A Comprehensive Guide to Petrol Notes in Wine

Are you familiar with the term "petri notes"? This is a specialized term frequently used in wine tasting, particularly when evaluating aged Riesling wines.

Experienced tasters can sometimes identify a wine's grape variety as Riesling the moment they detect this aroma in blind tastings. While petrol notes are relatively well-known among wine-related off-flavors, few people understand their exact nature or the causes behind their development.

"Petrol" is the British English term for gasoline. In the wine world, "petrol notes" refer to aromas reminiscent of gasoline, also described as kerosene, fuel oil, or aviation fuel aromas. This scent can be likened to the smell that permeates airport tarmacs around aircraft departure and arrival areas.

Wine and kerosene may seem completely unrelated, yet certain wines genuinely exhibit these aromas. This article will thoroughly explain why some wines develop gasoline-like scents.

What You'll Learn from This Article

By reading this article, you'll understand:

• The true identity of petrol notes and their formation mechanism
• Why these aromas appear in wine
• Measures wineries implement to prevent these aromas

Petrol notes, like paint thinner or wax aromas, are polarizing scents that clearly divide preferences. Whether you love or hate this aroma, understanding its nature will help you predict its presence before opening a bottle. By the end of this article, the fixed notion that "petrol notes = Riesling" will likely be significantly challenged.

Are Petrol Notes Unique to Riesling?

Petrol notes are often discussed as synonymous with Riesling. The association is so strong that some people can identify a grape variety as Riesling solely based on detecting this aroma in blind tastings.

But are petrol notes truly unique to Riesling?

The answer is "No."

While Riesling wines frequently exhibit this aroma, it's not exclusive to Riesling. Theoretically, wines made from any grape variety—whether red or white wine grapes—can potentially develop detectable levels of this aroma.

The perception that "petrol notes = Riesling aroma" is indeed nothing more than a stereotype that we must first understand and move beyond.

Conditions That Favor Petrol Note Development

Wines that tend to develop petrol notes share several characteristics:

• Wines made from Riesling grapes
• Wines fermented with specific yeast strains
• Aged wines
• Wines produced in warm regions
• Wines produced in hot vintages
• Wines stored in relatively high-temperature environments

The reasons behind these conditions will become clear in the following explanations. Note that if you dislike petrol notes, selecting wines that don't meet these criteria can significantly reduce your chances of encountering this aroma.

The Identity of Petrol Notes: The TDN Compound

Those studying wine may have heard that TDN causes petrol notes.

TDN stands for 1,1,6-trimethyl-1,2-dihydronaphthalene. Pay attention to "naphthalene" at the end of this long chemical name.

Naphthalene refers to a substance manufactured from naphtha, which is refined from crude oil. It's also known as a component in mothballs and insecticides. This fact should help you understand why compounds containing this substance produce gasoline-like aromas.

Interestingly, while TDN's human perception threshold is approximately 2μg/L, aged Riesling wines sometimes contain TDN levels exceeding 50μg/L.

This fact that "aged Riesling wines tend to contain large amounts of TDN" is the primary factor creating the equation "TDN (petrol notes) = Riesling."

TDN's Limitations: Not the Root Cause

TDN is clearly the compound responsible for petrol notes. Therefore, many sources explain TDN's presence as the cause of petrol notes. However, from grape growing and winemaking perspectives, while this explanation is accurate, it's incomplete.

This is because TDN is the direct cause of petrol notes, but it's not the fundamental reason this off-flavor appears in wine.

The True Cause: Carotenoids

The statement "TDN causes petrol notes but isn't the fundamental cause of the off-flavor" may be confusing. Let's clarify.

First, TDN is indeed the direct causative compound for petrol notes. (TDN itself possesses petrol-like aromas; it doesn't react with other substances in wine to produce this scent.)

However, most TDN found in wines with petrol notes barely exists in grape juice at harvest and during fermentation and other winemaking stages. Most TDN forms during the aging phase after fermentation (or after winemaking).

