The Science Behind Aroma Compound Molecules
Before you caught the wine bug, started tasting new grape varieties, and honing your palate, you likely saw wine as somewhat of a mystery. How can someone swirl, sniff, sip, and then rattle off 20+ different descriptors differentiating one wine from another? How can something made from grape juice result in thousands of different iterations produced around the globe? It can seem like wine enthusiasts and professionals are making it all up. Alas, they are not, and the truth behind decoding the complex matrix of wine tasting is 100% scientific.
Aroma Compounds In Wine
When we talk about aroma compounds, we’re referencing volatile molecules that exist in the pulp of the grape. When exposed to oxygen these volatile compounds are quickly released and we are able to detect them with our olfactory sense and subsequently with our sense of taste. This is why swirling a glass of wine is so effective—it causes the volatile aroma compounds to leap from the glass to be detected more easily.
3 Main Aroma Compounds
There are 3 different categories of aroma compounds that make up the bouquet of a wine. There are combinations of specific aroma compounds that exist intrinsically in some grape varieties, giving them markers, or a tell, helping a taster identify the characteristic grape variety when evaluating a wine blindly.
Aroma Compounds from Grapes
Simple aroma compounds develop inside the grape itself. These aromas will be present without fermentation.
Here are a couple of examples of aroma compounds in grapes:
- Methoxypyrazines: The green pepper/vegetal aroma and flavor compound that exists in grape varieties such as Sauvignon Blanc and Cabernet Franc.
- Rotundone: The distinctive black pepper aroma that is present in Syrah.
Aroma Precursors
This is the cool category of aroma compounds that can unlock the world of wine tasting for a wine lover. These building blocks are one of the reasons why wine is far more complex than fresh grape juice alone.
Aroma precursors are molecules that are linked to the sugar molecules in a grape as it is developing and ripening. Because these molecules are linked to the sugar, they are not volatile—they are absolutely aroma and flavorless. During fermentation when the yeast metabolizes sugar, aroma precursors are freed and able to be released and then detected.
A couple of examples of these aroma compounds that are only freed and detectable post-fermentation are:
- Thiols: Thiols are responsible for the box tree, passion fruit, grapefruit, and gooseberry in Sauvignon Blanc. Volatile thiols are more prevalent in Sauvignon Blanc, but other grape varieties contain trace amounts of thiols, such as Riesling, Colombard, Semillon, and Merlot. These thiols are in two general categories of 4MMP and 3MH.
- Terpenes: Terpenes give aromatic, fruity, and floral characteristics to a final wine. Linalool and Geraniol give classic grapey characteristics to different Muscat varieties. Classic descriptors for other terpenes (more than 50 different terpene molecules have been identified) are floral, rose, citrus, coriander, and spicy. Terpenes can also be found in Riesling, Gewürztraminer, and Torrontés.
Aromas From Fermentation
This is a class of aroma compounds known as esters. They are created from certain interactions with alcohol and acids. During fermentation, yeast feeds off of sugars present in the grape must and the byproducts of that process are alcohol, carbon dioxide, and over 200 aromatic esters.
Esters are primarily responsible for fresh, fruity flavors that are so important in young wines, especially youthful white wines.
The world of ester aroma compound molecules is vast, here are a couple of common examples:
- Isoamyl Acetate: The characteristic aroma of bananas and pears in wines develops from intracellular fermentation during carbonic maceration.
- Diacetyl: This ester is developed from the process of malolactic conversion, giving a wine a classic buttery aroma
Other Aromas
Even more, aromas can be introduced into the picture from sources such as fermentation and maturation vessels. American oak can give pronounced aromas of vanilla, dill, coconut, and sweet spices. New French oak generally imparts subtler aromas than American oak because it is tighter-grained. Common characteristics of French oak are cinnamon, smoke, chocolate, and coffee.
Aroma compounds develop at different rates and levels depending on the grape variety and the specific climate where the fruit was grown. This is part of what makes the world of wine so vastly complex. The grape variety itself, climate and soil (terroir), and the winemaking techniques chosen all combine to create a unique liquid fingerprint. The next time you’re mindfully tasting a glass of wine, be confident in the different characteristics you’re detecting—they’re really there!