particle application produced the sweetest wines, which is presumably not an effect of residual sugar.

The holistic presentation in a PCA in Figure 2 reveals in the loadings plot (a) a horizontal separation explaining 45.6 % of total variation by the off-notes dull/ATA (ortho- and retronasal) and smoky/honey on the right side versus the citrus/mineral and green notes on the left side. The vertical separation explaining 18.5 % of the total variance is governed by sweet and full-bodied wines in the upper half and, to a smaller extent, by ripe apple in the lower half.

In the score plot (b), replicated tasting of the two field/fermentation replicates of each treatment are mapped in proximity to each other. Early defoliation treatment plotted in the direction of dull/ATA and smoky/honey while showing low intensities of citrus/mineral and green/vegetative. Applying the reflecting particles kaolin (K) and calcium hydroxide (C) shifted the scores of the wines to the left, fruitier side. Overall late defoliation treatments were in the centre and lower part of the score plot indicating less dull/ATA and smoky/honey intensities but more pronounced citrus/mineral, green/vegetative and ripe apple notes. Reflecting particles modified the sensory properties of the wines to a smaller extent than during early defoliation, except for the calcium hydroxide application. The control without defoliation yielded wines with citrus/mineral and green flavours and the least sweetness and body.

Figure 2. Principal component analysis displaying the loadings of sensory attributes on the left (a) and scores of experimental wines on the right (b). Replicates combining field, fermentation and sensory variability are denoted with 1 and 2.

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3. Modification of aroma composition by sunburn mitigation treatments

For aroma analysis, all three replicates of the treatments were measured. According to ANOVA, 13 of the 17 analysed odourants varied significantly due to the treatments in Table 3. Variance due to field and fermentation replications was significant only for the esters ethyl-butanoate, ethyl hexanoate, ethyl octanoate and 2/3-methylbutyl acetate. Among the grape-derived aroma compounds, linalool varied the most between 26 and 63 µg/L and TDN between 0.9 and 3.2 µg/L. Both aroma compounds increased with defoliation at both time points (Figure 3). The application of reflecting particles lowered the TDN concentrations, but only calcium hydroxide in the early defoliation yielded a significant decrease in TDN. For linalool, a significant difference occurred only between the undefoliated control and all the other defoliation treatments. 4-Vinyl-guaiacol and β-damascenone, however, were not significantly modified by the treatments. Among the esters, variations of ethyl butanoate and ethyl hexanoate were most prominent, as they exceeded their published odour threshold by factors up to 20 and 70, respectively. However, no clear impact of mitigation strategies was observed. The impact of the treatments on 2-aminoacetophenone was the most significant one; however, the concentration in all experimental wines remained below the published odour threshold of 1.29 µg/L (Fischer and Sponholz, 2000). Nevertheless, the lowest concentration was found in the two treatments with the application of the calcium hydroxide suspension.

Figure 3. Concentrations of linalool and 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN) in the experimental wines and their general odour thresholds in the wine of 2.3 µg/L for TDN according to Ziegler et al. (2019) and 25 µg/L for Linalool according to Ferreira et al. (2000) (letters indicate significant differences among treatments according to Fishers Least Significant Difference (LSD) test).

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4. Temporal dominance of sensations (TDS)

Assessing the sensory modulation over time, the TDS graphs revealed the greatest differences among the defoliation treatments. The wines of early defoliated grapes all showed a higher sweetness dominance in the first 10 seconds. The dominating sourness lasted longer for the late defoliation treatments and non-defoliated control. Astringency dominance remained below the significance threshold, except for the application of reflecting particles on the late defoliated grapes, where astringency dominated after 30 and 40 seconds at a significant level (Figure 4). Analysis of variance (ANOVA) was applied to those dominance curves, which were above the chance level (p = 1/number of attributes, here: 12.5 %). Significance was determined for the parameters “maximum dominance rate” (D_max), “duration of dominance” and “area under the curve”, which were extracted from the curves for each attribute of the experimental treatments and replicates separately. Unfortunately, none of these parameters proved significant and are, therefore, not considered further.

Figure 4. Smoothed TDS curves of the different treatments. (a) no defoliation, (b)–(d) treatments with early defoliation, (e)–(g) treatments with late defoliation. Level of significance with α = 0.05, chance level 1/p with p = number of attributes (12.5 %) (17 J × 2 Rep.). Dominance curves not reaching the chance level are displayed transparently.

