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Plastic Corks, Screwcaps, and Mercaptans Many people are still confused as to why traditional corks are being replaced by plastic corks and screwcaps in most inexpensive wines. The simple answer is that the industry is looking for a cork replacement because of the high percentage of defective corks, specifically corks that carry a chemical called TCA. Corks that contains a particular harmless bacteria will produce TCA in the cleaning process during cork production. TCA contributes a moldy, wet cardboard smell and flavor to wines and robs them of their fruit. By most estimates, two to five percent of all corks contain some TCA. Alternative closures do not have a TCA problem, but it turns out that most of them have other problems. For wines that are meant to be drunk young, these problems will probably not surface. But even wines meant to be drunk young can sit around for a year or two, which is enough time for some bad things to happen. Think of a closed bottle of wine as a living (and breathing) container of organic and inorganic compounds. Think of the changes that can take place in such a closed system. Remember from high school chemistry that changes can occur aerobically (in the presence of oxygen) or anaerobically (in the absence of oxygen). The changes that occur will be different in these two situations. More specifically, the changes in the wine bottle (aging) will be different if there is some oxygen present (even if it is microscopically small) than if there is no oxygen present. In fact, wines age well with cork stoppers because cork allows a finite but very, very small amount of oxygen to migrate into the wine. Plastic corks do not. The liners of most screwcaps do not. Some screwcap makers are producing liners that closely mimic real corks in air transfer. What this all means is that plastic corks and most screwcaps form an impermeable barrier that creates an anaerobic condition in a wine bottle. In this situation, undesirable changes may take place as the wine ages even for a relatively short time. Next month we will present an article adapted from a longer one written by Jamie Goode. A fairly technical explanation of what’s going on, it may sound deadly dull or of interest only to wine geeks, but I think it’s important for wine lovers to have some knowledge of the issues involved. Mercaptans and other volatile sulfur compounds in wine There are around 100 volatile sulfur compounds that have been identified in wine, but only a few of them are important to this discussion. First of all, they are smelly! Even at low concentrations they can negatively impact a wine. Second, they are chemical chameleons, able to change their form depending on the environment they are in. They may be undetectable in one form, but may suddenly become noticeable in another, more smelly form. Third, they are not all bad. They are important contributors to varietal character in many wines, so winemakers should not just try to completely eliminate them. Reduction vs. Oxidation The production and aging of wines involve chemical reactions. Electrons change hands, and as one compound is oxidized (the electrons are transferred from the chemical components in the wine to the oxygen), another is reduced. We tend to think of oxygen as a bad thing for wine, but it's not that simple. If there is plenty of oxygen around, chemical components in a wine will be gradually oxidized. The end result is an oxidized (spoiled) wine. Wine spoils by oxidation; it does not turn to "vinegar." But some oxidation is necessary in the production of wine. During fermentation the yeasts need oxygen, and a little oxygen is helpful in the early stages of red wine development because it allows the oxidation of some ethanol to acetaldeyde (also known as ethanal). This helps with the development of tannins and pigmented polymers that are important in building structure and color (this is the theory behind micro-oxygenation). After this, wine development is largely reductive: that is, it occurs best in the relative absence of oxygen. However, as we shall see later, oxygen levels that are too low may be harmful to wine. We now need to understand the chemical term "redox potential." This is simply a measure of how oxidative or reductive a system, such as a wine in barrel or bottle is, and it is measured in millivolts (mV). High readings are oxidative; low reading are reductive. Typically, an aerated red wine will have a redox potential of 400–450 mV (high), whereas storage in the absence of air for some time will reduce this to 200–250 mV (low-middle). If levels get as low as 150 mV, there is a danger that 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 safer 200–300 mV. In white wines, this redox level will change much more rapidly than red wines, because red wines have a higher concentration of phenolic compounds such as tannins which are able to interact with oxygen and act as buffers. Another variable here is the level of free sulfur dioxide (sulfites) in the wine, which will act protectively by reacting with the products of oxidation. Yeast lees also absorb 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 to preserve fresh fruit characters. Reductive conditions\those in which oxygen is more or less excluded\can favor the development of smelly forms of sulfur compounds. This is where the term "reduced" comes from. If this "reduction" occurs before bottling, the addition of oxygen may correct the fault. But these sulfur compounds can develop in wine even in non-reductive conditions,. Further oxygen exposure may turn a smelly wine into a smelly oxidized wine. Equating the term "reduced" with the presence of volatile sulfur compounds is therefore an oversimplification. The use of this term is scientifically imprecise and can be misleading. "Reduction is a simplification, a language abuse," says Dominique Delteil, scientific director of the ICV in the south of France. "Tasters often link sensory sensations to chemical or physical states without being sure they are real or not. I prefer to call this concept "sulfur flavors" rather than "reduction." None-the-less we will continue to use reduction as a shorthand for sulfur flavors.
