The Growing Wine Defect: What Is Smoke Taint?

There is a wine defect that is currently attracting attention. It is smoke taint.

Smoke taint is a defect that occurs when grapes are exposed to smoke. In tasting, it is described with nuances such as "smoky," "burnt rubber," "bacon," "disinfectant," and "ash." Imagine the smoky aroma you would feel near a campfire or barbecue. This will help you understand this scent more easily.

Normally, you would not detect such aromas in wine. Therefore, it is treated as a defective odor—an off-flavor. You might detect similar aromas from wines that use heavily toasted oak barrels. However, smoke taint differs from such so-called roasted aromas. It carries much stronger burnt, smoky nuances.

Smoke taint began to be identified as an off-flavor in wine around the early 2000s. It is a relatively new defect. As such, it is also new as a research subject, with the first report published in 2007. However, since then, research cases have increased dramatically. Recently, it has begun to attract attention worldwide.

The background to this is global climate change.

Why is smoke taint attracting attention now? We will explain below.

Increasing Forest Fires

Smoke taint is a defect that occurs when grape berries and vines are exposed to smoke. Forest fires are the source of this smoke. For this reason, smoke taint was previously recognized as an off-flavor specific to production regions with frequent forest fires, such as California, Australia, South Africa, Chile, and Canada.

However, the situation has been changing recently. Against the backdrop of global climate change, rising temperatures, decreased rainfall and shifts in rainy seasons, and the occurrence of strong winds are taking place. The risk of large-scale forest fires occurring has rapidly increased even in regions where wildfires were previously not a concern.

In European regions, approximately 500,000 hectares are lost to fires annually in recent years. It is said that 85% of these occur in Mediterranean coastal countries. Moreover, both the number of incidents and the affected area are expected to continue increasing in the future. Furthermore, due to rising temperatures and advancing desiccation, it has been pointed out that even the boreal forests existing in Northern European regions are now exposed to the risk of forest fires.

Most of these fires occur between June and October. This corresponds exactly to the grape growing season through harvest period. Since smoke taint is caused by contact with smoke from fires, even if fires occur at some distance from grape production areas, smoke generated by large-scale fires can drift on the wind and become a potential risk.

Currently, it is said that situations where a certain degree of visibility can be maintained do not lead to damage. However, precise exposure conditions have not been identified, so this cannot be considered certain. Smoke taint has now become an issue that wine production regions worldwide cannot consider as someone else's problem.

The Cause: Lignin and Volatile Phenols

The substances that cause smoke taint perceived in wine are roughly understood. They are volatile phenolic compounds.

Among these, guaiacol (also known as 2-methoxyphenol, a substance with a Seirogan-like odor), 4-methylguaiacol, 4-ethylguaiacol, 4-ethylphenol, eugenol, and furfural are known to be primarily detected in grapes exposed to smoke. However, which of these substances is the definitive causative agent is still unknown.

Unidentifiable Causative Substances

The volatile phenolic compounds that cause the defect are produced when lignin, a component contained in plant materials that serve as fuel during forest fires, undergoes thermal decomposition. Lignin itself is an essential component when plants undergo lignification, so it is contained in virtually all plants. However, its composition varies by plant species. In other words, the state of lignin present differs not only by country or region, but also by forest location, and even by season and year.

What makes it difficult to identify the causative substances of smoke taint is that materials with different compositions are burned under different combustion conditions. Since the underlying conditions differ in each case, the volatile phenolic compounds generated from them do not remain consistent.

According to one survey, more than 30 types of volatile phenolic compounds have been detected in grapes that actually suffered smoke damage. Among these phenolic compounds, it is believed that some have adverse effects on wine aroma and taste while others do not, requiring differentiation based on the degree of impact.

Additionally, guaiacol and 4-methylguaiacol have traditionally been considered the main causative substances. However, these are merely measurement markers, and it is also suggested that the actual defect may be a complex defect caused by a broader range of substances.

Mechanism of Development

The process by which volatile phenols come to be detected in wine begins when grapes and grape vines are exposed to smoke generated by forest fires.

Generally, smoke taint is often thought to occur when causative substances present in smoke adhere to the surface of grape skins. For this reason, it is sometimes suggested that grapes should be washed after smoke damage occurs. However, in reality, the causative substances exist inside the grapes, not on their surface. Therefore, even if grapes are washed, it has already been proven that smoke taint development cannot be avoided.

Volatile Smoke Compounds Absorbed by Grapes

Compounds containing volatile phenols generated by the combustion of lignin contained in plant tissues become smoke that covers vineyards, including other substances as well. Subsequently, these compounds are absorbed from contact points into grape berries, or into parts such as leaves and stems.

The precise pathway by which compounds are absorbed is not yet understood. However, it is known that the movement of absorbed substances within plant tissues is minimal. Therefore, it appears that they are directly absorbed from places where smoke makes contact.

