Riesling is like Rosie the Riveter: it’s got some gentle features, a sweet personality, yet a perfume that smells distinctly like gasoline.
This gasoline odor comes from a big molecule called TDN (1,1,6-trimethyl-1,2-dihydronaphthalene) which belongs to the norisoprenoid family of aromas.
Norispoprenoids are a class of varietal aroma, meaning they originate in the berry. (Remember, varietal aroma families include terpenes, pyrazines, thiols, and norisoprenoids!)
Here are some common norisoprenoids and their corresponding aromas in wine:
ß-Damascenone: sweet and fruity, also an aroma enhancer (think of it like the MSG of wine aromas. A little bit will enhance other smells in the wine!)
Vitispirane: Floral, fruity, woody (similar to eucalyptus oil)
Norisoprenoids are formed from the breakdown of carotenoids, the yellow-ish color pigments that develop in berries in the vineyard. When it gets really sunny, the grapes need a way to protect themselves from UV light — just like humans. So they build up carotenoids to act as a natural sunscreen!
As berries reach maturity, these carotenoids break down into norisoprenoids. At this point, the norisoprenoids are bound to sugar molecules. If you’ve followed my other blogs on terpenes and thiols, this will sound familiar. When an aroma molecule is bound to a sugar molecule, it is “tied-up” and odorless. We say these aromas are actually “aroma precursors” because they will one day be cut free from the sugar molecule and give off an odor. TDN has a very low odor threshold, meaning that even a tiny tiny amount of it (~ 2 μg/L) is detectable by humans.
ß-Damascenone generally becomesodorous during fermentation when the yeasts enzymatically cut the link between the sugar molecule and the ß-Damascenone molecule. But TDN and Vitispirane don’t become odorous until the aging process when acid hydrolysis cuts the link between the norisoprenoid and the sugar molecule. This usually happens after extended aging in the bottle, and the effect is higher when storage temperatures are higher. One theory on why TDN is more pronounced in older wines is that in younger wines, other aromas like floral monoterpene aromas may mask TDN’s presence. As time goes on the fresh, floral, or fruity aromas may give way to deeper, less fresh aromas like gasoline.
TDN can also be higher in bottles that have a screwcap vs. bottles that have a cork. This is likely due to “flavor scalping” whereby a packaging material absorbs some of the volatile aromas of its contents.
The common gasoline odor in Riesling wines comes from TDN, which is in the norisoprenoid family of aromas.
Norisoprenoids form from the breakdown of carotenoids, which are like a natural sunscreen that builds up in berries to protect them from UV light.
The norisoprenoid ß-Damascenone (fruity, sweet smell) is released during fermentation by enzyme hydrolysis. TDN (gasoline) and Vitispirane (similar to eucalyptus oil) become odorous during wine aging in the bottle.
Screwcap closures and warmer bottle storage temperatures can lead to more pronounced TDN aromas.
If you’ve ever smelled a New Zealand Sauvignon Blanc, you’ve smelled thiols (“thigh-all”, rhymes with “style”). These are the aroma compounds that smell like grapefruit, guava, passion fruit, and yes, sometimes cat pee.
What are thiols?
Thiols are odorous compounds that contain sulfur. Our understanding of thiols is a recent development in the wine world. In 1995, some researchers in Bordeaux identified a volatile thiol, called 4MMP for short, which is a dominant aroma in Sauvignon Blanc. They found this compound after they realized that adding copper to the wine removed tropical fruit smells. Since they knew that copper reacts with sulfur to form an odorless compound, they worked backward and concluded that the tropical fruit aromas must be coming from a sulfur-containing volatile thiol.
Since then, many different volatile thiols have been found. The most common include:
4MMP (4‐mercapto‐4‐methylpentan‐2‐one): Boxtree, passion fruit, broom, black current
You can see that one aroma compound can produce a range of different smells depending on its concentration. In the right amount, volatile thiols will smell like tropical fruits. In the wrong amount, they can smell like cat pee.
Which wines have thiol aromas?
Sauvignon Blanc is the thiol king, but thiols are also present in Semillon, Gros and Petit Manseng, Merlot, Grenache, Cabernet Sauvignon, and rosé wines from Provence.
How do thiols form?
Thiols start as aroma precursors in the grape berries in the vineyard. These precursors are called cysteine S-conjugates or glutathione S-conjugates, which simply means that cysteine (an amino acid) or glutathione (a compound that contains cysteine) is bound to a sulfurous compound. For example, a precursor for 3MH is Cysteine-3MH.