Volatile sulfur compounds: a quick tour The characteristics of reduction can be quite variable, but the following may be found in wine.
A key point:
A LITTLE NITROGEN BACKGROUND MATERIAL:
Nitrogen is an element that is found in both the living portion of our planet and the inorganic parts of the Earth system. The nitrogen cycle is one of the biogeochemical cycles and is very important for ecosystems. Nitrogen moves slowly through the cycle and is stored in reservoirs such as the atmosphere, living organisms, soils, and oceans along its way. Most of the nitrogen on Earth is in the atmosphere. Approximately 80% of the molecules in Earth’s atmosphere are made of two nitrogen atoms bonded together (N2). All plants and animals need nitrogen to make amino acids, proteins and DNA, but the nitrogen in the atmosphere is not in a form that they can use. The molecules of nitrogen in the atmosphere can become usable for living things when they are broken apart during lightning strikes or fires, by certain types of bacteria, or by bacteria associated with legume plants. Other plants get the nitrogen they need from the soils or water in which they live mostly in the form of inorganic nitrate (NO3-). Nitrogen is a limiting factor for plant growth. Animals get the nitrogen they need by consuming plants or other animals that contain organic molecules composed partially of nitrogen. When organisms die, their bodies decompose bringing the nitrogen into soil on land or into the oceans. As dead plants and animals decompose, nitrogen is converted into inorganic forms such as ammonium salts (NH4+) by a process called mineralization. The ammonium salts are absorbed onto clay in the soil and then chemically altered by bacteria into nitrite (NO2-) and then nitrate (NO3-). Nitrate is the form commonly used by plants. It is easily dissolved in water and leached from the soil system. Dissolved nitrate can be returned to the atmosphere by certain bacteria in a process called denitrification.
Certain actions of humans are causing changes to the nitrogen cycle and the
amount of nitrogen that is stored in reservoirs. The use of nitrogen-rich
fertilizers can cause nutrient leading in nearby waterways as nitrates from the
fertilizer wash into streams and ponds. The increased nitrate levels cause
plants to grow rapidly until they use up the nitrate supply and die. The number
of herbivores will increase when the plant supply increases and then the
herbivores are left without a food source when the plants die. In this way,
changes in nutrient supply will affect the entire food chain. Additionally,
humans are altering the nitrogen cycle by burning fossil fuels and forests,
which releases various solid forms of nitrogen. Farming also affects the
nitrogen cycle. The waste associated with livestock farming releases a large
amount of nitrogen into soil and water. In the same way, sewage waste adds
nitrogen to soils and water.
Why sulfur compounds in wine is a hot topic Let’s first correct a popular misconception. Sulfur dioxide (SO2)
- the infamous "sulfites" - is not a volatile sulfur compound, and is
not really part of the "reduction" syndrome. SO2 is added to wine in
order to provide microbiologic stability and perhaps more importantly to prevent
oxidation. Actually, it’s a little more complicated than that. SO2 splits into
various molecular forms in wine; only the free SO2 has any protective activity.