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Discussion

According to our study, an early defoliation results in the plant adaptation to increased sun exposure by producing protecting secondary plant compounds, such as carotenoids and polyphenols, which may act as precursors for TDN, vitispirane and 4-vinylguaiacol as well (Ziegler et al., 2019). The increased content of these phytochemicals results in a more intense yellow colour of the wines. In contrast, late defoliation at berry closure fosters sunburn events because the grapes had insufficient time to adapt to sun exposure and high temperature. Application of both reflecting particle suspensions seems to contribute to mitigating sunburn events. At the same time, lower colour and less ATA perception in the wines of these grapes suggest a diminished formation of protective carotenoids and polyphenols. This could be rationalised by the protective effect of the reflecting particles themselves. For the petrol off-flavour in TDN, this effect occurred when particles were applied after early defoliation, but not in the case of kaolin after late defoliation. Ongoing sun exposition after véraison decreases carotenoid and polyphenol levels. This catabolism also leads to a higher content of the degradation products, such as the odour-active TDN, vitispirane and 4-vinylguaiacol (Marais et al., 1992, Yuan and Qian, 2016).

Figure 5. Multi-factor analysis (MFA) of data from sensory and aroma analysis: On the left (a), the loadings of significant sensory attributes, the concentration of odourants and scores averaged over both replications (vectors). On the right (b), the scores of each treatment replicate. Individual scores 1 and 2 reflect variability due to field, fermentation and sensory replications.

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The multiple factor analysis (MFA) with the significant sensory and aroma analysis data (Supplementary Table 4) of each replicate in Figure 5 allows the holistic interpretation of the treatment effects in regard to the sensory properties of the wines in combination with their aroma composition.

A positive correlation between hexyl acetate and a citrus/mineral and green/vegetative odour impression seems to be obvious. The odour of hexyl acetate, in general, enhances the fruity aroma of a wine, while one of its precursors, hexanol, is described as herbaceous (Forde et al., 2011, Dennis et al., 2012). Wine made from the non-defoliated control and the treatment with late defoliation combined with Ca(OH)₂ application yielded the highest concentration of hexyl acetate and hexanol and accordingly were evaluated with more intense green/vegetative and citrus/mineral odour. According to Dennis et al. (2012), multiple C₆ alcohols and aldehydes are grape-derived precursors for hexyl acetate. Thus, the post-fermentative concentration and fruity impression depend on the sum of the precursors present in the initial grape juices. At the same time, precursors which have not been used for ester formation by the yeast may influence the aroma properties of the wine as well.

Ethyl acetate is shown on the right side in the upper half of the MFA. Ethyl acetate has a typical odour of nail polish remover and is a result of the esterase activity of yeasts (Waterhouse et al., 2016). According to Table 3, ethyl acetate varied between 50 and 68 mg/L, surpassing its published odour threshold of 12.3 mg/L in 10 % water/ethanol mixture (Escudero et al., 2004). Lower concentrations of ethyl acetate may contribute to a fruity impression, while higher concentrations mask fruity and other attributes. According to the MFA in Figure 5, ethyl acetate fostered the dull/ATA and smoky/honey intensities while masking the citrus/mineral and green/vegetative nuances. The fact that ethyl acetate had only a masking effect but was not perceived as such with its solvent character despite concentrations exceeding the odour threshold in a model wine by a factor of four can be rationalised by the recently published huge increase of the odour threshold of ethyl 2-methyl butanoate ester when established in a model wine (11.5 µg/L) and a regular white wine (2332.8 µg/L) (Gottmann et al., 2023). The deviation in ethyl acetate among the treatments is unlikely to be caused by bacteria or Hanseniasporum uvarum metabolism, as only sound grapes were processed, and a strong Saccharomyces cerevisiae yeast strain was inoculated. However, different levels of phytochemicals may have contributed to the hyperosmotic stress, which triggers minor ethyl acetate formation in Saccharomyces cerevisiae (Bely et al., 2005; Ferreira et al., 2006).

Based on the above-discussed results (ANOVA, PCA, MFA, TDS), it seems likely that enhanced fruity odour and retronasal perception due to elevated ester concentrations may have contributed to a more intense sweet perception, although residual sugar varied only by less than 1 g/L, having no sensory impact (Table 1). According to day-to-day experience, fruits with a higher ripeness and sugar content correlate with increased fruity ester concentrations. Vice versa, it is well known that a stronger sweet taste does enhance retronasal fruitiness perception (Matysiak and Noble, 1991).

This hypothesis is corroborated by our examination of the wines by GC-olfactometry (not reported) that ethyl esters generally exhibited fruit characteristics of sweet fruits such as strawberry in ethyl butanoate and pineapple in ethyl hexanoate. This interaction between fruity esters and sweet perception is also backed by our TDS results in Figure 4, where wines of increased fruitiness showed a stronger sweet dominance early on and a lower and later onset of acidity dominance in the course of the temporal examination. In case of the early defoliation treatment, the application of reflecting Ca(OH)₂ and kaolin particles into the grape zone enhanced the perception of fruitiness and ester concentrations, while the detrimental dull/ATA attribute was suppressed.