In other words, directly controlling TDN during cultivation or winemaking stages is impossible because TDN barely exists at these points.

The key point is that TDN exists like a byproduct. In this sense, TDN isn't the fundamental cause of the off-flavor.

So what is the fundamental causative substance? That would be carotenoids.

The Conversion Process from Carotenoids to TDN

Carotenoids are a collective term for natural pigments including β-carotene, famous for being found in carrots. As a "collective term," carotenoids don't refer to a single specific substance but represent a group of nearly 800 different compounds.

According to Wikipedia, carotenoids are defined as follows:

"Carotenoids are a group of natural pigments displaying yellow, orange, and red colors. Over 750 types of carotenoids have been identified from microorganisms, animals, and plants. Carotenoids serve diverse physiological functions in nature, playing particularly important roles in auxiliary light-harvesting, photoprotection, and antioxidant activities in photosynthesis."

Grapes contain carotenoids, not TDN.

These carotenoids decompose during grape development, fermentation, or wine aging, and their breakdown products undergo enzymatic reactions and resynthesis to form a group of aromatic compounds called norisoprenoids, which includes TDN, within grapes or wine.

TDN isn't the only substance synthesized using carotenoids as a starting point (norisoprenoid). However, as the amount of carotenoids in grapes increases, the relative probability of TDN synthesis increases, resulting in a higher likelihood of petrol notes appearing.

Norisoprenoids: Not Necessarily Negative

TDN is one type of norisoprenoid, but not the only one. Norisoprenoids also include β-damascenone and vitispirane, which contains α- and β-ionones.

β-damascenone, in particular, not only possesses fruity aromas itself but also enhances other fruity aromatic components, making it a valuable component in both red and white wines.

In other words, all wines contain norisoprenoids formed using carotenoids as starting materials. This means all wines potentially contain TDN.

Regarding TDN specifically, small amounts can be viewed positively as adding complexity to wine. This aspect ultimately comes down to personal preference in luxury goods.

The Role and Formation Reasons of Carotenoids

From the previous explanation, many of you have likely realized that reducing TDN requires reducing carotenoids, its source compound.

How specifically can we reduce carotenoids? To understand this, we need to comprehend why carotenoids exist in grapes.

The reason carotenoids exist lies in their functions, as mentioned in the Wikipedia quote above.

Carotenoids serve several key roles:

• Auxiliary light-harvesting function in photosynthesis
• Photoprotection
• Antioxidant activity

In other words, they assist photosynthetic efficiency by absorbing sunlight through their pigmentation while simultaneously reducing damage from that collected light.

This shares the same aspects as phenols, which we've previously discussed. In regions where sun damage becomes severe—areas with strong ultraviolet radiation—carotenoids tend to be produced in greater quantities.

Conversely, if grape damage from ultraviolet radiation can be prevented through some means, carotenoid production can be relatively suppressed.

Beyond strong ultraviolet radiation, other factors promoting carotenoid production include:

• High temperatures
• Drought conditions
• Deficiency of specific nutrients

As these factors show, stress factors are strongly involved in carotenoid production.

Why TDN Develops More Easily in Riesling

We explained that carotenoid properties resemble those of phenols. Both are UV-protective substances that increase in production when ultraviolet radiation intensifies—a common characteristic.

Actually, these two substances share another commonality beyond this: the location where each is produced and accumulated.

Both phenols and carotenoids basically accumulate in grape skin portions. However, there are rare exceptions to this rule. Riesling is one such exception.

In typical grape varieties, carotenoids concentrate and accumulate in skin portions. However, according to research papers, Riesling accumulates carotenoids in the flesh as well as the skin.

If one grape variety accumulates carotenoids only in the skin while another accumulates them in both skin and flesh, the carotenoid concentration per unit weight will clearly be higher in the latter.

And carotenoid content directly correlates with expected TDN production.

