Overripe Grapes, Undesirable Aromas

The impact of climate change on the aromatic quality of wine is increasingly becoming a subject of concern among viticulturists and winemakers. This issue was explored in depth by Vicente Ferreira, a researcher from the Laboratory of Aroma Analysis and Enology (LAAE) at the University of Zaragoza, during his presentation titled "Aromatic Quality and Climate Change: Some Key Insights and Warnings." Held as part of the Enoforum Web event, Ferreira's talk focused on how the over-ripening of grapes, driven by rising global temperatures, affects the volatile compounds in wines, which in turn can negatively influence their sensory profiles.

At the heart of Ferreira's analysis was the notion that over-ripening occurs when grapes are left on the vine past their ideal point of ripeness. While some winemaking processes, such as those involved in the production of dessert wines, purposefully seek this effect to achieve certain desirable traits, the phenomenon can also occur unintentionally as a result of extreme weather conditions or specific agricultural practices. When not carefully controlled, over-ripening leads to the development of compounds that significantly alter the aromatic makeup of the wine. Ferreira emphasized that two primary groups of volatile compounds are produced during this process: aldehydes and lactones, and alkylketones.

Aldehydes and lactones are notable for imparting herbaceous, metallic, and mushroom-like aromas. In moderate concentrations, these compounds can contribute to the complexity of a wine, but excessive amounts often lead to undesirable sensory qualities. For instance, compounds such as γ-no-lactone, which evokes coconut aromas, or masoya lactone, reminiscent of plum, may give a rich aromatic profile to wines made from overripe grapes. However, if these compounds dominate, they can mask or distort the wine's natural characteristics. Alkylketones, on the other hand, can introduce cucumber-like and vegetal notes, which are typically perceived as faults, especially in higher concentrations.

During fermentation, these volatile compounds are not entirely eliminated. In fact, some may become more pronounced as the wine ages. This is due to the gradual transformation of non-volatile acids present in the wine into volatile aromatic compounds over time. Lactones, in particular, evolve during barrel aging, contributing to the wine's complexity, but also posing a risk if they overpower the wine's more delicate aromas.

Another group of compounds that Ferreira discussed are Strecker aldehydes, which form when amino acids interact with quinones, a reaction that becomes more prevalent in grapes that have undergone desiccation or excessive sun exposure. These aldehydes bring aromas of honey, potato, or bread. While such notes can add depth to a wine, they may also signal spoilage or deterioration if not properly managed.

The role of sulfur dioxide (SO₂) in preventing these unwanted developments was also highlighted. As a stabilizing and antioxidant agent, SO₂ is crucial in minimizing the formation of these undesirable compounds. Ferreira pointed out that maintaining SO₂ levels above four parts per million is essential to prevent rapid oxidation and the subsequent production of aldehydes. If SO₂ levels fall too low, aldehydes bound to the sulfur can be released slowly during aging, resulting in a gradual decline in the wine's aromatic quality.

Over-ripening poses numerous challenges for winemakers, as the changes it induces in the chemical composition of the grape must can significantly affect the winemaking process and, ultimately, the wine's quality. Throughout his presentation, Ferreira stressed that mitigating these effects requires careful decision-making both in the vineyard and the winery. In the field, growers must closely monitor the ripening process to ensure grapes are harvested at the optimal time, avoiding excessive over-ripening. In the winery, managing fermentation temperatures and maintaining appropriate SO₂ levels are critical steps to preserving the wine's aromatic freshness and preventing the formation of unwanted volatile compounds.

The reality of climate change has made over-ripening more common, particularly in regions where temperatures have steadily risen over the past few decades. This trend is forcing wine producers to rethink their vineyard management strategies and vinification processes. Without proper adaptation, wines could lose their freshness and complexity, ultimately compromising both their quality and aging potential. Ferreira's presentation serves as a crucial reminder of the evolving challenges climate change poses to the wine industry and offers insight into potential strategies for adapting to these new realities.

As vineyards across the world grapple with these changes, the need for innovation in vineyard management and winemaking practices has never been more apparent. The delicate balance between ripeness and over-ripeness, once a matter of experience and judgment, is now further complicated by the unpredictable and intensifying effects of global warming. Winemakers are tasked with not only preserving the traditional characteristics that define their wines but also navigating the new chemical landscapes that over-ripening and volatile compounds introduce. It's a challenge that demands both respect for the past and a willingness to embrace scientific advances, as the future of wine in a warming world may well depend on both.