It reacts with oxygen very slowly, and its role is to bind up the products of
oxidation in wine so that the oxidation isn’t apparent, even though oxidation
of the wine components will already have occurred. This is a complicated, but
important story, but it isn’t terribly relevant to our discussion. Now the really controversial bit. Sulfur compounds in wine - "reduction" - have been in the press lately. The Daily Telegraph carried a story titled "Screwcaps Blamed For Tainting Wine" which was also picked up by other news outlets. This was prompted by the results of the faults clinic from the 2006 International Wine Challenge (IWC). "In a number of cases the IWC chairmen validated a link between screwcap use and an unfavorable, vegetal/rubber flavored compound presumed to be a complexed sulfide," reports Sam Harrop MW, one of the four IWC chairs. "At first glance a percentage of 4.9% of total faults may not seem high, but when examined in the context of total screwcap figures, a more worrying rate of 2.2% [of all screwcapped wines] emerges. In the context of the 2006 IWC cork taint figure of 2.8% [of all natural cork-sealed wines], this fault type is significant and should be given more attention by wineries using screwcap." However, Harrop is keen to emphasize that he’s not equating the two: "While the IWC figures for screwcaps are a concern, there is no question in my mind that the continued incidence of cork taint is still a more serious issue." The potential problem with sulfides in screwcapped wines first came to the world's attention with the closures study by the Australian Wine Research Institute (AWRI) in 1999 and which has reported at regular intervals as a 1999 Semillon from the Clare Valley has developed in bottles using 14 different closures. Included in this study was a metal-lined screwcap. The liner is important here: the oxygen transmission properties of a screwcap are determined by the nature of the liner. The almost universally used liner in Australia and New Zealand has a metal layer in it (usually tin; sometimes aluminum). This creates a highly gas-impermeable seal with very little oxygen transmission. These liners are instantly recognizable because of their metallic appearance. The other commonly used screwcap liners for wine appear white. These are known as saranex-only liners and allow more oxygen transmission, although probably a bit more than is needed just to avoid reduction and likely more then we'd want of a closure for wines destined for keeping more than a few years. After the wines had been in the bottle for 20 months, the AWRI reported that the tin-lined screwcaps performed as expected: with their tight seals they kept the wine freshest. Screwcapped bottles scored highest for fruity aromas, maintained the highest levels of free sulfur dioxide, and showed the least color development. But they also scored highly for "struck flint/rubber" in the sensory analysis. This observation persisted at least through 63 months post bottling. Subsequent trials, which have examined the performance of metal-lined screwcaps, have reached consistent results, as have studies using sealed ampoules where there is no oxygen transmission at all: "reduction" seems to be a problem. The obvious explanation is that the low redox environment of the screwcap-sealed wine is causing some unwanted sulfur chemistry to occur, with sulfur compounds shifting from a less smelly (and thus unnoticed) form to a more smelly (and noticeable), reduced form.What are we to make of this? Is it a real world problem on a par with cork taint, or is it just a minor technical problem - a teething issue that just needs a bit of tweaking? The latter position has been consistently adopted by proponents of screwcaps, including many individuals and wineries that have signed up to the International Screwcap Initiative. They have invested emotional energy as well as several years' production of their wines, so their natural response to these sorts of data is to either fight or deny them. Others have been gunning for screwcaps to fail\none more so than the cork industry who see their livelihood threatened. Since the publication of the first AWRI report in 2001, there has been just a trickle of data on the subject of screwcap reduction. But little by little a clearer picture has emerged, and the evidence suggests that the issue of mercaptans in screwcapped wines is problematic enough that some caution should be exercised in their use. First we have consistency of observation: where people have been looking carefully at screwcapped wines, these mercaptans (or what people believe to be mercaptans from sensory analysis) have always been found. Then we have anecdotal observation by interested parties. Australian wine writer Campbell Mattinson reported on a tasting in which he encountered a number of reduction problems with screwcap wines. A comparative tasting of 24 white and red wines, screwcapped and cork-sealed, was reported by Ralph