Why Grape Aromas Differ from Wine Aromas | Volatile Compounds and Precursors

Aroma is one of the essential elements that cannot be overlooked when enjoying wine.

When we pour wine into a glass, we don't immediately put it to our lips. First, we bring the glass close to our nose and enjoy the aromas that rise from it. Sometimes we swirl the glass and focus even more intently on the changing aromas. Only then do we finally taste the wine. This sequence of actions might well be called a ritual for appreciating wine.

Wine is said to contain more than several hundred different types of aromas. This might seem like an overwhelming amount, but the human olfactory system can reportedly distinguish ten times that number—thousands of different aromas. Numerically speaking, it's not impossible to identify every aroma contained in wine.

In contrast, the types of tastes we perceive when wine touches our palate are basically limited to just five. Hundreds of aromas versus five tastes. It's no wonder that wine is considered a beverage of aromas, and that we try to enjoy them fully through this "ritual."

But where exactly do these hundreds of aromas rising from a wine glass come from?

Wine is an alcoholic beverage made from grapes. However, when we eat grapes or drink grape juice, we rarely experience the complex aromas found in wine.

The reason lies in volatile compounds and compounds called precursors (flavor precursors), which are formed when these volatile compounds bond with other substances.

Wine Aromas and Their Categories

Wine contains hundreds of different aromas. Not all of these aromas necessarily come from grapes. While wine is made exclusively from grapes, some of the aromas it contains, such as oak aromas, come from sources other than grapes.

The aromas contained in wine are broadly classified into two types and three categories based on when they are created: aromas and bouquet. Aromas are divided into primary aromas and secondary aromas, with grape-derived aromas classified as primary aromas.

Secondary aromas include those derived from microorganisms such as yeasts and lactic acid bacteria that are active during the winemaking process. Oak-derived aromas and those from aging are categorized as bouquet.

This might be somewhat confusing, but primary aromas are those based on aromatic components contained in grapes, while secondary aromas are those created from grape sugars. Bouquet refers to aromas that are not derived from grape-based materials.

Note: The above classification follows the JSA system. In WSET, oak-derived aromas are classified as secondary aromas, with aging aromas categorized as tertiary aromas.

Whether primary aromas, secondary aromas, or bouquet—and even in cases other than wine—the true identity of all aromas is chemical substances. Regardless of what type of aroma it might be, everything can be explained in terms of compounds. Furthermore, the aromas humans perceive are low-molecular-weight compounds.

Odors and Volatile Compounds

For humans to perceive something as an odor, whether it's a primary aroma, secondary aroma, or bouquet, it must equally exist in gaseous form. Our olfactory organs capture rising gases to perceive odors; they don't directly smell liquids or solids. This means that compounds responsible for odors must be volatile. The reason odors are identified as low-molecular-weight compounds is that smaller molecular weight makes volatilization easier.

Among volatile compounds that possess odors, some volatilize easily while others do not. Generally, those that volatilize easily tend to achieve higher concentrations in air, making them more perceptible.

However, the threshold value—the minimum concentration at which an odor can be perceived—varies greatly among different compounds. Therefore, even highly volatile substances may not be perceptible due to factors such as low original content. Additionally, a certain percentage of people are genetically unable to perceive specific volatile compounds, so even when smelling the same wine, not everyone necessarily perceives identical aromas.

Temperature also significantly affects compound volatility. Higher temperatures generally increase compound volatility, but when many compounds volatilize simultaneously, they can mask each other, paradoxically making odors harder to perceive.

However, regardless of the situation, if we can perceive aromas from a glass, those aromas exist in the wine as volatile compounds (also called volatile organic compounds or volatile hydrocarbons, but these terms are essentially synonymous).

In contrast, precursors (more precisely called flavor precursors, but referred to simply as precursors in this article) are compounds in a non-volatile state due to other components acting as weights attached to volatile compounds. Expressed more chemically, they are compounds that have lost volatility due to increased molecular weight from bonding with additional compounds.

Precursors Are Tethered Balloons

Imagine a balloon or hot air balloon with a string tied to it, and at the end of that string, a large stone is attached. Because this stone is heavy, the balloon cannot take flight and remains in place.

This tethered balloon represents a precursor.

The balloon part of the precursor corresponds to the volatile compound. Since it possesses volatility, if the string connecting it to the stone were somehow cut, it would immediately fly up into the air. Conversely, as long as the string remains uncut, it cannot become gaseous, and humans cannot perceive its odor.

The method for cutting the string varies depending on the type of stone attached to the end. The most common pattern is when sugar acts as the weight. Picture a small, light balloon (volatile compound) tied to a large, heavy bag of sugar.

Glycosides: Common Among Precursors

In chemistry, the string in compounds where sugar is attached is called a glycosidic bond. A glycosidic bond is a covalent bond formed when carbohydrate (sugar) molecules and other organic compounds connect through dehydration condensation. Compounds with this bond are called glycosides or glycoconjugates. These are precursors.

Breaking down glycosides with glycosidic bonds to separate them into sugars and volatile compounds is relatively straightforward. Enzymes called glycosidases can cut the string. In winemaking, yeasts possess these enzymes, so without any special intervention, glycosidic bonds undergo hydrolysis during alcoholic fermentation, and components that existed as precursors in the juice are liberated as volatile compounds. In other words, they become gases that take flight and can be perceived by humans as aromas.

