Wine occasionally develops unpleasant odors, defects, or off-flavors—unwelcome, sometimes distinctly repulsive aromas. Among the various unpleasant odors found in wine, "reduction" stands out as one of the most representative.
Without fear of misunderstanding, encounters with wines containing reduction are everyday occurrences. At the International Wine Challenge (IWC), one of the world's major wine competitions, an astounding 26-29% of wines judged to have defective aromas between 2006 and 2008 were attributed to reduction. Furthermore, according to a report by the OIV (International Organisation of Vine and Wine), global wine losses caused by reduction in 2015 reached 28 billion euros.
Reduction is that common a defect.
Why does such a frequently occurring defect continue to reach the market without improvement? What exactly is reduction? Let's examine this phenomenon from the ground up.
The True Nature of Reduction: Volatile Sulfur Compounds
While the term "reduction" is particularly common in the wine industry, this phenomenon is not limited to wine alone. Reduction occurs across all food products.
The substances responsible for reduction are a group of compounds called volatile sulfur compounds (VSCs). In other words, reduction and sulfur odors refer to the same phenomenon.
Reduction affects not only wine but also other alcoholic beverages like beer, as well as food and beverages in general. In food science, since animal products generate hydrogen sulfide through spoilage (hydrogen sulfide is known as the smell of rotten eggs), this phenomenon is sometimes recognized as putrefaction odor rather than reduction.
It's worth noting that putrefaction and fermentation are essentially the same phenomenon, distinguished only by whether they are beneficial or harmful to humans.
The first recorded reference to reduction in wine dates back to 1873, when German biochemist J.L.W. Thudicum wrote in "reduced sulphur scud" that "hydrogen sulfide (H₂S) is first generated and makes wine terribly malodorous."
Types and Effects of Reduction
While we've described reduction as an off-flavor, this isn't necessarily always the case. The presence of VSCs in wine doesn't automatically make that wine defective. What matters is the type of VSCs present and their concentration levels.
Since VSCs are sulfur-based compounds, they generally constitute a group of substances with sulfur-like odors. While sulfur might evoke thoughts of hot spring aromas for many Japanese people, few would associate it with grapefruit fragrance. Surprisingly, one of the compounds responsible for grapefruit-like aromas in wine is actually a sulfur-based compound—a VSC.
VSCs encompass a diverse range of substances, and molecular weight is a crucial factor among these compounds.
Generally, high molecular weight VSCs are not considered causes of defective odors; rather, they play a role in imparting desirable aromas to wine. For example, 3-sulfanylhexan-1-ol, also abbreviated as 3SH, is a type of thiol belonging to polyfunctional thiols with a molecular weight exceeding 100g per mole. This is genuinely a volatile sulfur compound, yet it produces grapefruit and passion fruit-like aromas.
In contrast, low molecular weight VSCs are problematic compounds that bring negative odors to wine. Most of the causative substances when reduction is considered a defective odor in wine are these low molecular weight VSCs. Wines containing such substances emit odors reminiscent of rotten eggs, sewage, cabbage, or burnt rubber—smells that cause discomfort when detected.
In wine aroma descriptions, a distinction is sometimes made using "aroma" for non-offensive reduction notes and "odor" for offensive ones, separating pleasant "reduction aroma" from unpleasant "reduction odor."
The Relationship Between Oxygen and Reduction
The word "reduction" is commonly understood as the antonym of "oxidation." Consequently, reduction odors are often thought to result from wine being placed in oxygen-free conditions—the opposite of oxidative odors.
This understanding is not incorrect. The reason so many wines were flagged for reduction at IWC judging is attributed to the recent trend toward more reductive winemaking practices, supported by advances in equipment and manufacturing technology, combined with the increased use of low oxygen-permeability closures like screw caps for bottle sealing.
However, the causes of reduction in wine are complex, with multiple pathways identified. Some of these pathways actually involve oxidation as a prerequisite.
When wine comes into contact with oxygen, the concentration of sulfur compounds typically decreases. Therefore, wines placed in oxidative environments show reduced absolute amounts of sulfides, and consequently, reduced production of VSCs that use these as raw materials. However, alongside this trend, some components undergo oxidation to become precursors of low molecular weight VSCs, creating potential sources of reduction. Reduction doesn't necessarily result solely from the presence or absence of oxygen.
This aspect might be better understood by considering oxidation and reduction reactions in their more chemically precise sense. In chemistry, oxidation and reduction represent electron transfer, where the presence of oxygen is not particularly relevant.
Generation of Reduction During Fermentation
The fermentation process is the most crucial stage in winemaking. In fact, most sulfur odors are generated during this process, as yeast is heavily involved in sulfide production.
The conditions under which yeast produces large quantities of sulfur compounds during fermentation are largely understood. Perhaps for this reason, it's often believed that sulfur odor generation can be relatively easily avoided by paying attention to these key points.
Indeed, proper management of yeast assimilable nitrogen levels required by yeast can suppress the occurrence of these defective odors to some extent.
However, it's known that low molecular weight VSCs—the problematic causative substances—are also generated during the maturation period after bottling.
If problematic sulfur odors were generated only during alcoholic fermentation, some treatment could be applied before bottling. As a result, not so many wines would be flagged for defects at IWC. However, reality tells a different story. This situation suggests that the problem occurs at times when wineries cannot address it.
Difficult-to-Remove Causative Substances
Low molecular weight VSCs include various substances, but the three most representative are hydrogen sulfide (H₂S), methanethiol (MeSH), and dimethyl sulfide (DMS).
