Why Does Riesling Smell Like Gasoline?

Riesling is like Rosie the Riveter: it’s got some gentle features, a sweet personality, yet a perfume that smells distinctly like gasoline.

Rosie the Riveter (Photo by NC AFL-CIO)

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 terpenespyrazinesthiols, and norisoprenoids!)

Here are some common norisoprenoids and their corresponding aromas in wine:

  • TDN: petroleum
  • 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!

A grape cluster applies carotenoid “sunscreen” to itself.
Carotenoids, the building blocks for norisoprenoids, are like a natural sunscreen for white grapes. (Drawing by Anna Sprenger)

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.

To Summarize:

  • 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.

Why Some Wines Smell Like Tropical Fruits

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.

A guava, passionfruit, and grapefruit personified, holding up glasses of white wine
(Shown are guava, passionfruit, & grapefruit aromas) Drawing by Anna Sprenger

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

3MH (3-Mercaptohexan-1-ol): Passion fruit, grapefruit, gooseberry, guava

3MHA (3-Mercaptohexyl acetate): Passion fruit, grapefruit, box tree, gooseberry, guava

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.

Much like terpene floral aromas, thiols do not become odorous until fermentation when yeasts break down the precursors and release volatile thiols.

Many bottles of Sauternes wine sit on top of a wine barrel in a barrel cellar
A Sauternes wine tasting (Photo by Charlotte Adams)

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!

The mechanism for thiol precursor formation after berries are attacked by the fungus Botrytis cinerea (from Thibon et al, 2011)
The mechanism for thiol precursor formation after berries are attacked by the fungus Botrytis cinerea (from Thibon et al, 2011)

Do thiol aromas change with age?

A bottle of 1893 Sauternes from Chateau Lafaurie-Peyraguey
A very old bottle of Sauternes! Photo by Charlotte Adams

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.

To Summarize:

  • 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.

Why Some Wines Smell Like Green Bell Peppers

What are pyrazines?

Pyrazines are a nitrogen-containing compound. They are a type of varietal aroma, meaning they originate in the grape berries in the vineyard.

A green bell pepper dressed up as a queen to show that this aroma is the “Pyrazine Queen.”
Drawing by Anna Sprenger

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?

A diagram shows two hypotheses for how IBMP originates (one in the leaves and one in the berries).
Two theories about the origin of IBMP in grapes from Lei et al, 2018

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.

A cluster of grapes at véraison with some green berries and some red berries.
Berries changing color at véraison (Photo by Rohit Tandon on Unsplash)

Berries that get a lot of light before they change color (the growth period called “véraison”) tend to have lower IBMP levels. This is because sunlight is thought to photochemically degrade IBMP in green berries. So, removing some leaves around the grape clusters and exposing them to sunlight before véraison can lead to a wine with fewer vegetal aromas.

But if the leaves are removed after véraison, it could be too late. Light exposure post-véraison has been shown to have little effect on IBMP levels.

So in other words, berries that get more sunlight early on in the summer typically have less IBMP. This can explain why wines from cool, cloudy climates tend to have more green bell pepper aromas.

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!

To summarize:

  • 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.

How to Know if a Wine Will Be Sweet

Two slices of cheesecake sit on a plate next to glasses of yellow and red wines.
Photo by fran hogan on Unsplash

I used to work at a French wine bar where we had a dry Riesling on the menu. All the time, I would hear customers say, “This one is dry even if it’s a Riesling? Is that a typo?”

Wine drinkers want to know with confidence whether a wine will be sweet or dry before tasting it. But how can you really tell? First, you have to understand how wines become sweet.

Wine Sweetness is Determined by Three Things

1. The sugar level of the berries at harvest.

Light green grape berries hang on a vine.
Photo by Nacho Domínguez Argenta on Unsplash

Think of the sugar level in the berries at harvest as the limiting factor for sweetness. Unless the winemaker is chaptalizing (a fancy term for adding sugar to the wine tank before or during fermentation, which is outlawed in many places) or using sugary, color-correcting additives, the amount of residual sugar in the wine will be limited by the sugar level in the grape juice.