Additionally, carotenoids in Riesling tend to have relatively high lutein content. Lutein produces more TDN compared to other carotenoids.

In other words, even if containing the same amount of carotenoids, Riesling, with its relatively higher lutein content, tends to have higher TDN formation probability compared to other varieties.

About Lutein Lutein is one substance classified as a carotenoid, like β-carotene. As mentioned in the text, lutein produces more TDN than β-carotene and other carotenoids.

These facts explain why TDN—petrol notes—develop more easily in Riesling.

Strategies to Prevent Petrol Notes

Methods to prevent petrol notes in wine broadly fall into two approaches:

1. Suppressing carotenoid production
2. Suppressing TDN formation

Let's examine each in detail.

Viticultural Approach: Suppressing the Cause

The first approach involves reducing the content of carotenoids, the fundamental causative substance.

Since carotenoids already peak in content before grape harvest, all strategies to suppress them involve viticultural measures.

The key is eliminating or reducing stress factors affecting grapes.

As mentioned previously, stress factors causing carotenoid production are "excessive sunlight (ultraviolet radiation)," "drought," "excessive temperatures," and "specific nutrient deficiencies." Individual elimination or reduction of these factors can be expected to decrease carotenoid production.

Specific measures include:

• Adjusting leaf removal timing and extent
• Implementing irrigation
• Understanding and appropriately adjusting soil nutrients (primarily nitrogen levels)

Winemaking and Storage Approach: Suppressing Development

The second approach focuses on preventing TDN formation from carotenoids already present in grapes.

The key to TDN formation lies in carotenoid breakdown. If carotenoid breakdown can be prevented or kept to minimal levels, future TDN production can consequently be suppressed.

Factors affecting carotenoid breakdown include oxygen, temperature, and light. Additionally, certain metabolically active yeasts and grape clone types are known to increase TDN production.

Therefore, important approaches to suppress TDN formation include:

• Avoiding problematic yeast strains and clones
• Lowering storage temperatures during (bottle) aging, transportation, and storage

Since yeast influence is difficult to control, focusing on temperature management during storage represents a more practical approach.

The reason low-temperature management is necessary during aging, storage, and transportation is that higher temperatures increase enzyme activity, accelerating breakdown and resynthesis processes.

Once TDN forms, it doesn't disappear. Therefore, if TDN has already formed in bottles before purchase, even subsequent rigorous low-temperature storage will have limited effectiveness.

Summary: Petrol Notes—An Off-Flavor Likely to Increase

At the beginning of this article, we listed wines prone to developing petrol notes:

• Wines made from Riesling grapes
• Wines fermented with specific yeast strains
• Aged wines
• Wines produced in warm regions
• Wines produced in hot vintages
• Wines stored in relatively high-temperature environments

Having read this far, you should now be able to explain why petrol notes appear more easily in each type of wine.

Due to recent climate change, average annual temperatures and maximum temperatures are rising in various global regions, including Germany. This trend leads to increased content of protective substances like phenols and carotenoids in grape fruits, meaning the risk of petrol note development increases.

Reports already indicate that even relatively uneaged wines, particularly Rieslings made in high-temperature regions like South Africa and Australia, are showing increased frequency of perceptible petrol notes.

Petrol notes can certainly be perceived as pleasant aromas by some people. Additionally, since it's widely recognized that aged Rieslings naturally develop this aroma without it being particularly unusual, many people don't consider this aroma an off-flavor.

However, this aroma formation can be somewhat avoided through viticultural and winemaking methods. Most importantly, when petrol notes appear, wines lose their fruitiness.

While I don't oppose treating this aroma as one aspect of aging changes, since wine is made from grapes, losing fruit flavors to gain gasoline aromas seems counterproductive.

Given that this aroma will likely become more prevalent in grape varieties other than Riesling, I believe winemakers should recognize this as an off-flavor and work to minimize it as much as possible.