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.

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