But, still, if the yeasts convert all of this sugar into alcohol, the wine will be dry. This brings us to number 2…

2. When and if the winemaker stops fermentation before it is complete.

One classic method for making sweet wine is by stopping fermentation before all of the sugar has turned into alcohol. If fermentation is allowed to continue unimpeded, the yeasts will consume all of the sugar in the tank and turn it into alcohol. Thus, the wine will be dry; there will be no residual sugar. If fermentation is stopped (by adding sulfur, dropping the temperature of the tank, or adding a spirit to kill the yeast), sugar will remain in the wine.

3. Sugary additives, if used.

By sugary additives, I mean sweet grape juice or color-correcting syrups. An example of the former method is the German wine “Sussreserve,” which is made by adding sweet grape juice to still white wine in order to increase its sugar content without increasing its alcohol percentage.

In other instances, thick, deeply pigmented color and flavor correcting syrups are sometimes added to wines post-fermentation. But because ingredients are not required on wine bottles, it’s hard to say if the wine will have them. The best rule of thumb is to assume that big brand wines under $10 like Barefoot, Yellowtail, Cupcake, and Sutter Home use these additives in order to standardize their production from one year to the next.

Keeping that in mind, here is a myth-busting list of some common sweetness indicators.

Dark wines are sweeter: FALSE

Color in wine comes from soaking the juice or wine with the grape skins, which draws out color pigments from the skins. If color indicated sweetness, then all red wines would be sweet and all white wines would be dry.

Deep purple juice falls from a container.
Photo by Lasseter Winery on Unsplash

The reason we associate dark color and sweetness is that cheap red or rosé wines often contain Mega Purple. This syrup increases their roundness, covers up any vegetal aromas, and increases the wine’s sweetness. It’s a marketing tactic, and unfortunately, it’s taught an entire generation of new wine drinkers that dark wines (and particularly dark rosés) are all going to be sweet.

If you don’t trust me, try Tavel rosé from the south of France. It is a dark pink color, which comes from a long skin maceration time of red grapes. This style is much more deep and tannic than most rosés, but there is nothing about its color that indicates sweetness; these wines are dry.

Fruity and floral aromas mean the wine will be sweet: FALSE

Two glasses of sparkling wine sit next to a bouquet of pink and white roses.
Photo by Євгенія Височина on Unsplash

If a wine smells fruity, will it be sweet? Not necessarily. A wine can smell like pineapples and be bone dry. If a wine smells floral, will it be sweet? Not necessarily. These “aromatic” wines can be made sweet or dry. Aromas and sweetness are independent of each other.

Sweetness depends on grape variety: SOMEWHAT

Many people believe that Muscat (Muscato) or Riesling are inherently sweet wines. This is not true. Any grape variety can be made into a dry wine by fermenting all of the sugar into alcohol. I have tasted plenty of dry Muscats and Rieslings.

But it is true that Muscat and Riesling are often made in a sweet style. It is not wrong to associate these wines with sweet styles, but it is wrong to think that the wines are sweet because of the grape variety. They are sweet because of a winemaking choice. Other grapes that are often made into sweet wine include Gewürztraminer, Semillon, and Chenin Blanc.

Chardonnay, Sauvignon Blanc, Merlot, and Cabernet Sauvignon grapes are almost used to make dry wines. Of course, there are exceptions to these rules, particularly with low-priced, mass-produced wines. Yellow Tail Merlot is at 8.4 g/L sugar, which is right on the cusp of being called an off-dry (slightly sweet) wine, which starts at 9 g/L.

You can tell if the wine is sweet from the label: TRUE

Wine labels are one of the surest ways you can tell if the wine will be sweet.

Sparkling Wines

  • Brut Nature: 0–3 g/L (also called Brut Zero, meaning no sugar added)
  • Extra Brut: 0–6 g/L
  • Brut: 0–12 g/L
  • Extra Dry: 12–17 g/L
  • Dry: 17–32 g/L
  • Demi-Sec: 32–50 g/L
  • Doux: 50+ g/L

Notice that Brut Nature, Extra Brut, and Brut have overlapping ranges of sweetness. A Brut wine could have 3 g/L sugar, or it could have 10 g/L. If you want to be sure that your wine will be bone dry, opt for Brut Nature.

Wine Folly put it into perspective with this graphic, filling each glass with its corresponding amount of sugar in grams for a 5 oz pour.