One classic thiol-dominant wine is Sauternes, a sweet wine from the Bordeaux region that is made from Semillon grapes attacked by the fungus Botrytis cinerea. Thiol precursors in the grapes develop after the fungus sets in. To fight the attack of Botrytis, the grapes will defend themselves by producing a toxic aldehyde, hexanal. The problem is that hexanal is not only toxic to Botrytis, but also to the grape. So to lower the toxicity, glutathione will bind to the hexanal. Enzymes break this glutathione-hexanal compound down into a cysteine S-conjugate. This cysteine-3SH is the precursor for 3SH, which smells like grapefruit. In short:thiols can develop as a natural, biochemical response of the grape berry to ward off an attacking pathogen!
Do thiol aromas change with age?
Once these thiols have formed, they are not totally stable. In the presence of oxygen, they can bind with quinones, the compounds that turn wine brown. This “traps” the thiols and they become odorless. To prevent this aroma loss, sulfites, ascorbic acid, or glutathione can be added to grape juice before fermentation. Instead of binding to the thiol, the quinone will bind to the sulfite/ ascorbic acid/ glutathione. This is why it’s important to limit oxygen exposure at bottling, and/or to add an antioxidant. Wines that are aged on the lees (the dead yeast cells) are also more protected from aroma loss. The reason is that hydrogen sulfide (H2S) is released from the lees, and H2S will bind to quinones faster than thiols, preserving the thiol aroma.
Thiols generally smell like tropical fruits. At high concentrations, they can smell like cat pee.
Thiols are the dominant aroma in Sauvignon Blanc wines, especially from New Zealand. But other white grapes (Semillon, Gros/Petit Manseng) and red grapes (Grenache, Merlot, Cabernet Sauvignon) can also have thiol aromas.
Thiols start out as precursors in berries in the vineyard. They do not become odorous until fermentation when the yeasts degrade the precursor and release the odorous component.
Botrytized wines like Sauternes have very high thiol aromas since thiol precursors are built up in the berry as a means of self-protection after the fungal attack.
Oxidized wines can lose their thiol aromas. This is why adding an antioxidant like sulfur, glutathione, or ascorbic acid is important in white wines. Aging on the lees can also reduce oxidation and aroma loss.
Pyrazines are a nitrogen-containing compound. They are a type of varietal aroma, meaning they originate in the grape berries in the vineyard.
The most distinctive pyrazine smell is green bell pepper. The compound responsible for this smell is IBMP, which is the shorthand name for 3-Isobutyl-2-Methoxypyrazine.
Besides green bell peppers, pyrazines can also have a leafy green, broccoli stalk, or pea pod smell. Pyrazines are really odorous compounds; it only takes in the order of 2 parts per trillion for human noses to be able to detect the green bell pepper odor that comes from IBMP.
Which wines have pyrazines?
Cabernet Sauvignon, Cabernet Franc, Merlot, Carménère, Malbec, and Sauvignon Blanc.
Basically, anything from Bordeaux is going to have some green pepper smell, especially Left Bank blends dominated by Cabernet Sauvignon. Sauvignon Blanc from New Zealand will also have this aroma. A good rule of thumb is that wines from cooler climates will have more of these green, vegetal flavors (keep reading to find out why!)
How does IBMP form?
There are two contrasting hypotheses about how IBMP forms. One thought is that it originates in the leaves of the vine and then travel to the berries via phloem vascular tubes, the tubes that carry and distribute nutrients throughout plants. The other thought is that it originates in the berries themselves. Maybe it’s a combination of both!
How can we manage vegetal aromas in the vineyard?
Managing the intensity of pyrazine aromas in wine can be done by playing with sunlight exposure at the right time.
Can anything be done in the cellar to get rid of this smell?
Yes, thermovinification, or fermenting the grape juice at a really high temperature for a short period of time, can help. Since IBMP is volatilized at 50°C (122°F), quickly increasing the temperature of the wine can help to get rid of some of those vegetal aromas.
Do vegetal aromas go away with time?
Probably not. It’s been shown that for a red Cabernet Sauvignon and a white Sauvignon Blanc, after three years of aging in a dark cellar, there was no change to IBMP levels. This is why it’s so important to control in the vineyard!
Pyrazines give off vegetal aromas in wines.
IBMP, the most common pyrazine in wine, has a green bell pepper smell.