Additionally, it has been revealed that the volatile phenol content had already increased in wine made from grapes harvested from vines that had been exposed to smoke for only 30 minutes. This suggests that such absorption processes occur almost instantaneously.

At the same time, it becomes clear that any measures taken in smoke-damaged vineyards after forest fires would already be too late.

Compounds That Become Non-Volatile

The causative substances absorbed into grape plant tissues initially exist as aglycones. However, they subsequently rapidly combine with sugars present in grape tissues (glycosylation) to become non-volatile glycosides. What is generated at this stage is a complex carbohydrate called a glycoconjugate. This substance exists in grapes and grape juice as a so-called precursor.

To put it very simply, at the point when substances are absorbed by grapes, they are volatile, so humans can detect them as odors. However, once absorbed and converted to precursors, they lose their volatility. Therefore, humans cannot detect these substances by smell until they become volatile again.

This is one of the reasons why smoke taint damage tends to become severe. Winemakers cannot easily know the extent to which grapes have been contaminated by smoke at the time of harvest.

Unavoidable Acids and Saliva

The causative substances that come to exist as precursors in grapes enter wine through the winemaking process. Some of the precursors that mix into wine are decomposed during fermentation by acids or enzymes produced by yeasts. They again become volatile aromatic organic compounds and begin to emit odors.

For example, the threshold for detecting guaiacol is 0.0074 ppm, an extremely minute amount. Therefore, when the degree of contamination is severe, smoke taint becomes perceptible at this point.

In one verification case, when volatile phenol glycosides serving as precursors were present at 20 mg/L, more than 100 μg/L of volatile phenols were reported to be released into the wine. This corresponds to more than 0.1 ppm, indicating the presence of volatile phenols in wine that far exceed the threshold for guaiacol.

On the other hand, even when smoke taint defects are not detected during fermentation, this defect may emerge later. Non-volatile volatile phenolic compounds are decomposed and volatilized not only by acids and yeast enzymes, but also by enzymes contained in human saliva.

Therefore, they may be decomposed and volatilized by acids contained in wine during storage and aging periods. They may also be decomposed and emit odors in the mouth when drinking the wine.

To completely remove this defect from wine, the only method is to remove all target substances, including precursors in their non-volatile state. However, such methods have not yet been put into practical use.

A Defect with No Effective Countermeasures

Like other wine defects, once smoke taint occurs, it is extremely difficult to remove completely and is considered virtually impossible. Currently, measures considered effective do not focus on removing the defect that has occurred, but rather on reducing it to make it less noticeable.

However, even such measures are ineffective when the degree of defect is severe. The current situation is that producers must either sell the wine knowing it contains off-flavors or give up on sales and dispose of it.

One reason why responding to smoke taint is difficult lies in the fact that the contamination status of grapes cannot be easily determined. Detecting substances existing as precursors requires expensive precision measurement equipment such as chromatography. Therefore, it is not realistic for individual wineries to own such equipment themselves. Moreover, even when requesting testing from inspection agencies, costs accumulate.

Nevertheless, if wineries proceed without particular testing, spend considerable time and effort to finish the wine, and then discover that it contains smoke taint—especially if the degree is severe enough to require disposal—the economic damage suffered by the winery becomes considerably large.

Recently, in some countries and regions where forest fires occur frequently, insurance policies responding to smoke damage have begun to be sold. However, similar to cases of hail damage, the burden on wineries is substantial, and the number of wineries that can choose such options is limited.

Currently, winemaking techniques that can reduce the degree of smoke taint and methods that can remove volatile phenols contained in wine to some extent are being developed. However, all of these have significant effects on wine style and remain in the range of being "better than nothing" as technologies.

Wine containing severe smoke taint carries an even stronger burnt sensation than wine with extremely heavy toast from oak barrels. Therefore, it simply cannot be sold as a commercial product.

If such wine could be improved even slightly using currently considered methods and brought to market, that would certainly have meaning. However, if this still cannot restore the wine's original aroma and flavor, it remains a fact that this would still be fatal for producers.

Climate Change and Human Factors

It is said that more than 80% of forest fires occurring worldwide are based on human causes. Previously, even when such causes existed, the environment was fortunate enough to avoid major incidents, but this is changing against the backdrop of recent climate change.

Once the environment changes, as long as causes exist, fire occurrence can be said to be inevitable.

Recently in the wine industry, efforts to attract tourists to areas around vineyards as part of wine tourism have been actively pursued. While these efforts generate positive effects in promoting wine sales, the number of fire accidents caused by cigarette littering and similar actions is also showing an increasing trend.

Such environmental changes are also raising the possibility that vineyards will encounter unexpected smoke damage. Once fires occur, there is not only the risk of vineyard destruction itself, but also the risk that wine becomes unsellable through smoke taint.

Such risks can no longer be considered someone else's problem for wineries around the world.