Six flute glasses filled with table sugar show the level of sweetness in each category of sparkling wine.
Photo from Wine Folly

Still Wines

The label of most still wines will indicate whether it is a dry or sweet style. The three main indicators to look for are ABV %, residual sugar (RS), and style descriptors.

Alcohol by Volume (ABV, %)

A wine high in alcohol is unlikely to be sweet, as the high alcohol percentage comes by turning sugar into alcohol. Sweet wines tend to be lower in alcohol since fermentation is stopped before all of the sugar can be turned into alcohol.

The exception to this rule is fortified wines. These wines are made by adding a spirit to the wine. If the spirit is added to stop fermentation, like for Port, the wine will be high in ABV but still sweet.

Residual Sugar (RS)

If the amount of residual sugar in the wine is marked on the wine label, you have the clearest indicator of all!

Dry < Off-Dry < Medium Dry < Medium Sweet < Sweet

  • Dry: 0–9 g/L
  • Off-Dry/ Slightly Sweet: 9–18 g/L
  • Medium Dry/ Semi-Sweet: 18–50 g/L
  • Medium Sweet: 50–120 g/L
  • Sweet: 120+ g/L

This Wine Folly chart puts it into perspective, with some common wine styles for reference.

A graphic showing wine still wine sweetness levels.
Photo from Wine Folly

Style Descriptors

Wine style descriptors that indicate the wine will be sweet include demi-sec, Amabile, semi-secco, doux, dolce, moelleux, Beerenauslese, Trockenbeerenauslese, Sauternes, Tokaji, and ice wine.

In the end, the fail-proof method is still to ask your wine shop clerk or server. There’s no shame in double-checking! Hopefully, you will see that color, grape variety, and aroma give no inherent indication of sweetness level. The best way to tell if the wine is sweet is by reading the label and getting comfortable with the names used to classify sweet wines. Cheers!

Why Some Wines Smell Like Flowers

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.

A drawing of four common terpene aromas (geranium, lemon, rose, lavender) next to their chemical symbols.
Some common terpene aromas: geranium, lemon, rose, and lavender. (Drawing by Anna Sprenger)

What are terpenes?

Terpenes are found in the skin of the grapeThey 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

Aromas of 5 different grapes wafting up to noses. Squiggly lines on a scale of 1–5 represent how aromatic the grapes are.
Aromatic scale of different wine grape varieties (Drawing by Anna Sprenger)

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!

A table showing grape varieties synonymous and not synonymous with Muscat.
Clearing up confusion about Muscat synonyms (Illustration from Canva)

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.

Aroma precursors being “revealed” by yeast enzymes during fermentation (Drawing by Anna Sprenger)

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.

To summarize:

  • 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.

How Wine Aromas Are “Revealed” Over Time

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

A diagram of wine aroma precursors and their positive or negative impact on wine aromas.
Raymond Baumes, Wine Chemistry & Biochemistry, Chapter 8 (2009)

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!

Aroma Revelation

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).

Aroma precursors being “revealed” by yeast enzymes during wine fermentation
Aroma precursors being “revealed” by yeast enzymes during fermentation (Drawing by Anna Sprenger)

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!

Friends drinking wine outside. A woman takes a sip of red wine.
Photo by Kelsey Chance on Unsplash

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.

To summarize:

  • 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.

Why Doesn’t Wine Just Smell Like Grapes?

Photo by Helena Lopes from Pexels

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.

Vilela and Cosme, 2016

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. 

Photo by CHUTTERSNAP on Unsplash

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!

Untangling Terroir

a dirt row in a vineyard
Photo by Daniel Salgado on Unsplash

Terroir is one of the most widely used wine words, but it’s also probably the most ill-defined wine word. It’s a classic example of a French concept that doesn’t translate directly into English. Certainly, this leads to confusion, but I can tell you from experience that it is a poorly understood word even among French speakers.

The term terroir comes from the Latin word terra, meaning land or earth. In a sense, the word means “of the earth,” but in today’s vernacular, it has a more complex meaning. Whereas the vintage effect is the change in a wine’s quality and typicity from one year to the next based on weather, the terroir effect is the change in wine quality and typicity based on where the wine is grown.