Pyrazine aromas are most common in Bordeaux varieties, like Cabernet Sauvignon, Merlot, Cabernet Franc, and Sauvignon Blanc.
Sunlight exposure before véraison can decrease IBMP levels in berries. This is why wines from cloudy, cool-climates tend to have more vegetal aromas.
Thermovinification, or quickly heating the wine to a very hot temperature, can volatilize IBMP. This reduces vegetal aromas in wine.
Pyrazine aromas in wine do not seem to go away after wine aging.
If you’ve ever smelled a Muscat or Gewürztraminer wine, you probably noticed that it smelled like flowers. Where does this smell come from? Terpenes.
What are terpenes?
Terpenes are found in the skin of the grape. They constitute many of the floral, citrusy, or spicy compounds in essential oils. They are also responsible for the floral smell of marijuana.
Terpenes are made up of hydrogen and carbon, and they can produce different smells depending on the arrangement and length of those molecules. Sometimes you will see them called monoterpenes (containing 10 carbons), sesquiterpenes (containing 15 carbons), or diterpenes (containing 20 carbons).
The most common terpenes in wine, including linalool, geraniol, and nerol, are monoterpenes. You’ll find these aromas in Muscat, for example. Rotundone, responsible for the pepper aroma in Syrah, is a sesquiterpene.
Wines described as “aromatic” have a high level of monoterpenes
The Muscat family of grapes (i.e. Muscat Blanc, Muscat of Alexandria, Muscat Ottonel, and over 200 other varieties) is among the most aromatic. Grapes like Riesling or Viognier are somewhere in the middle, and non-aromatic wines like Chardonnay or Pinot Gris have almost no monoterpene aromas.
Muscat, Moscato, Muscadelle…what’s the difference!?
If you like aromatic wines, you’ll like Muscat. Stick to the names on the left!
Terpenes get “revealed” during fermentation
Terpenes develop in the vineyard but their aromas become volatile during fermentation. It’s been shown that the more mature the grape when it is picked, the greater the terpene content in grape skins. And since terpenes are located in the skin of the grape, terpene aromas in wine can be enhanced by skin maceration (soaking the juice/wine with the grape skins).
Before fermentation, terpenes generally exist in their precursor stage when they are bound to a sugar molecule. We cannot smell terpenes in this bound form.“Free” terpenes do exist in grapes but in very small quantities — bound terpenes are 2–8x more common. This means that most terpenes need to be released from those sugar molecules to become odorous.
The aroma gets volatilized (released) via an enzyme that comes from yeast during fermentation, from acid enzymes during aging, or even by bacterial enzymes in your mouth as you are drinking the wine! For more info on how this works, see my aroma precursors blog.
Terpenes give off floral, citrusy, or spicy aromas in wine.
Terpenes are made up of carbon and hydrogen. Monoterpenes, the most common type, are made up of 10 carbons.
Since terpenes are located in the skin of the grape, terpene expression in wines can be enhanced by skin maceration.
If a wine is called “aromatic” it means it has a high level of monoterpenes.
Monoterpenes are most present in Muscat grapes, somewhat present in grapes like Riesling, and pretty much non-existent in neutral grapes like Chardonnay.
Terpenes exist as precursors bound to a sugar molecule. They are volatilized (released) during fermentation thanks to yeast enzymes.
If you’ve ever let a bottle of wine sit out for too long on your counter, you’ll know that wine aromas can change over time — and not always for the better. But oftentimes these changes make the wine more interesting to drink.
I like to think of these changes as the ones we can “see” and the ones we can’t see. If a wine ages in new oak barrels, it’s going to taste and smell oaky. We can see the barrel, and we can anticipate its effect.
But then there are the changes that we can’t see. These are the tiny little chemical reactions taking place within the wine itself. Those reactions that take place during winemaking or aging can “reveal” new aromas from wine aroma precursors.
So much potential
An aroma precursor is a primary aroma that is currently non-volatile, meaning it doesn’t give off an odor, but it could be volatilized into a smell detectable by the human nose. You can think of these as potential aromas, the building blocks for what we can actually smell. In other words, they are the “ingredients” for the aroma, but they need the right conditions to change in a way that our human noses can perceive them.
Volatile aroma precursors develop in the grape berry in the vineyard. It starts around the stage “fruit set,” the stage when the fruit appears on the vine as tiny little green beads. These aroma precursors continue to develop until berry maturation when it’s time to harvest. Because they are mostly stored in the grape skins, extended maceration time can lead to more intense aromas, especially for floral aromas.
