Riesling aroma composition in light of changing global temperatures – delving into the effects of warmer nights on the volatile profile of riesling grapes

Abstract

Context and purpose of the study: Climate is a key parameter when the modulation of berry and subsequent wine composition is considered. Recent decades have already seen an increase in global surface temperatures, with a more pronounced effect on night temperatures. In Australia, very warm monthly minimum temperatures (two standard deviations higher than the historical average) increased from a 2% to 11 % frequency of occurrence, and very cool monthly night temperatures have declined by about a third (Barlow and Daly, 2017). Night time temperatures are known to influence transcriptomic responses in ripening grapes (Rienth et al., 2014), however, the effect on grape chemical composition, in particular on the aroma compounds, remains to be elucidated. Aroma compounds such as the terpenes and norisoprenoids are key to the quality of white wine varieties such as Riesling. Understanding both the synthesis and loss of these desirable compounds due to the effects of warmer night temperatures, is critical to understanding the need for implementation of suitable mitigation strategies to help cope with the effects of warming projected in the future.

Materials and Methods: Four sites in the Canberra wine region (Australian Capital Territory and New South Wales, Australia) were chosen based on climatic data and, historic cool night index. As such, sites were catalogued as having either warmer, cooler or intermediate temperature nights. Temperature, humidity and light sensors were installed from the véraison stage to monitor meso‐ and microclimatic parameters throughout the ripening period. Berries were collected every two weeks from véraison until commercial harvest for chemical analysis. Midday stem water potential was also measured at sampling to assess water stress levels. Chemical analyses included total soluble sugars, titratable acidity, pH, yeast assimilable nitrogen, carotenoids, and free and bound volatile compounds.

Results: Higher temperature summations significantly depressed the synthesis of important aroma compounds such as norisoprenoids and terpenes, with carotenoid concentrations also being significantly decreased. Conversely, the concentration of aldehydes such as E-2‐octenal and E-2‐nonanal were positively correlated with higher temperature summation throughout the overall ripening season. Night temperature appeared to have a more pronounced effect, particularly on the synthesis of terpenes, during the later stages of berry development, as previously observed by Rienth et al. (2014). At harvest, warmer night temperatures resulted in lower concentrations of terpenes (e.g. linalool and α‐ terpineol) and the C6 alcohols (e.g. 1‐hexanol) whilst a direct correlation to heat summation was less significant.