Aroma Compounds

Pyrazines 

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Have you ever smelled a glass of red wine and experienced the surprising scent of grass or bell pepper? Has anyone ever described a glass of wine as being vegetal? When that happens what you are really smelling are the Pyrazines, which are organic compounds in the stems, leaves, and unripe berries of grapes.  While Pyrazines are present in the grapes themselves, they begin to lose their strength during veraison and are generally eliminated when the grapes are fully ripe. It is generally believed that Pyrazines are a defense system for the grape, preventing it from being consumed before it is fully ripe and the seeds are ripe enough for successful germination. 

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The Winemaking

Grapes undergoing veraison and losing the strong Pyrazine characteristic

Vegetal flavors in a wine are hit or miss depending on the wine drinker's palate. Some winemakers choose a stronger fruit flavor and higher alcohol content and leave their fruit on the vine to ripen to a point where there are no Pyrazines left to alter the flavor of the wine. Many winemakers feel that vegetal component adds a deep complexity to their wine and they go to great lengths to preserve some of its characteristics. Further still, the presence of Pyrazines is often a regional characteristic with darker or colder regions struggling to ripen the grapes enough to eliminate the Pyrazine flavors. This is why you don't see Cabernet Sauvignon grown and produced successfully in colder climates. 

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The Result 

Pyrazines help us to know our bottle of wine. When you are considering buying a red wine made from Bordeaux grape, read the back of the label and see where it was made. Is it generally sunny or generally cold? If it's dark and cold, then you know your bottle may have some vegetal characteristics to it. So the next time you smell bell pepper or asparagus in your red wine, have a cheers to the Pyrazine and enjoy its unique characteristics. 

Aroma Compounds

Volatile Sulfur Compounds

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Have you ever smelled a glass of wine and thought whoa! That smells stinky or sour? Odds are you are smelling the volatile sulfur compounds that are present in wine. What are volatile sulfur compounds? There are generally two types of sulfur compounds in wine. The first are sulfites, which indicate the presence of excess sulfur dioxide and smells like the familiar rotten eggs that we all associate with sulfur. The second are sulfides, which consists of volatile sulfur compounds that can be created through chemical reactions by the yeast during the fermentation process.  

Hydrogen Sulfide 

The Science

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Must is freshly pressed grape juice that contains the skins, seeds, and stems of the fruit. Usually, by itself must contains so few sulfur compounds that they are unrecognizable to the human palate. The sulfur compounds begin to come out during the fermentation process.

While the yeast are breaking down the sugars and converting them to ethanol, a process named sulfide reduction sequence also occurs. During the sulfide reduction sequence, amino acids are created which allow the yeast to break these sulfur compounds into generic sulfide.  This process generally goes off without a hitch, and the result is wine free of any harsh sulfur smells. Occasionally, the must does not have the requisite nitrogen required by the yeast to complete the sulfide reduction sequence. When this occurs, the resulting chemical reaction produces hydrogen sulfide and gives the wine sulfur aromas. 

The Result 

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Sulfur compounds are yet another struggle that the winemaker must face while producing their wine. They are one of the many reasons why chemistry plays such a big role in winemaking. If a winemaker is unaware of the lack of nitrogen in the must, then a whole batch of wine can be spoiled and smell of rotten eggs, garlic, or onion. 

Sulfur Compound Aromas

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In a wine, sulfur can take on the aromas of rotten eggs, leeks, onions, chives, garlic, scallions, or a burnt match. 

Aroma Compounds

Minerality

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Have you ever heard anyone tell you that a wine smells like a wet stone? What about some nice chalk like notes? These scents come from a variety of elements involved in the creation of the wine. A cool climate Terroir consisting of limestone, and granite soils containing sulfur is generally reasoned to be the source of minerality in wine, but experts aren't so sure that's the final answer. Grape vines consist of over a dozen essential mineral nutrients as a part of their growth cycle, but in tests done using gas chromatography have determined that any single mineral makes up less than .03% of the wine and is far too small of an amount for us to taste or smell. It's more likely what we are experiencing is the scent of Petrichor, or the smell we commonly think of as fresh rain on rocks. 

Petrichor

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The term was coined in 1964 by two CSIRO researchers, Isabel Joy Bear (Australian) and Roderick G. Thomas (British), for an article in the journal Nature. In the article, the authors describe how the smell derives from an oil exuded by certain plants during dry periods, whereupon it is absorbed by clay-based soils and rocks. During rain, the oil is released into the air along with another compound, geosmin, a metabolic by-product of certain Actinobacteria, which is emitted by wet soil, producing the distinctive scent - Bear, Isabel Joy; Thomas, Roderick G. (March 1964). "Nature of argillaceous odour".

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The Results

When you smell minerality or wet rocks in your wine you are smelling the remaining traces of organic compounds that were on the vines when the grapes were crushed that is native to the terroir where they were grown. These organic compounds are highlighted by acidity which is more easily retained in wines grown in cooler climates with a large diurnal cycle. So the next time you take a whiff of your favorite cool climate chardonnay and smell the acid and hints of flint and chalk, take a moment and cheers to the terroir and the scent of Petrichor! 

Aroma Compounds

Esters

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People often wonder why they smell what they smell in a glass of wine. Turns out, there are aromatic compounds present in all foods that our brains associate with specific recollections. One such series of these compounds is Esters. The recognizable scents in white fruits and flowers? Those are the Esters you're smelling. When you are holding a ripe banana you aren't surprised to get a whiff of banana. But when you smell hints of banana in a glass of wine it can take you by surprise. It can be even more of a surprise when your friend doesn't smell the banana and instead smells pears or apples. Why does that happen? Let's examine Esters. 

A carboxylate ester

The Science

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The reason for this disparity is a compound known as Esters. Esters are derived from a carboxylic acid and an alcohol. Glycerides, which are fatty acid esters of glycerol, are important esters in biology, being one of the main classes of lipids, and making up the bulk of animal fats and vegetable oils. Esters with low molecular weight are commonly used as fragrances and found in essential oils and pheromones. 

 

The Results

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Since the differing Esters are made up of a unique chemical compounds, we experience them all differently. Each of our noses are more sensitive to some Esters rather than others, which is the reason why I smell banana when you smell pears. Temperature and oxidation can have a significant effect on the strength of an Ester. That is why a cold wine can change scents dramatically when it warms  up in the glass. So the next time you smell bananas and your friend smells pears, have a toast to your unique noses and the scents of Esters. 

Ester Scents 

Pineapple, Pear, Strawberry, Jasmine, Pine, Apple, Honey, Banana, Cinnamon, Lemon, Nail Polish Remover, Geranium, Lavender, Sage, Peach, and White Flowers.