While many aroma precursors in grapes exist as glycosides with glycosidic bonds, not all do. One representative example is thiol-based precursors.

Precursors Beyond Glycosides

In Sauvignon blanc, volatile thiol compounds play an extremely important role as varietal character aromas. Among these, the most important compounds are 4-mercapto-4-methylpentan-2-one (4MMP) and 3-mercaptohexan-1-ol (3MH), both of which exist in grape juice only as precursors. When eating Sauvignon blanc grapes directly or drinking them as juice, there is virtually no aroma.

These thiol-based precursors exist in grape juice not as glycosides bonded with sugars, but as structures called S-cysteine conjugates. Since they are not glycosides, their bonds cannot be cleaved by glycosidases, but they can be cleaved using another enzyme that yeasts possess called β-lyase.

While the chemical reactions that liberate volatile compounds from glycosidic bond precursors and thiol-based precursors are completely different, both result in previously imperceptible aromas becoming perceptible through alcoholic fermentation. In both cases, enzymes possessed by yeasts are involved, which is why the aromas perceived before and after fermentation are completely different. However, the efficiency with which precursors can be broken down varies greatly depending on the type of yeast.

Notably, β-lyase, the enzyme that breaks down thiol-based precursors, can use vitamin B6 as a coenzyme. When well-ripened Sauvignon blanc grapes are held in the mouth for a while, the characteristic aromas of Sauvignon blanc wine suddenly become perceptible. This occurs because precursors are broken down in the mouth, causing thiol compounds to become volatile.

Terpene Compounds That Define Aromatic Grape Varieties

Wine grape varieties are sometimes divided into aromatic varieties and non-aromatic varieties. In some cases, non-aromatic varieties are further divided into non-aromatic and neutral varieties. This classification is determined by the amount of terpene compounds contained in the grape juice.

Varieties containing 6mg or more of terpene compounds per liter of juice are called aromatic varieties, those with 1-4mg are called non-aromatic varieties, and those with 1mg or less are called neutral varieties.

The aroma of terpene compounds is typically characterized as muscat-like. Some terpene compounds exist in grape juice as monoterpenes, which have very low molecular weight and high volatility. Since they are not precursors, grapes containing these volatile compounds exhibit the same aromas even before being made into wine.

Aromatic varieties are those with high terpene compound content that possess strong aromas even without fermentation, despite being primary aromas. Taking muscat as the representative example, they are sometimes called muscat-type varieties, while non-aromatic varieties are called non-muscat-type varieties. For example, Riesling is classified as an aromatic variety and is sometimes called a muscat-type variety, but as a grape variety, it has no connection to muscat.

Reference article: Considering Characteristic Aromas of Riesling | Monoterpenes and Norisoprenoids

Terpene Compounds Also Exist as Precursors

While it's easy to mistakenly think that terpene aroma equals muscat aroma, terpene compounds, like other volatile compounds, also exist as precursors. The most common are glycosides called terpene glycosides, but glycosides where sugars are bonded to monoterpenes also exist.

Terpenes are compounds based on isoprene (C5) as the fundamental unit, with names and characteristics changing according to the number of bonded isoprene units. Monoterpenes have 10 carbon atoms, consisting of 2 bonded isoprene units, and include fragrances like menthol and geraniol. Derivatives of terpenes with functional groups are called terpenoids or isoprenoids, and vitamin A is one such terpenoid. Therefore, terpene compounds do not necessarily equal muscat aromas, nor do they necessarily equal non-precursor volatile compounds.

Incidentally, glycoside-type precursors in grapes exist in multiple forms beyond terpenes, with more than 200 types identified.

Summary | How to Fully Extract Grape Aromas into Wine

In most wine grape varieties, aromatic components exist as precursors. Since most precursors require enzymatic breakdown, they have the characteristic of being relatively unaffected by factors like heat. Pre-fermentation heat extraction is a winemaking technique that takes advantage of this characteristic.

However, even if many precursors are extracted into the juice through heat extraction or extended maceration, this alone has little meaning. Precursors only become aromatic after being broken down.

To efficiently and completely extract precursors brought into the juice as wine aromas requires appropriate quantities and types of enzymes. To obtain such enzymes intentionally, yeast selection is generally essential, as some yeast strains possess almost no glycosidase activity.

However, this necessity runs counter to recent trends of avoiding commercial dry yeasts. When wines fermented with wild yeasts show weak varietal character aromas derived from precursors, it's likely that the active yeast strains lacked the necessary enzymes.

How aromatic should wine be? This decision broadly and directly affects winemaking approaches, from extracting precursors present in grapes to determining how much to break them down. It's also important to know which grape varieties contain which aromatic components in what states and quantities.

That said, adjusting the amount of volatile compounds that have been liberated and exist in wine to match preferences is not simple. Some compounds, even after being liberated from precursors, quickly decompose and do not remain in the wine.

How should the overall wine aroma be designed, and how should this be achieved? What constitutes an adequate range? These are difficult decisions that winemakers must face.

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