Types of aroma precursors
There are many types of wine aroma precursors. Some have desirable effects on the wine, like cysteine conjugates (which just means something joined to the amino acid, cysteine). Others are bad, like phenolic acids which can lead to a horse or leather smell. The diagram to the left summarizes it well.
To keep it simple, I’ll focus on one of the main aroma precursor forms, glycoconjugates. All this means is a sugar (glyco-) bound to something else (conjugate). This conjugate is also called an “aglycon” and can be in the form of phenolic acids, monoterpenes, alcohols, etc. These aglycons, in pink below, are the volatile aroma precursors!
So how does the aroma get volatilized so that we can smell it? It is believed to happen either during the process of fermentation or during aging via enzymatic or acid hydrolysis. Simply put, hydrolysis means the breaking of a bond in the presence of water.
In enzymatic hydrolysis, an enzyme cuts the bond between the precursor and the sugar molecule. This happens mainly during the fermentation stage because the enzyme, glycosidase, comes from the yeast Saccharomyces cerevisiae. But this enzyme can also come from other yeast strains and even from lactic acid bacteria (the microbe responsible for malolactic fermentation).
In acid hydrolysis, the acid cuts the bond between the precursor and the sugar molecule. This happens mainly during the aging process. Once the precursor and the sugar molecule are separated, the precursor can then change chemically during the wine aging depending on the wine conditions (i.e. temperature and pH). This helps to explain why aromas can change during wine aging.
It’s a personal thing
There is a study that was published in 2018 describing how the bacteria in our mouths might actually be able to volatilize wine aroma precursors as we’re drinking!
Researchers found that when the study participants used mouthwash to kill mouth bacteria, and then took a sip of wine, the volatilization of the aroma didn’t take place. This suggests that oral bacteria do have a role to play in aroma revelation! Once the aroma is volatilized in the mouth, it is smelled via retronasal olfaction (by the passageway from your mouth to your nose). For a refresher on taste vs. smell vs. flavor, check out this post.
The fact that our perception of wine aromas can be altered based on our mouth bacteria means that wine tasting is quite literally an individual experience. If someone tells you you’re wrong because you’re not tasting their ridiculous aromas of “gushing blackberry” or “smashed minerals”, don’t feel like you’re not part of the club. You could just have different mouth bacteria — and different poetic interpretations!
Also, just because a volatile aroma precursor is cut from the sugar molecule, it won’t necessarily become detectable by human noses. Each aroma compound has a threshold under which we cannot detect the smells (roughly in the parts per billion range). And each person has their own smelling ability. Some people have noses like dogs, others can barely smell at all. This is another way that wine is — and will always be — a personal thing.
Aromas can start as non-volatile “potential” aromas.
The precursors are often bound to something else (a sugar or an amino acid, for example). To perceive them, they have to be cut free (i.e. “volatilized.”)
The volatilization process can take place via enzymatic hydrolysis during fermentation, acid hydrolysis during wine aging, or even by bacteria in our mouths while drinking.
Volatile aromas can continue to develop and change during wine aging, and this will depend on parameters like temperature and pH of the wine.
Some free volatile aromas might exist in a quantity too small to be detectable by the human nose. There is a unique odor threshold for each volatile aroma. Plus, each person has their own smelling ability.
After visiting a winery with friends, I once made the pretentious mistake of scoffing when someone asked what kind of flavoring was used to give the wine an almond smell — as if it was as simple as putting a flavor shot in a Starbucks drink. But when my laughter subsided, I realized they weren’t joking. I got down from my high horse and realized that this is a perfectly normal assumption. How else could fermented grape juice smell like almonds?
This is the magic of wine. From grapes we can get aromas of almonds, pears, lavender, rocks, pineapple, even gasoline.
But let’s back up.
What is an aroma? How is an aroma different than a flavor or a taste? The way scientists describe these words is different than how we use them in everyday language.
An aroma is what you smell, either with your nose or with your mouth. (Yes, your mouth. We’ll get to that below.)
A taste is what you perceive in your mouth to be sweet, salty, sour, bitter, and umami. This is different from the mouthfeel, which is, literally, the way something feels in your mouth. Mouthfeel is a sensation, not a taste.
A flavor is a multisensory experience that occurs in your mouth when you eat or drink. It includes taste, aroma, and tactile sensations. Some scientists suggest that flavor even includes visual and auditory cues that we perceive when eating or drinking.