Humans: A part of or apart from terroir?

a hand holding a bunch of grapes
Photo by Jodie Morgan on Unsplash

Terroir is defined in the Merriam-Webster Dictionary as “the combination of factors including soil, climate, and sunlight that gives wine grapes their distinctive character.” While this definition could include other factors, like human influence, its omission represents the major debate in the wine world. Are humans a part of or apart from terroir?

In my Introduction to Viticulture & Oenology class at the University of Bordeaux’s ISVV, I was taught that terroir can be considered from two angles. There is the agro-terroir angle, which is everything that concerns nature (i.e. hydrology, topography, soil type, etc.). Then there is the socio-terroir angle, which recognizes humans as an integral part of the quality of the wine. From this perspective, humans work to reveal the wine’s typicity, leading to high-quality wine.

As you can see, it’s impossible to separate human influence from terroir because there is no wine without human intervention. A Jura vin de paille wine has Jura terroir because humans put grapes outside to dry in the sun (a process known as passellirage), which increases their sugar content and leads to a wine with notes of dried apricot and peach. The typicity of Jura is because of the winemaking style in this region.

In other words, terroir is made up of the aspects of the natural world that grow the grapes AND the influence of humans both in the vineyard and the cellar. If it seems like terroir is a loaded word, it’s because it is. It means everything that influences the wine based on where and how the grapes are grown, from the climate to the soil to the way the vines are pruned.

What is “terroir wine”?

Most of the time, when we talk about terroir, we talk about the agro-terroir angle. We talk about how the soil and the climate influence the taste and smell of the wine. We talk about how limestone soils produce a terroir ideal for Pinot Noir. We even talk about certain wines being “terroir wines.” But aren’t all wines terroir wines?

Researcher Cornelius van Leeuwen from Bordeaux’s ISVV doesn’t think so. In his article, “The Concept of Terroir in Viticulture,” he makes a clear distinction between “terroir wines” and “branded wines.” He says that terroir wines are made from vineyards of a traceable origin, year after year. Their characteristics come from influences of climate and soil on vines. On the other hand, branded wines are produced at a larger scale, and the grapes cannot be traced back to the same vineyards year after year. Their characteristics come from influences of blending and “oenological processing” in the cellar.

How big is terroir?

Dr. van Leeuwen touches on an important issue in the conversation of terroir: scale. Is terroir regional? Certainly, Oregon winemaker Sam Parra from Parra Wine Co., who makes exclusively single-vineyard wines, would say no. He would say that the scale of terroir is at the level of the vineyard. That each vineyard has a different terroir.

But if we go over to J Vineyards & Winery in California, we find some single-vineyard wines and some wines that are blended from fruit all over the Russian River Valley. Their 2016 STRATA Pinot Noir is the latter, but they still sell this wine as a terroir wine. To J Vineyards & Winery, terroir is perhaps not only the single-vineyard scale, but on the regional scale or the appellation scale.

And the guys at Barefoot? The word terroir isn’t even mentioned once on their website.

Microbial terroir

Researchers from the University of California at Davis published a controversial article in 2013 that coined the term “microbial terroir.” Their work showed that there were regional distributions of grape microbiota in California’s Napa, Sonoma, and Central Coast growing regions. They suggest that regional differences in terroir may actually be explained by these regional microbiota distributions. In 2016, this same team found that they could actually correlate the vineyard microbiota with the chemical composition of the finished wines.

Not everyone is convinced. A review in 2017 highlighted the fact that the 2016 UC Davis study didn’t take into account the sensorial perception of the wines. So, while we can say that the chemical compositions of the wine are influenced by vineyard microbiota, if these microorganisms aren’t actually influencing the smells or tastes of the wine, can we say that they are a driver of terroir expression?

Terroir’s impact on wine aromas

This year, perhaps the most comprehensive article ever to be written on the topic of terroir’s impact on wine aromas was published by a big team of researchers from France, Texas, and Germany. Led by the terroir guru himself, Dr. van Leeuwen, this article summarizes the four main ways in which terroir acts on aromas: 1) air temperature, 2) solar radiation, 3) vine nitrogen status and 4) vine water status. This fourth point results from soil water holding capacity, evapotranspiration, and rainfall.

a diagram showing how air temperature, solar radiation, vine water status, and vine nitrogen status influence wine aromas
van Leeuwen et al. (2020)

As you can see, various factors of climate and soil are what drive the terroir effect. For example, air temperature and solar radiation, two climate influences, drive green and peppery flavors. The compound responsible for green pepper smells, IBMP, generally decreases as temperature and sunlight increase.

