One of today’s most polarizing topics in wine is reduction. Are reductive aromas faults to be eradicated? A lovely aspect of wine character? Does it depend on the intensity or the eye of the beholder? Or is there a broader way to think about reduction altogether?
Winemaking practices in the late 20th century sought to avoid and undo reduction, while in the 21st century, there is an increasing population that welcomes reductive character. Some have moved beyond the binary “like or dislike” question, and begun to consider what reduction means about a wine, and how to manage or encourage it in ways that can increase a wine’s lifespan.
Speaking chemically, reduction is the opposite of oxidation. In wine, the term is often used in a colloquial sense to refer to the presence of myriad volatile sulfur compounds (VSCs) that tend to result from the absence of oxygen. This article considers the branch of VSCs often considered to be off-aromas, or “defects.” Varietal thiols—such as the various tropical aromas in Sauvignon Blanc—are also VSCs, but are for the most part created in the grape and are not the type of VSCs considered in this article.
There are many reductive compounds that can affect wine aroma, but some of the most important are hydrogen sulfide (H2S, which often smells like rotten eggs), mercaptans (like methanethiol and ethanethiol, which smell like anything from cabbage and burnt rubber to pleasant earthiness), and dimethyl disulfide (which may smell like truffles, blackcurrant, canned corn, or molasses).
VSCs, in addition to their own aromas, affect the expression of other aromas in wines. “Reductive aromas mask delicate fruit and confectionary aromas, and reductive wines seem less fruity or floral,” says Marlize Bekker, Ph.D., a senior lecturer in food chemistry at the University of Queensland in Australia. “Certain reductive wines may also be wrongly assessed as having smoke taint, when in fact they may have high ‘flint’ aroma.”
With an understanding of the incredibly complex (and often unknown) factors that result in how any given aromatic compound is actually expressed, it’s easy to see how these aromas can be so controversial.
Yeast’s role in creating reductive aromas
A major source of VSCs is yeast metabolism of certain nitrogen-containing compounds during fermentation, which can occur when grape must has low yeast assimilable nitrogen (YAN), or nitrogen compounds in forms that yeast can utilize, which are critical for their functioning. There are many causes of low YAN in must, such as nutrient-poor soils and vine stress.
Though adding nitrogen to soils can help boost YAN, changing soils’ nitrogen levels can impact a wine’s structure in other ways. “Keeping nitrogen levels lower in the vineyard can result in a higher level of anthocyanins,” says Bruce Zoecklein, Ph.D., a professor emeritus of enology at Virginia Tech. He points to Burgundy, a region where wines can be reductive but winemakers still keep vineyard nitrogen levels low. “Pinot Noir isn’t overly endowed with color, so you don’t want to jeopardize that by boosting soil nitrogen,” he says.
Many winemakers test YAN levels and use various products to raise YAN levels in the must to an optimal amount. Diammonium phosphate (DAP) is a common solution as it is cheap, though there are other options as well.
From left to right: Marlize Bekker, Ph.D., a senior lecturer in food chemistry at the University of Queensland in Australia (photo courtesy of Marlize Bekker); and Seth Long, the owner and winemaker of Morgen Long Winery (photo courtesy of Seth Long).
However, it’s not clear-cut, according to Dr. Zoecklein. "There’s only about a 70 percent correlation between having a low YAN and the production of VSCs,” he says. “If it were 100 percent, adding nitrogen would solve this issue. When you measure YAN, you’re measuring ammonia and FAN (free amino nitrogen) amino acids … It seems the important component here is not simply the total, but the qualitative nature of the YAN—specifically, the particular mix of ammonia and these FAN amino acids."
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