In a recently published article titled "Wine Off-Flavors | UTA / ATA," I discussed how aged nuances detected in what should be young white wine may indicate the presence of UTA, one type of off-flavor.
In that article, I introduced the addition of ascorbic acid as one method for preventing UTA.
This article examines in detail the effects that adding ascorbic acid has on wine.
Why Add Ascorbic Acid?
To begin with, why is the addition of ascorbic acid effective in preventing UTA?
Ascorbic acid is commonly known as vitamin C. Strictly speaking, the L-form of ascorbic acid is what we call vitamin C, but for the purposes of this article, you can understand ascorbic acid and vitamin C as equivalent.
When you examine the ingredient labels of foods and beverages, you will often find vitamin C listed. This is not added to provide acidity but is added in most cases as an antioxidant.
This function as an "antioxidant" is precisely the property of ascorbic acid needed to prevent UTA.
UTA Generated Through Oxidation
As described in the "Wine Off-Flavors | UTA / ATA" article, 2-Aminoacetophenone (AAP), the substance responsible for UTA, is formed when Indole-3-acetic acid (IAA) co-oxidizes with SO₂ added to wine. Trace amounts of oxygen are converted to superoxide and hydroxyl radicals, and IAA is decomposed into AAP via formylaminoacetophenone.
In other words, UTA occurs as a result of oxidation reactions.
While the explanation of this formation pathway may seem highly complex, put simply: if AAP is a product of oxidation, it will not form if oxidation does not occur in the first place. Indeed, UTA has not been confirmed to occur in red wines, which contain high levels of phenolic compounds with antioxidant properties.
Based on this principle, adding ascorbic acid, which has strong antioxidant properties, enhances the wine's antioxidant capacity and consequently prevents the occurrence of UTA.
Ascorbic acid binds immediately with superoxide (O2-), hydroxyl radicals (OH), and hydroperoxyl radicals (HO2), making it an excellent antioxidant widely used in many foods.
Additionally, as an "antioxidant" for use in wine, ascorbic acid has a better image than sulfur dioxide, which has been used traditionally, and is not subject to misconceptions such as causing headaches. For these reasons, cases of adding ascorbic acid as a substitute for sulfur dioxide can be observed.
Mechanism of Action and Effects on Wine
From here, we will examine the effects that adding ascorbic acid has on wine.
The rate at which ascorbic acid itself oxidizes depends on the wine's pH value (the oxidation of ascorbic acid first prevents the oxidation of other components, and as a result, the wine's antioxidant capacity is considered to have increased). The lower the pH value, the slower the oxidation rate of ascorbic acid.
Additionally, metal ions such as copper and iron present in wine significantly affect the oxidation rate of ascorbic acid.
Ascorbic acid is decomposed, for example, by hydrogen peroxide into tetrahydroxydiketohexane. Beyond this, it is known that in acidic liquids without air, furfural and carbon dioxide are generated, while in the presence of air, water, 3-hydroxy-2-pyrone, and 3-furonic acid are formed.
Furthermore, research has reported that numerous decomposition products are generated.
Each of these many decomposition products is thought to affect wine.
In particular, some decomposition products of ascorbic acid are colorless while others are brown, and the color of these decomposition products affects the wine's color. In citrus juices, for example, furfural, a decomposition product of ascorbic acid under anaerobic conditions, is considered the cause of color changes during storage.
Temperature and oxygen levels affect the decomposition of ascorbic acid and the types of products formed. Increases in both factors accelerate decomposition rates and have a greater impact on the formation of brown-colored metabolites.
Effects on Wine Color
As already mentioned, some decomposition products of ascorbic acid are brown in color, and this affects the wine's hue.
The degree of this effect varies depending on grape variety, but the tendency is more pronounced in wines with no or low sulfur dioxide addition. On the other hand, it is also known that color changes cannot be completely suppressed even if SO₂ is added.
Furthermore, color changes due to ascorbic acid have been confirmed in sparkling wines as well.
Effects on Aging, Taste, and Aroma
It is also anticipated that the addition of ascorbic acid may delay wine aging. In wines where UTA is not a concern, simply delaying the wine's optimal drinking window is normally judged negatively.
Additionally, in cases where wines were subjected to sensory evaluation immediately after ascorbic acid addition, changes in taste and aroma due to ascorbic acid were noted in approximately 50% of wines, and nearly all wines received lower ratings than wines without ascorbic acid addition.
Based on these results, it is considered that ascorbic acid addition should be avoided except in cases where UTA is strongly suspected.
Once AAP has formed in wine, subsequent addition of ascorbic acid cannot remove it, nor can it mitigate its effects. In other words, adding ascorbic acid after UTA has been detected merely results in further meaningless deterioration of wine quality.
How to Decide on Ascorbic Acid Addition
The addition of ascorbic acid is one of the difficult decisions faced during the winemaking process.
For wines where UTA occurrence is anticipated, adding ascorbic acid can clearly improve the evaluation. However, conversely, adding it to wines without UTA will lead to a significant reduction in that wine's evaluation.
Moreover, due to the properties of ascorbic acid, if strong reductive aromas occur in the wine, copper-based countermeasures cannot be employed.
While the negative impact on wine from UTA occurrence is certainly significant, being hesitant about addressing issues like reductive aromas—which typically manifest earlier than UTA—out of concern for UTA is also a major problem.
Some negative effects, such as color changes caused by ascorbic acid addition, can be avoided by simultaneously adding the necessary amount of SO₂ along with ascorbic acid. However, conversely, this also means that ascorbic acid cannot be used as a substitute for those who wish to avoid using SO₂.
Furthermore, it has been suggested that even in wines where SO₂ or ascorbic acid has been added, UTA may still occur after these substances have completely decomposed. While UTA is certainly a threat, the addition of ascorbic acid cannot necessarily be called the correct answer in winemaking.
Conclusion
While the addition of ascorbic acid certainly has significance as a countermeasure against UTA, it must be understood that this is limited to the extent of preventing early AAP formation.
In some cases, the efficacy of ascorbic acid is emphasized as an antioxidant alternative to sulfur dioxide. However, as we have seen, this is not necessarily something to be praised unconditionally.
With this understanding, winemakers must make the decision whether to add this substance to wine despite the possibility that it may reduce wine quality.