So let’s take a few examples to understand this better. When you drink red wine, you are feeling and tasting the tannins. This is because tannins are both astringent (a feeling) and bitter (a taste). However, tannins are non-odorous compounds; they do not have a smell.
But if you consider the aromas of that glass of red wine, you’ll start to understand how the words flavor, taste, and aroma can get mixed up easily.
First, you smell the wine. You are taking in its aromas. You might get smells of blackberry and lavender. When you take a sip of the wine, you might notice the taste of mint. But this might actually be the flavor of mint; that is, the taste and the aroma of mint.
Why? Because we smell through our mouths too, not just our noses. Smelling through your nose is called orthonasal olfaction, and smelling through your mouth is called retronasal olfaction.
There is a passageway that allows smells to travel from your mouth to your nose, where it is processed by the olfactory bulb in your brain. What we so often think of as being a taste is actually a smell. This explains why when you’re sick and have a stuffy nose, your ability to “taste” food diminishes. In fact, you weren’t really tasting the food after all; you were smelling it.
Now that you’ve got down the science of sniffing, let’s talk about where wine aromas come from. There are three different kinds of aromas found in wine: primary, secondary, and tertiary.
1. Primary aromas come from the grape variety.
This could be fruit smells, flower smells, or herb smells. For example, Cabernet Sauvignon is known to have a green pepper smell. Viognier is known to have a rose smell. Sauvignon Blanc can have tropical fruit smells. These aromas come from various chemical compounds in the grape itself.
2. Secondary aromas come from the fermentation process.
These are mostly the smells that come from the yeast strain. This is what gives that yeasty, bready smell. It can also be buttery or milky (from malolactic fermentation). Sometimes, when wines are left to age on the lees (the dead yeast cells), this can impart more secondary aromas.
3. Tertiary aromas come from the aging process.
These are the smells that get revealed during aging, like leather, mushrooms, tobacco, nuts, and even dried fruit or complex spices. Tertiary aromas arise due to the wine’s interaction with oxygen, which can seep into the wine in tiny amounts through the barrel or the cork (if already bottled). If the wine is aged in barrels, the oak will also impart an aroma to the wine.
Another distinction to make in sniffing science is the difference between wine aromas and wine bouquets. Generally, when people talk about aromas, they’re referring to the primary aromas. When people talk about the wine bouquet, they’re talking about the secondary and tertiary aromas. This is why you’ll hear the term “bouquet” used more for aged wines.
It’s important to note that not all of these aromas are good. Sometimes there can be off-flavors in the wine, and these can arise from undesirable types of yeast (such as Brettanomyces), acetic acid bacteria, a lack of nitrogen in the grape must, or even from the cork.
And there are also some instances when the aroma doesn’t quite fit into the category of primary, secondary, or tertiary.
Remember that almondy wine? It was a Sauternes wine, which is a type of wine made in the Bordeaux region from white grapes that have been attacked with Botrytis cinerea, a fungus also known as “Noble Rot.”
The almond smell could have been from a compound called Sotolon, which arises in Botrytised wines during the aging process, likely due to the interaction of oxygen on the wine. In this case, yes, it would be a tertiary aroma.
But it might also have been due to something entirely different. When the Botrytis fungus attacks the berry, the berry responds by producing benzyl alcohol, which is toxic to the Botrytis. In return, the Botrytis produces the enzyme necessary to degrade the benzyl alcohol into benzaldehyde — the compound responsible for the bitter almond aroma.
This benzaldehyde aroma is not really from the grape itself under healthy conditions. It is the biochemical response of the grape to the fungus, and the subsequent counter-attack by the fungus, that leads to this smell.
In the end, wine aromas are incredibly complicated. Scientists don’t have all the answers, but we do know there are plenty of natural mechanisms within the berry itself, during fermentation, or during aging that influence wine aromas and flavors.
And to my friend’s credit, sometimes these aromas are actually from a flavored syrup. It might not be as straightforward as a sugary Starbucks flavor shot, but it’s not a wild deviation. There is an infamous wine syrup that’s probably in most red wines under $10. This sugary grape juice concentrate, Mega Purple, is meant to impart a smooth sweetness and intensify the wine’s color.
In my next posts, I’ll be doing a deeper dive into some of the common primary aroma families (those that arise from the grape variety), as well as wine faults. If you’re wondering how wine can smell like green peppers, guava, or even rotten eggs, follow my Medium blog, The Wine Press!