Soil nitrogen is also very important for the expression of volatile thiols in wine, the compounds responsible for grapefruit and passionfruit aromas in Sauvignon Blanc. When there is not sufficient nitrogen, these aromas decrease. The main reason is that nitrogen is necessary for the formation of glutathione, a precursor to volatile thiols. While glutathione itself is non-aromatic, during alcoholic fermentation, the yeast break the bond between the glutathione and the thiol, which reveals the thiol aroma. Thus, low nitrogen in the vineyard means less formation of volatile thiol precursors, and eventually, volatile thiols.

This article also suggests that by characterizing the four main drivers of terroir expression on a vineyard site (air temperature, solar radiation, vine nitrogen status, and vine water status), it is possible to predict aroma typicity from different terroirs. For example, since the literature shows a positive influence of solar radiation on dried fruit aromas, the general conclusion would be that vines that get a lot of sunshine will likely have higher dried fruit aromas. (Remember the vin de paille from Jura with dried apricot notes?)

Since terroir is one of the most used wine words out there, defining terroir and understanding how it impacts wine is one of the most important areas of research in wine today. The most important thing to remember is that place matters; where the grapes are grown undeniably changes the wine’s expression. And in a changing climate, understanding what factors influence terroir expression is paramount.

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The Vintage Effect

Photo by Ice Tea on Unsplash

Around late August in the Northern Hemisphere, grapes start to make their way into cellars. If they’re white, they start to be pressed, or if they’re red they start to be loaded into tanks to macerate. Thus begins a long series of steps that will turn juice into wine. There will be hoses and pumps, transferring liquids from one tank to the next. There will be pieds de cuve, fermentation starter packs full of nutrients and yeast. There will be big plungers, used to punch down the floating skins in a red wine tank in order to extract more color pigments, tannins, and aromas into the fermenting wine. And before winter, the reds will be put in barrels, ready to go into the long stretch of hibernation we call “aging.”

All of this might lead to good vintage. But that is dependent on one one factor alone: the grapes. A good vintage means, quite simply, good grapes.

As one winemaker from Alsace told me this fall, “If the grapes are grown well, my job in the cellar is trivial.” Sharing his sentiments is Stéphane Ogier of the Northern Rhône’s Côte-Rôtie. “The wine is made in the vineyard. We can do nothing without this basis.” These winemakers understand that the output is only as good as the input; no amount of Mega Purple or chaptalization can make low quality grapes into high quality wine.

What drives high quality grapes?

Certainly, the grape variety must be a good fit for its soil and climate. If the variety tends to be very vigorous, it should be placed in low fertility soils to balance this vigor. If it tends to have very low vigor, it should be placed in nutrient rich soils to give the vine enough energy to produce high quality fruit. Concerning climate, the goal should always be for the grapes to achieve ripeness as late as possible in the growing season in order to achieve varietal aroma development. For instance, if a grape tends to ripen quickly, such as Chardonnay or Pinot Noir, it should be planted in a cool climate in order to try to draw out the ripening period and make sure those secondary metabolites are developed.

Assuming that the plant material is well chosen for the climate and soil, grape quality variation is mostly influenced by two different effects: the vintage effect and the terroir effect. The vintage effect is the variation in the quality and typicity of a wine from the same vineyard year after year due to weather. The terroir effect is the variation in the quality and typicity of wines based on where they are grown (i.e. according to their terroir).

The Vintage Effect

When considering the same wine producer who is growing grapes on the same vineyard year after year, the variation in vintages will be predominantly from the weather. Here are the five main weather parameters that influence the quality of grapes, and thus the finished wine.

Spring frost

Spring frost is a major determinant of the quality of the season’s harvest. If frost occurs after budburst, it can damage the tissue of the developing bud. This can make the buds unviable, meaning that they won’t continue to develop. If this is the case, sometimes the secondary buds will develop. These buds will produce only about 25% of the fruit that the primary buds would have produced, and the fruit will be of lower quality. But remember, as I wrote in my blog on the fundamentals of vineyard science, these buds were intended to enter the game in the following growing season. So to top it off, spring frost could also impact the next vintage because the current vintage is “stealing” the buds intended to be developed for the next year.