These compounds are generated through multiple reactions, but they fundamentally use sulfur-based substances as starting points. Logically, if sulfur could be prevented from entering the winemaking process, reduction would not occur.
However, sulfur is an essential component for yeast metabolism. Therefore, even if a completely sulfur-free environment could be created, while sulfur odor generation would certainly be prevented, winemaking itself would become impossible. As long as yeast is used for winemaking, the risk of reduction generation cannot be completely eliminated at its root.
The sulfur that serves as raw material for VSCs generation comes from S-methylmethionine contained in grapes and sulfur-based agents applied during grape cultivation. These sulfur compounds are present in grape juice at concentrations of 160-400 mg/L or higher. Additionally, sulfur dioxide (SO₂), added as an antioxidant during the winemaking process, also serves as a sulfur source.
Characteristics and Mechanisms of Wines Prone to Reduction
Sulfur components that enter grape juice through various pathways are absorbed by yeast during alcoholic fermentation and synthesized into sulfur-containing amino acids methionine and cysteine. At this stage, sulfur-containing amino acids are not directly biosynthesized from sulfide raw materials. Intermediates are first generated in the sulfate reduction pathway, which are then incorporated as raw materials into amino acid biosynthesis processes. The production of these intermediates varies greatly depending on yeast strains and environmental conditions, typically ranging from 10-100 mg/L.
These intermediates become problematic.
Not all sulfides generated as intermediates are used for amino acid biosynthesis. Sulfides that fail to be properly incorporated into biosynthesis processes become raw materials for H₂S and MeSH production.
Furthermore, some sulfur-based compounds accumulated within yeast cells, including sulfur-containing amino acids, are metabolized into VSCs depending on conditions. From these factors, wines using yeast that produces large amounts of sulfur-based compounds face correspondingly higher risks of reduction development.
Reduction Characteristics in Champagne
Even when selecting yeast strains that produce smaller amounts of sulfur-based compounds, extended aging with yeast after alcoholic fermentation increases the likelihood of yeast cell breakdown, supplying sulfur compounds to the wine. This means wines employing Sur Lie methods, and sparkling wines made from such base wines that undergo extended contact with lees after secondary fermentation, become more prone to reduction.
The strong tendency for reduction in natural wines, which often use wild yeast for fermentation and maintain extended contact periods with yeast, stems from the same reason. Additionally, these types of wines rarely use fermentation aids during fermentation, creating environments where yeast production of sulfur-based compounds easily increases—another reason for the higher probability of encountering reduction in such wines.
Limitations of Copper Treatment
Copper addition has been considered extremely effective for treating reduction that develops in finished wines.
This method is indeed effective, and if a bottle at hand has reduction, gentle stirring with a pure copper rod can remove such odors.
However, recent research has revealed that heavy metals contained in grape juice may increase the long-term risk of reduction development in wine. This includes copper. In other words, prescribing copper to remove reduction may potentially cause more severe reduction in the future.
This also suggests that using grapes from organic or biodynamic cultivation, which tend to require higher copper usage for disease control, may increase the likelihood of reduction development.
Effective Methods for Avoiding Reduction
When reduction occurs, metal ion addition remains an effective response method. However, as we've seen, this approach may increase reduction risks in the future.
An effective method that doesn't rely on metal ions is micro-oxygenation, also called controlled oxygen addition.
A major factor in reduction development is wine being placed in oxygen-deficient environments. Therefore, artificial oxygen supply during winemaking processes aims to avoid situations that promote reduction development. Verification results confirm that increasing oxygen supply correspondingly decreases H₂S and MeSH content.
However, the fundamental premise is that when precursors exist in wine, long-term storage will cause gradual increases in H₂S and MeSH concentrations within bottles. Therefore, the most effective means of avoiding reduction is managing to prevent these precursors from existing in wine.
Most of these precursors form during alcoholic fermentation. Yeast stress has particularly significant impact. Specifically, placing yeast in nutrient-poor environments increases sulfur compound production. Therefore, proper management of temperature and yeast assimilable nitrogen during alcoholic fermentation, with appropriate additions as needed, becomes effective not only for smooth fermentation progress but also for preventing unwanted off-flavor development.
Conclusion: Considering the Boundary Between Defect and Acceptance
Wine is inherently reductive in its very existence. During alcoholic fermentation, when simple grape juice becomes wine, carbon dioxide is continuously generated, with some dissolving into the liquid.
In wines immediately after completing alcoholic fermentation, particularly white wines, flint-like aromas are very commonly detected. The cause of this aroma is sulfur-based compounds.
Like 3SH mentioned earlier, this flint aroma is rarely considered an off-flavor unless extreme. Conversely, sulfur odors reminiscent of hot springs or rotten eggs are immediately judged as defective odors. Having standards for where to draw the boundary between these two cases is important. Simultaneously, we need to consider the criteria for distinguishing between cases where we deliberately use positive expressions like "reduction aroma" to emphasize the presence of such aromas, versus cases where we neither judge flint-like aromas as reduction-related nor mention them at all, despite detecting them.
The presence of reduction aromas not recognized as defective odors is so familiar that, when consciously sought, they can likely be found in a surprisingly large number of wines.
When you discover such aromas, reconsider where those aromas belong. In many cases, they are not grape varietal character aromas. They are not yeast aromas either. They might be aromas that attached during processing—aromas that, considering the original raw materials, should never have been present in that wine. While they may not be unpleasant, they might still represent winemaking defects.
Aromas that naturally accompany winemaking. Aromas so commonplace we've grown accustomed to them.
The act of consciously identifying sulfur odors will surely provide clues for discovering such aromas and, furthermore, prompt questions about how to position these aromas.
