You tasted a white wine at a winery or wine shop and liked it enough to purchase it. However, you felt it was still a little early for drinking, so you decided to let it rest a bit longer in your home cellar.
Six months later, at the dinner table. You knew it would be better to let it age a little more. But you couldn't resist and finally brought out that wine. You opened the cork, thinking it was still probably too early, but the moment of enjoyment had arrived. Yet something was different.
It had an oxidized nuance, as if it had been aged for years. Was your storage method wrong? You had properly controlled the temperature and looked forward to opening it.
Have you ever had such a disappointing experience?
It might be UTA.
What is UTA?
UTA is a type of off-flavor found primarily in white wines. Wines affected by this condition have only been aged for a few months to about a year. Despite this short aging period, they already exhibit oxidative nuances characteristic of long-term aging. It may also be described as having naphthalene odor, bean-like smell, hybrid odor (foxy flavor), or dirty laundry smell.
The varietal characteristics of the grape are mostly or completely masked and become imperceptible.
UTA is an abbreviation of the German term "Untypische Alterungsnote." In English, it is called "Atypical Aging (ATA)," which literally means abnormal aging of wine.
It was first identified in Germany in 1983. However, at that time, it was treated merely as an unusual unpleasant odor. It was then confirmed again in the Franken region in 1989, and from 1991 onward, it began to be observed in other grape-growing regions as well. Today, it is an off-flavor found in wines worldwide, not limited to Germany.
The Cause of UTA is AAP
The substance causing UTA has already been identified. It is 2-Aminoacetophenone (AAP). This substance has recently been found to be a causative component of foxy flavor, alongside methyl anthranilate.
Aminoacetophenone, which causes UTA, is not inherently present in grapes. According to current research, it starts with tryptophan (TRP), one of the amino acids, which leads to the formation of indole-3-acetic acid (IAA). From there, AAP is thought to be produced through several stages of reactions.
This IAA is one of the plant growth hormones called auxins. It is also a substance that plays an important role in plant growth.
In addition to this pathway, the following substances have been found to cause AAP formation:
- Indolmilchsäure (indole-lactic acid)
- Indol-brenztraubensäure (indole-pyruvic acid)
- Indol-3-essigsäureethylester (indole-3-acetic acid ethyl ester)
Whether UTA is perceived in wine depends on the wine's style and grape variety. However, when the AAP concentration reaches approximately 0.5 to 1.5 μg/L, it becomes detectable to human sensory organs.
SO₂ Mediates AAP Formation
IAA, which becomes the causative substance of UTA, is one of the important growth hormones for plants, as mentioned above. Therefore, even if it is generated from tryptophan, if it is consumed during the plant's growth process, it does not become a problem afterward.
When IAA is recognized as the causative substance of UTA, an off-flavor in wine, its formation involves SO₂ (sulfur dioxide/sulfurous acid), which is known as an antioxidant used in wine.
Research conducted in 1996 revealed the following: The AAP formation reaction begins with the co-oxidation of sulfurous acid and IAA. Trace amounts of oxygen are converted to superoxide and hydroxyl radicals. Then IAA is degraded to AAP via formylaminoacetophenone.
There are also cases where AAP is generated from indolmilchsäure, indol-brenztraubensäure, and indol-3-essigsäureethylester, which do not start from IAA itself. Even in these cases, the involvement of SO₂ is similarly required.
The Root Cause is Stress on Grapes
We have seen that the chemical substance causing UTA is AAP, and that IAA is involved in its formation.
On the other hand, the cause as a phenomenon that tends to trigger UTA lies in the stress that grapes experience.
- Harvesting too early
- Excessive yield
- Excessive drought stress
- Nutritional deficiency
- Cultivation and winemaking methods that expose grapes to stress
When grapes experience strong stress through these factors, enzymatic reactions are said to increase the amount of IAA produced within the grapes. Additionally, solar radiation in high-temperature environments also affects the increase in IAA production. The IAA that is excessively produced and remains in the grapes reacts with sulfurous acid added during the winemaking process, generating AAP and triggering UTA.