In the early summer period between flowering and fruit set, it is crucial that the vine get enough water. This is because water stress at this stage can lead to poor fruit development, lowering the harvest quantity and quality.

But a little water deficit around the period of véraison, when the berry skin starts to change color, can actually be a good thing, since this stress signals to the vine that it should work on creating ripe, attractive berries for seed dispersers if it wants to spread its genes. This accelerates its sugar accumulation and switches on the grape’s secondary metabolism, which produces those key elements to wine quality, like phenolics (tannins, anthocyanins, and flavonoids) and aroma precursors. However, it’s important to note that summer water stress is, in general, more tolerated by red varieties than by white varieties.

Once the berry starts to ripen, it’s important for it to be dry. We want a dry harvest — not only because picking in the rain is no fun — but because mold at this stage in the game is devastating to wine quality.

Also, while irrigation can be used in some countries, like the US, it is forbidden in other countries, like France. Thus, the timing of rain is extra important where irrigation is unpermitted.


Temperature is another key factor that leads to a good vintage. In the spring, the air temperatures around the vines need to remain around 10°C for three days in a row in order for budburst to occur. After budburst, it’s extremely important that the temperatures don’t drop below freezing (see point 1 on spring frost).

Grapes grow in regions around the world whose growing season temperatures are between 13–21°C (55–70°F). Prolonged temperatures above 30°C (86°F) are problematic, and photosynthesis can stop all together when temperatures exceed 35°C (95°F). If photosynthesis is reduced or halted, this would drastically reduce the sugar accumulation in the berries, leading to an unripened, withered grape.

Beyond sugar accumulation, it can impact the development of phenolic compounds responsible for color pigmentation in red grapes. One study has shown that high temperatures around 35°C, applied one week before veraison, reduced anthocyanin concentrations in the skin of the Cabernet Sauvignon to less than half of what accumulated at 25°C.

And in general, higher temperatures lead to more jammy, cooked fruit aromas and flavors rather than herbal or vegetal aromas. One study tried to mimic what Bordeaux wines would taste like in 2050 due to higher temperatures driven by climate change. It found that the wine was denser and more bitter, with overripe fruit aromas.


Photo by Rui Marinho on Unsplash

Sunlight also drives good vintages. The right amount of sunlight is crucial for the sensory profiles of the berries, and thus the finished wines. One study showed that shaded Syrah grapes made wines of a less intense color, lower flavor compounds, and lower phenolics, leading to a less tannic and less fruity wine.

For white varieties, higher exposure to sunlight can actually induce a natural sunscreen, carotenoids, which break down into volatile aromas. In Riesling, for example, higher sunlight in the vineyard can lead to the breakdown of carotenoids into norisoprenoids, leading to a stronger petroleum smell. In moderation, this odor is a desirable trait of Riesling, but in excess, it can lead to an unpleasant medicinal smell.

It’s also been shown that for Sauvignon Blanc berries, higher sunlight exposure before véraison can lead to lower levels of methoxypyrazines, the family of aromas responsible for the green bell pepper or pea pod smell. In other words, shaded berries lead to a wine marked by vegetal aromas.


Photo by Cody Angus on Unsplash

A bad vintage could also be due to crop damage brought on by hail. Summertime hail is caused by drafts of wind that push raindrops higher into the atmosphere where temperatures are colder, causing the rain to freeze and drop as hail. For grapevines, hail is disastrous. In moderate hail storms, it can damage the fruit, leading to bruising or open wounds that are more susceptible to disease and pathogen infestation. In extreme hail events, the vines can be totally stripped of their fruit or flowers, leading to a lower harvest and loss of money. Nets can be used to cover vines and prevent hail damage, but these nets can be costly and time consuming to put up.

While the winemaking surely plays into the final product, what happens in the vineyard limits the winemaking’s effect. The best winemaker can do little in the cellar to turn low quality grapes into high quality wine. There are many vineyard practices that can lead to higher quality grapes, but sometimes, a little bit of luck is needed when facing Mother Nature and her weather whims.