Have you ever opened a bottle of wine you were looking forward to drinking, only to encounter a sulfur-like odor?
When mild, this aroma is sometimes described as flint or struck match and can even be viewed favorably in wines made from certain grape varieties. However, when this odor becomes stronger, it can resemble rotten eggs. The primary cause of this smell is oxygen deficiency in the wine.
In mild cases, the odor naturally dissipates if the bottle is left open for a while after uncorking. However, there is a much faster way to eliminate it: using metal.
This article explores the metals that promote oxidation in wine.
Removing Sulfur Odors with Metal
The sulfur-like aroma detected in wine is called sulfide odor or reduction odor. The way reduction odor is perceived depends on its intensity. In mild forms, it can be positively evaluated as a characteristic aroma. However, when the intensity increases to a certain degree, it is treated as a wine fault or an unpleasant odor. In fact, this reduction odor is one of the most frequently encountered off-odors in wine.
The reason sulfide odors have become so common lies in the modernization of winemaking equipment. The equipment used for winemaking has become increasingly airtight. As a result, contact between wine and oxygen during the production process can now be minimized to an extreme degree. Consequently, the oxygen that was previously supplied naturally to wine is no longer provided, creating an environment where reduction odors develop more easily.
When extremely strong reduction odors are present in wine, remediation is difficult. However, when the intensity is mild, there are several methods to remove this odor from the wine. The simplest approach is to uncork the bottle earlier to allow the wine to breathe, or to swirl the wine in the glass after pouring. Since reduction odors occur due to insufficient oxygen in the wine, supplying the missing oxygen can restore the wine to some degree.
That said, this method only works for wines with mild symptoms. Additionally, this approach takes time. For wines with slightly stronger symptoms, the odor often cannot be completely eliminated. In such cases, metal can serve as an effective remedy. Exposing wine to metal can dramatically remove the unpleasant sulfur-like odor. This is because metals, particularly copper and iron, have the characteristic of strongly oxidizing wine.
Metal-Induced Oxidation in Wine
Even without deliberately adding metal to remove reduction odors from a glass or bottle, some wines naturally contain copper and iron in ionic form. These metals oxidize multiple components present in wine.
Several phenomena are considered part of wine maturation. For example, the browning of wine as it turns a brownish color, the oxidation of polyphenols including tannins, the oxidation of alcohol, and the formation of acetaldehyde. These phenomena are often influenced by such metals.
Furthermore, metals present in wine react with amino acids and melanoidins also present in the wine. These reactions affect the color, taste, and aroma of wine. Moreover, these effects are not limited to components already present in the wine. It has already been confirmed that components in food ingredients used in cooking can also react with wine during meals.
Iron Determines Compatibility with Seafood
Seafood is one of the prime examples of food ingredients affected by metals in wine. In 2008, researchers at what was then Mercian Corporation, now Kirin Holdings Company, discovered and published findings that iron in wine reacts with certain components in seafood to produce compounds that create a distinctive fishy odor.
According to this research, ferrous iron ions (Fe²⁺) in particular react with lipid peroxides present in seafood to produce specific carbonyl compounds. Six types of carbonyl compounds are generated, and these compounds are strongly suggested to be the cause of the characteristic fishy odor perceived when pairing seafood with wine.
Therefore, this research revealed the following: even white wines, which are generally considered compatible with seafood, as well as red wines, are unsuitable for pairing with seafood when they contain iron.
It should be noted that this reaction, which causes the fishy odor in seafood, is also the oxidation of lipid peroxides by iron ions. Copper and iron ions are catalysts that strongly promote the oxidation of many substances.
The Complex Relationships Among Metal Ions
It is known that copper, iron, and manganese contained in wine and other food ingredients become extremely powerful oxidizing agents when ionized. However, the question of whether the mere presence of these metals causes other substances to oxidize is not such a simple matter.