Another hypothesis, especially regarding the late defoliation treatments, can be derived based on the lower right half of the MFA (Figure 5). 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN) is solely responsible for the formation of the petrol off-flavour, while the structurally very similar vitispirane yields a camphor, woody and herbaceous flavour (Ziegler, 2020). The degradation of TDN precursors from the carotenoid catabolism is strongly accelerated by low pH values (Gök et al., 2022). Thus, the observed correlation between sour taste and TDN concentration in the MFA could be explained by the acidic hydrolysis of the TDN progenitors. However, due to a narrow range of pH values between 3.1 and 3.3, sourness perception did not vary significantly and, thus, was not included in the MFA exploration (Figure 5). Overall, the enhanced TDN and vitispiran concentration due to later defoliation at berry closure can be explained by a stronger carotenoid degradation.

The observation in the TDS analysis over time that late defoliation yielded less sweet and fruity dominance during the first ten seconds is mostly due to more dominant sourness and later astringency. Vice versa, enhanced perception of sweetness, which was also triggered by a higher concentration of fruity esters, and fruitiness by itself may have lowered the dominance of sourness. Interestingly, the application of Ca(OH)₂ did not result in lowered acidity perception due to the formation of neutral calcium tartrate or calcium malate salts.

Carrying viticultural field replicates forward as grape processing/fermentation replicates and finally even as sensory/analytical replications yielded a joined evaluation of the field, fermentation, and sensory variance. Despite the accumulated variance, many sensory and analytical parameters varied significantly due to different treatments, although the experimental error increased. This highlights the robustness of the here reported significant findings.

Further vintages should be observed to confirm the results of the study and draw more conclusions on the relationship between sunburn damage, preventive measures and aroma composition of white wine. Further on, additional treatments, such as the use of shading nets, should be investigated. The application of reflecting particles should be improved by using more efficient wetting agents, which will achieve a more uniform and complete distribution of the reflected particles on the grape surface.

Conclusion

Assessing the results of the warm vintage 2020, the defoliation measures had a larger impact on the sensory characteristics of the wines than the application of the reflecting particles, similar to the reports of previous studies by Ou et al. (2010). Early defoliation resulting in an early toughening of the grapes creates wines with less fruity aroma but with a tendency towards an increased smoky taint and the atypical ageing flavour reminiscent of acacia flower and fusel alcohols. The effects of the early defoliation were mitigated by applying the reflecting particles, and the application even increased the fruity aroma and gave a sweeter taste impression. Late defoliation reduced both the green notes of the non-defoliated treatment as well as the smoky taint and atypical ageing note occurring with early defoliation. However, these wines exhibited a slight petrol taste confirmed by increased levels of TDN (1,1,6-trimethyl-1,2-dihydronaphthalene) and vitispirane. In contrast, early defoliation promoted the formation of floral compounds such as linalool, 2-phenylethanol, and β-damascenone. Thus, the study supports Friedel et al. (2015), stating that defoliation measures greatly influence the aroma composition of the wine.

The year 2020 was a very hot and dry year, with a total of 20 hot days between June and August measured at the experimental station. It may serve as a model for future weather scenarios in case of ongoing climate change. Thus, the reported findings will serve the wine industry well to act in the prevention of increasing incidents of sunburn in the future.

In light of these results, early defoliation in combination with the application of reflecting particle suspensions such as kaolin or Ca(OH)₂ not only lowers the sunburn risk but also contributes to wines with an enhanced sweet taste. The application of reflecting particles diminishes the dull and ATA impressions observed in grapes from early defoliated grapes and also leads to a more fruity smell of the resulting wines. Late defoliation, however, increases the sunburn risk and results in wines with a more mature aroma that is not diminished by applying a kaolin suspension, while Ca(OH)₂ yielded more citrus and green-flavoured wines. While defoliation is a long-term measure, protection by reflecting particles is a short-term treatment, which can be applied shortly before a forecasted heat wave occurs.

Acknowledgements

We would like to thank Jonas Waber and Jochen Bogs for their work in the vineyard. The contribution of Sandra Klink and Anette Schormann in sensory analysis and Michael Wacker in aroma analysis is highly appreciated. The wine-making team of Pascal Wegmann-Herr, Florian Schraut, and Jonas Müller, as well as the analytical team in the laboratory of Patrick Nickolaus, Susann Krautwald, Michaela Deguenther, and Jutta Keiser, gave valuable support. We would also like to thank our project partners at the TU Braunschweig in the working group of Peter Winterhalter for good cooperation and discussion of the results.

Funding

This research project was supported by the German Ministry of Economics and climate actions (via AiF) and the Forschungskreis der Ernährungsindustrie e.V. (FEI, Bonn, Germany)—Project AiF 21095 N.

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