How to Avoid UTA
Several countermeasures have been considered for avoiding UTA.
Since the substance causing this off-flavor is known, the fundamental countermeasure is to suppress the amount of IAA, its precursor. In other words, reducing the stress experienced by grapes is the highest priority measure.
As measures in this direction, the following methods can be considered:
Cultivation Measures
- Eliminate nutritional stress through fertilization
- Avoid drought stress through irrigation
- Harvest when sufficiently ripe
- Adjust yield in advance to avoid excessive harvesting
Winemaking Measures
- Avoid excessively long maceration
- Avoid using yeast with excessively fast fermentation rates
- Keep fermentation temperature and tank aging temperature low
- Avoid oxidation as much as possible
- Avoid using winemaking equipment that causes strong stress
However, climate change and rising temperatures have been continuing on a global scale in recent years. There have also been changes in rainfall patterns. Due to these circumstances, the risk of grapes being exposed to heat stress and drought stress in the vineyard has increased. Additionally, the need to secure sufficient acidity in harvested grapes has made it difficult to avoid advancing harvest timing.
Currently, conditions are inevitably becoming more conducive to UTA occurrence.
UTA Does Not Occur in Red Wine
In recent years, UTA has been increasingly discussed. However, while reports have been increasing for white wines, there have been no cases of UTA reported in red wines to date. The same applies to white wines that have undergone maceration or to the fraction pressed at the final stage.
This reason is thought to lie in the radical mechanism that is the fundamental cause of UTA formation.
The antioxidant and anti-radical potential of wine plays an extremely important role in UTA formation. What is considered to have significant influence in this regard are polyphenols, which are abundant in red wines and some white wines.
That said, it is almost impossible to freely control the amount of polyphenols contained in wine. It has also become clear that implementing the cultivation and winemaking measures mentioned above does not necessarily prevent UTA occurrence.
Additionally, cases where white wine contains high concentrations of polyphenols are rare. The impact on wine from increased polyphenol content is not insignificant. In other words, attempting to avoid UTA in white wine through polyphenol concentration like in red wine is not a realistic approach.
Against this background, a method that has been proposed as a more accessible and cost-effective countermeasure is to improve the antioxidant capacity of wine using ascorbic acid (vitamin C).
Preventing UTA with Ascorbic Acid
Ascorbic acid is also known as an antioxidant alongside sulfurous acid. There are many cases where it is added to wine. In Germany, for example, addition to wine is legally permitted up to a limit of 150 mg/L.
In some cases, there are movements to avoid adding sulfur dioxide to wine and substitute it with ascorbic acid. However, while ascorbic acid exhibits excellent effects as an antioxidant, unlike sulfur dioxide, it has no microbial suppression effects. Therefore, from a winemaking perspective, it is not suitable for direct use as a substitute for sulfur dioxide.
Now, consider the case where the target wine is known to have a high risk of developing UTA. In that case, it makes sense not to add sulfur dioxide, which triggers UTA, and instead add ascorbic acid.
However, what if the wine originally had low or no risk of developing UTA? If only ascorbic acid is added without adding sulfur dioxide, it increases the risk of the wine being affected by microorganisms. At the same time, the excessively strong antioxidant action can also interfere with the proper aging of the wine.
The addition of ascorbic acid certainly demonstrates a clear effect in preventing UTA. However, adding it indiscriminately to wine expecting only this effect can be a method with a high possibility of being counterproductive.
To avoid these risks while accurately preventing UTA occurrence, it is necessary to measure the antioxidant capacity of must or wine. Then, the necessity of ascorbic acid addition must be determined on a case-by-case basis. However, measuring the antioxidant capacity of wine involves very high costs. For this reason, simplified tests have now been developed to predict the possibility of UTA occurrence and are being implemented as needed.
The next article will explain the specific effects that ascorbic acid has in wine and the evaluation of wines with ascorbic acid addition.