For instance, iron ions are responsible for oxidizing the lipid peroxides in seafood mentioned earlier. Other metals such as copper and zinc rarely cause this reaction; oxidation of lipid peroxides proceeds only with iron. However, not just any iron ion will do—ferrous ions (Fe²⁺) are specifically required. Ferric ions (Fe³⁺) also cause a similar reaction, but compared to ferrous ions, the reaction is several times weaker, and the degree of fishy odor perceived is milder. In other words, even if wine contains metals, if it contains only copper, for example, pairing that wine with seafood will not produce an unpleasant fishy odor.
Additionally, trivalent manganese ions have stronger oxidizing properties than iron. However, manganese ions do not react with hydrogen peroxide, which iron ions do oxidize. On the other hand, when manganese ions are present simultaneously, the oxidizing action of iron ions is enhanced, and simultaneously the redox cycle of divalent and trivalent manganese ions is accelerated. Here, while manganese ions enhance the influence of iron ions, iron ions also affect manganese ions. Similarly, copper ions promote the oxidation of iron ions. Therefore, when copper ions are present simultaneously, the impact of oxidizing action by iron ions becomes greater.
Each metal ion acts somewhat selectively. However, they simultaneously influence each other's redox cycles. These relationships are extremely complex.
Metal Content and Its Origins
It is already known that even trace amounts of various metal ions in wine can affect wine quality.
For example, copper promotes oxidation of components in wine at concentrations as low as 0.05 mg per liter. Additionally, it has been reported that to avoid generating fishy odors when pairing with seafood, or to suppress the effect to negligible levels, the content of ferrous iron must be reduced to 1 mg per liter or less. However, the actual amount of metals contained in wine is often higher than these values.
The reasons each metal is present in wine differ by metal type. Copper can originate from copper-based compounds used as winemaking materials, but the primary source is pesticides used in grape cultivation. Iron, on the other hand, primarily enters wine through soil in vineyards and iron components used in vineyard equipment. Previously, dissolution from iron winemaking equipment was also a cause of iron presence in wine, but in recent years, contamination through this route has nearly disappeared.
When soil contains no iron and no iron products are used in winemaking equipment, iron content can be kept to around 0.1 mg/L or less. Conversely, in vineyards with soil rich in iron oxide, iron exceeding 5 mg/L may be detected in the juice. Copper content tends to be proportional to disease pressure from inclement weather. Thus, it is known that metal content can vary greatly depending on vineyard location and vintage year.
Metal-Induced Destabilization of Wine
It is widely known that oxidation is not desirable for wine. In this sense, it is not difficult to imagine that the presence of metal ions, which promote wine oxidation, is unfavorable for wine. However, the most significant problem arising from metals being present in wine is that they destabilize the wine.
Natural wines, also called natural wines and recently gaining popularity, are sold without added SO₂. However, SO₂ is added to most wines. SO₂ serves as an antioxidant to prevent wine oxidation.
Consider a scenario where there are two wines containing the same amount of metal ions, one with SO₂ and one without. In this case, oxidation of wine components caused by metal ions actually proceeds faster in the wine containing SO₂. When metal ions are not present, wines with added SO₂ have higher quality stability. However, when metal ions are introduced, this relationship reverses. Wines with added sulfites, which should have higher quality stability, undergo accelerated oxidation and consequently are more likely to become unstable wines. Additionally, such wines are affected in their maturation progression and condition.
Knowing which metal ions are present in wine and in what quantities is not easy. In reality, even wineries often do not have clear information about this. Therefore, even though wineries may be adding appropriate amounts of SO₂ and attempting to produce stable quality wines, the actual outcomes may not align with these intentions.
To avoid such problems, it is necessary to send finished wine to testing facilities to measure metal content. Alternatively, preventive metal removal treatments must be performed without knowing the actual content. However, measuring metal content involves considerable cost. Moreover, even when performing preventive removal work, most winemaking techniques for removing metals from wine damage the taste and aroma of the wine. Therefore, very few wineries actively undertake this.
For example, even if consumers want to select iron-free wines to pair with seafood dishes, it is difficult for them to know this information in advance. However, most producers likewise do not know the types or amounts of metals in their own bottles. Recently, labeling requirements for wine have expanded in Europe as well. As a result, consumers can access more information. However, metals are not yet included in such labeling requirements. The state of metals in wine may be the greatest black box in winemaking.
