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Wine Flaws: Reduction Jamie Goode

One of the difficulties in discussing the topic of wine “flaws” is that not all flaws are created equal. While some are clearly unacceptable in any context—the mustiness imparted by cork taint being the most obvious example—others need to be examined on a case-by-case basis. The spicy, earthy, animally, phenolic character that is produced by Brettanomyces yeast infection, more commonly known as “brett,” is probably the best example of a fault that is a problem in some cases, but a desirable quality in others. It’s all a question of degree, context, and perhaps personal preference.

Reduction is another “wine fault” that isn’t at all clear-cut. For starters, even experienced tasters may not be sure what it is, and therefore lack confidence in their ability to spot it. Also, some of the compounds responsible for reduction are actually positive aspects of varietal character in some cases. The same level of reduction might be considered positive in one wine and negative in another. It isn’t a simple matter.


The term itself is, in fact, a little misleading. Reduction generally refers to the characteristics imparted to wine by a group of sulfur-containing compounds. Indeed, many commentators prefer to use the term sulfur-like off-odors (SLOs) or volatile sulfur compounds (VSCs) rather than reduction. “Reduction” is used because these smelly sulfur compounds tend to develop under conditions that occur when oxygen is excluded.

Simply speaking, reduction is the opposite of oxidation: in a chemical reaction, one partner is reduced (gains electrons) while the other is oxidized (loses electrons). “Redox potential” is a chemical measure that describes how oxidized or reduced a system is. The redox potential determines which reactions occur in a wine, because the electrons tend to rearrange themselves into a more favorable order. SLOs can actually be present in a wine in an oxidized state, but they are associated with reduction because at low redox potential, the sulfur-containing compounds tend to be reduced to forms for which we have a lower sensory threshold.

An aerated red wine will typically have a redox potential of 400-450 millivolts, but long-term storage in the absence of air will reduce this potential to 200-250 mV. If levels get as low as 150 mV, then reduction problems can occur. Exposure to oxygen through winemaking practices such as racking, topping up barrels, and filtering increases the level of dissolved oxygen in the wine and increases the redox potential, which will then return to a level of 200-300 mV. The redox level changes much more rapidly in white wines than in red wines, because red wines have a higher concentration of phenolic compounds that can interact with oxygen and thus act as buffers. Another variable is the level of free sulfur dioxide in the wine, which will protect it by reacting with the products of oxidation. Yeast lees also scavenge oxygen and protect the wine in a similar fashion, helping to lower the redox potential and create a more reductive environment.

In modern winemaking, reductive conditions are encouraged: the protection of wines from oxygen by the use of stainless steel tanks and inert gases helps preserve fresh fruit characters. But these reductive conditions can also favor the development of pungent sulfur compounds during fermentation. When yeasts are stressed—for example, when they don’t have enough oxygen, the temperature changes rapidly, or there isn’t enough nitrogen in the must—they can generate higher levels of compounds such as hydrogen sulfide, dimethyl sulfide, and mercaptans (see table). Yeasts can also produce these compounds from precursors such as sulfur or sulfur dioxide in the must. To keep their fermentations as healthy as possible, winemakers watch for signs of SLOs and, if they appear, try to change the conditions by adding nutrients or oxygen to the must. Wine will always contain some SLOs; it’s a question of keeping these compounds at a level where they don’t have a negative sensory impact.


After bottling, the redox state of the wine will be influenced by a number of factors, including the condition of the wine at bottling, the free sulfur dioxide levels, the oxygen pickup during the bottling process, the amount and composition (air or inert gas) of the head space, and the type of closure. One of the reasons reduction has become such a hot topic lately is because of the growing use of screwcaps, which allow much less oxygen transmission than corks. The low redox potential in wines sealed with screwcaps can catch winemakers by surprise: they may bottle a wine free of noticeable SLOs, only to find that the SLOs still present in the wine have been modified by the low-redox environment into a smellier form, the result being “reduction” noticeable to tasters. One such reaction would be the reduction of disulfides to mercaptans (thiols). Therefore, winemakers who intend to use screwcaps need to change the way they prepare their wines for bottling. The wines must be free of any reductive tendencies, and the amount of sulfur dioxide used in bottling must be reduced. Copper fining trials are often done before bottling, but because copper removes mercaptans, not disulfides, winemakers try to use as little as necessary to clean the wine up.

