Reductive strength in wine: beyond faults and aromas

In the world of enology, the topic of reductive aromas in wine, particularly those stemming from volatile sulfur compounds (VSCs), has been a subject of considerable debate and evolving perceptions. From being viewed as faults to be eradicated, to being appreciated as characteristics that add complexity and depth, the understanding of reductive aromas in wine has undergone significant transformation. This exploration delves into the chemical foundations of reductive aromas, their impact on wine's sensory profile, and the shifting paradigms in winemaking practices.

Chemical Nature of Reduction in Wine Reduction, chemically speaking, is the antithesis of oxidation. In winemaking, this term colloquially references the presence of various VSCs that are typically formed in the absence of oxygen. These compounds include hydrogen sulfide (H2S), mercaptans like methanethiol and ethanethiol, and dimethyl disulfide, each contributing distinct aromas ranging from rotten eggs to truffles and blackcurrant. These compounds, while often considered defects, can significantly influence a wine's aromatic expression, masking fruit and floral notes, and can sometimes be confused with smoke taint.

Yeast plays a pivotal role in the creation of VSCs. During fermentation, yeast metabolizes nitrogen-containing compounds, particularly when grape must has low levels of yeast assimilable nitrogen (YAN). The causes of low YAN are manifold, including nutrient-poor soils and vine stress. Addressing this, winemakers often augment YAN levels using substances like diammonium phosphate (DAP). However, this approach is not without its complications, as different yeast strains have varied nutritional needs and not all yeasts can utilize DAP effectively, leading to potential reductive faults.

Viticulture and vinification practices also contribute to the formation of reductive aromas. In the vineyard, the use of elemental sulfur, particularly in organic vineyards, can lead to sulfur residues on grape skins, which during fermentation may convert to H2S. Fermentation temperature is another critical factor. Some winemakers intentionally ferment at higher temperatures to stress the yeast, thereby encouraging the formation of reductive aromas that they find desirable.

An interesting aspect of reductive aromas is their relationship with a wine's reductive strength and, consequently, its aging potential. The reductive strength of a wine, which is its ability to absorb oxygen without oxidizing, is influenced by factors like phenol content and lees aging. This aspect is particularly important in the context of wine longevity. A low redox potential in a wine implies a higher capacity to reduce compounds, thereby providing a buffer against oxidation and potentially increasing the wine's lifespan.

Winemakers employ various techniques to manage VSCs, including aeration and the use of copper. However, these methods have their limitations and can sometimes exacerbate the problem. The understanding now is leaning towards tolerating these compounds in the short term, recognizing their potential role in enhancing a wine's longevity.

In red winemaking, exposing young wines to oxygen early in their life can increase their reductive strength. While it may seem counterintuitive, this process allows the wine to become more resistant to later oxygen exposure. In white wines, reductive strength is often bolstered through lees aging, particularly when gross lees are retained without stirring.

The understanding of reductive aromas in wine is evolving from being seen purely as faults to being recognized for their role in a wine's development and aging potential. This shift in perception challenges the traditional binary view of liking or disliking these aromas and invites a more nuanced appreciation of their significance in the overall character and longevity of a wine.

The science behind reductive aromas in wine reveals a complex interplay of chemical reactions, yeast metabolism, and winemaking practices. While once considered undesirable, these aromas are now increasingly recognized for their contribution to a wine's complexity and aging potential. This evolving understanding underscores the dynamic nature of winemaking and the continuing exploration of the myriad factors that contribute to the character and quality of wine.