Kyte Powell in the Australian newspaper The Age (2/21/2006). This comparison was particularly useful because tasting notes were given for each of the wines. In reading these notes, two points are emphasized. First, that the wines taste quite different in almost every case. Second, that the number of descriptors indicative of the presence of mercaptans in the screwcapped wines is striking. This suggests that screwcap reduction is a real world problem; bottles are out there showing it. New Zealand winemaker (Stonecroft, Hawkes Bay) and PhD chemist Alan Limmer has been a thorn in the side of the screwcap lobby. He has written widely on the subject, bringing his knowledge of wine chemistry to bear. Limmer believes that screwcap reduction is not a problem that can be completely eliminated by better winemaking as many have claimed. "In essence we are talking about thiol accumulation post-bottling from complex sulfides which do not respond to pre-bottling copper treatment," claims Limmer in response to the assertion that fining with copper removes reduction defects. "This reaction occurs in 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 sulfides present in the wine at bottling necessitate a very small level of oxygen ingress through the closure, otherwise they can become reduced to thiols. Because sulfides are less smelly, a wine that is clean at bottling may taste reduced after bottling if the closure doesn't permit enough oxygen ingress. So the use of a closure such as cork, which does allow a little oxygen ingress (but not too much), is a necessary concession to the vagaries of sulfur chemistry. Of course, we'd rather not have the sulfides in the wine at all, which would then avoid problems with reduction to mercaptans at a later stage. But as Limmer points out, "Controlling fermentation so as not to produce the complex sulfides is beyond our means currently. This sulfide behavior during fermentation is more controlled by the yeast genetics than the winemaker," he explains. "It is not the winemaker's fault these compounds exist in the wine at bottling. We can minimize it to some extent by providing optimum nutrient conditions for fermentation and employing some specific winemaking regimes. But the research tells us this only has a slight impact on the complex sulfide pattern produced by the yeast." Limmer reinforces his point: "The patterns are quite specific to each yeast type almost irrespective of nutrient conditions. Every wine contains these complex sulfides." Even if we could eliminate all sulfur compounds from wine, that would be undesirable because some sulfides are important for varietal character in Sauvignon Blanc and other grapes.
CONCLUSION: The new cork taint? It is also possible that low level mercaptans affect more than 2.2% of wines sealed with screwcaps. "They impact... towards the end of the palate," claims Limmer, "imparting a mineral or bitter/hard/astringent aspect. This has the appearance of shortening or closing up the palate, so the wine does not display a fine fresh long finish, but ends abruptly and somewhat harshly." This describes something I’ve certainly noticed in side-by-side comparisons of cork and screwcap-sealed wines. Is it happening all the time, but going more-or-less undetected? Gregor Christie of membrane cork company ProCork has been concerned enough about this problem that he has sent wines for testing at ETS laboratories in California. Clearly, Christie has a commercial imperative for showing that ProCork is superior to tin- lined screwcaps in this regard, but even given this motivation, the results are interesting. Christie took the 2002 Clare Valley Semillon used in the commercial closure trial run by the AWRI, comparing ProCork with natural cork and screwcap and submitted bottles sealed with all three closures to ETS for testing for a range of volatile sulfur compounds. For methyl mercaptan, which has a perception threshold of 0.3 parts per billion (ppb), both the cork- and ProCork-sealed bottles were below detection limit. The screwcapped bottle, however, showed a level of 0.6 ppb. A sense of perspective is called for. There’s a real danger that the message distilled by journalists from all this technical talk becomes a misleading "screwcaps taint wine" story. The picture emerging is a complex one, but such a simplification would be dangerous if it caused producers to back away from adopting alternative closure solutions, which would then remove any incentive for the cork industry to put its house in order and do all it can to reduce taint levels. However, complications like this mercaptan issue should put pressure on winemakers to be more curious about the closures they are using. They should ask more questions about issues like oxygen transmission and insist on seeing independently validated data on closure performance rather than accepting manufacturer’s testimonials or sales pitches unquestioningly.
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