Wine chemist Alan Limmer, who is also the winemaker for New Zealand’s Stonecroft and Hawkes Bay, has pointed out that screwcap reduction cannot be eliminated simply by taking more care in winemaking. “In essence we are talking about thiol accumulation, post-bottling, from complex sulfides which do not respond to prebottling copper treatment,” Limmer claims. “This reaction occurs to all wines containing the appropriate precursors, irrespective of closure type. But the varying levels of oxygen ingress between closures leads to significantly different outcomes from a sensory point of view.”

Limmer’s explanation for screwcap reduction is that if sulfides present in the wine at bottling are not exposed to a low level of oxygen ingress through the closure, they can be reduced to thiols. In his view, the use of a closure such as cork, which allows a little oxygen ingress (but not too much), is a necessary concession to the vagaries of sulfur chemistry. “Controlling ferments to not produce the complex sulfides is beyond our means currently,” he explains. “This sulfide behavior of the ferment is more controlled by the yeast genetics than the winemaker.”


So we come to the key questions: How do you recognize reduction? And how do you decide when it is a fault? There is no simple answer to either. In its worst manifestations, reduction is easy to spot: hydrogen sulfide gives off rotten-egg, drain-like aromas that spoil the wine. Any such wine is clearly faulty and would be recognized as such by both sommeliers and customers. A winemaker would have to display a great degree of incompetence to release such a bottling.

In my experience, however, the most commonly encountered forms of reduction are more subtle. The various SLOs will have different sensory impacts at different concentrations, making it hard to identify the signature impact of a specific sulfur compound.

The form I see most often is “struck match/flint,” presumably due to mercaptans. This type of reduction can be attractive in the right sort of context, but many people find it objectionable. In its most subtle form, it enhances the minerality of white wines; for example, it can add to the complexity of white Burgundies. Related to this is the cabbage or cooked-vegetable character that comes from either mercaptans or disulfides. Again, this is sometimes found in white Burgundies, and although it’s initially off-putting, it can add complexity. Reduction also manifests itself in a slightly rubbery quality that is most often spotted in red wines. I’ve found strong smoky, roast-coffee aromas in some reds that I would classify as reduction. It should be added that low-level reduction may have only a minor sensory impact on the aroma of a wine, but can affect the fruit expression and add hardness to the palate. Thus, it is also an issue of mouthfeel.

A distinction needs to be drawn between reduction and sulfur dioxide. Sulfur dioxide is added to almost all wines as a preservative and to counter the effects of oxidation. Only occasionally, however, is it added at levels where it crosses the sensory threshold, in which case the wine will have a slightly acrid, sulfury edge. I find this effect in a few traditionally styled German and Loire whites, where it can be a bit unpleasant, but does diminish with time in bottle and aeration of the wine.


In some circumstances, SLOs can make a positive contribution to a wine. As New Zealand winemaker James Healey points out, “In Champagne, the bready-brioche character from aging for a period on lees is a result of a certain type of reduction, in association with autolysis and liberation of the contents of yeast cells into the wine. The reductive characters from fermentation of Chardonnay juice containing highish solid concentrations result in accentuated nuttiness and improved texture after aging on lees for some time. And the ‘cat’s pee’ or sweaty character that develops during fermentation of Sauvignon Blanc from cooler climates is the result of a certain reduction-related compound.”

Consultant winemaker Dominique Delteil provides contrasting examples of the effects of reduction. “First, a very ripe Languedoc Syrah macerated to reach licorice aromas, and then aged in oak. In that wine, hints of ‘burnt match’ could be very interesting from a sensory point of view. They will match the ripe fruit-vanilla style. Most wine drinkers will appreciate that because those aromas are in a very sweet aromatic environment, so they won’t express as dominant. Second, a cool-climate, unripe Cabernet Sauvignon. Let’s suppose that this wine has exactly (chemically speaking) the same amount of the sulfur compounds that gave the interesting light ‘burnt match’ in the above Syrah. In this wine, those chemicals will give a different sensation that the same taster will translate as ‘leek,’ ‘green bean,’ and eventually ‘garlic.’”

It’s clear that reduction is a complicated but important topic for wine professionals. Recognizing SLOs, and then understanding when they are faults and when they are acceptable, is a useful skill to acquire—and one that takes considerable tasting experience to master.