By Jamie Goode
People often lament the weakness of the human sense of smell, comparing the olfactory world we live in with that of dogs or other animals who seem to experience a much richer array of smells.
But while there’s no doubting that many animals can smell things that we can’t, consider the world of gastronomy. Many people are prepared to spend a lot of money and effort to eat and drink well. And most of what we think of as ‘tasting’ is actually smelling. We value smell very highly it seems!
When food or drink is in our mouth, we are only getting relatively limited information from the actual sense of taste: most of it is being detected by smell, through what is known as retronasal olfaction. This is where aroma molecules get to the olfactory receptors in the nose by the back of the mouth. Wine is the ultimate form of gastronomic pleasure, because people with the money to do it will happily spend hundreds or thousands of dollars pursuing the smell of fine wines. This suggests that the human sense of smell is very important to us.
So what makes wine smell the way it does? There are a number of ways of unpacking this topic.
Some of the aromatic compounds in wine are actually present in grapes, survive fermentation, and make it into the wine. But grape juice doesn’t smell much like wine, so this is a relatively small subset of what’s present in the wine. Others are made by the yeasts during fermentation from precursors present in the wine. Some are made de novo by the yeasts. And a few might come from the wood of a barrel the wine is aged in, or other microbes that grow in the wine.
Whatever way they get there, wine is full of many aromatic compounds. It’s hard to know how many, but around 800-1000 seems to be a good guess. Not all of them are above the threshold level where suddenly we are able to smell them, or identify them, but might still be having an effect on the global smell of the wine. The concept of ‘threshold’ is important here.
At the receptor level, we detect smells through a suite of some 400 different olfactory receptors, housed in olfactory receptor neurons in a patch of cells the size of a postage stamp in the back of our noses. The thing is that each of us has a different subset of these receptors, and as a result, we all have different thresholds for different aroma molecules. There are different sorts of thresholds, too. There’s the detection threshold, when we recognize that there is a smell there, and then there’s a recognition threshold, where we actually recognize what the smell is. Take a particular smell molecule at a specific concentration, and some people might smell this as a strong smell, others might sense it as a weak smell, and still, others might not smell it at all.
Three Groups of Flavor Compounds
So what makes a wine smell the way it does? Sensory scientist Vincente Ferreira breaks the smell of wine up into a few categories, and this conceptual framework is a useful one.
Ferreira states that wines don’t have a single characteristic aroma, but “rather they have a palette of different aromas, which are difficult to define and which surely are perceived differently by different people.”
He separates the various flavor compounds of wines into three different groups. As we think about the smells we describe when we sniff a wine, we can’t dial down to single aroma molecules to explain these. Instead, we have to think about groups of molecules working together, both in combination, and also where one affects the perception of another through effects such as masking, or enhancement.
The first group is the global “wine aroma,” the result of some 20 different aromatic chemicals that are present in all wines to make a global wine odor. Of these 20 aromas, just one is present in grapes (β-damascenone), with the rest being made by yeasts. These include higher alcohols (e.g. butyric, isoamylic, hexylic, phenylethylic), organic acids (acetic, butyric, hexanoic, octanoic, isovaerianic), ethyl esters from fatty acids, acetates and compounds such as diacetyl, and—of course—alcohol itself.
Alcohol is important here. As the level goes up it makes many of the aroma compounds less likely to leave the solution, and so higher alcohol wines tend to be less aromatic. Fruity compounds such as esters are effectively masked by higher levels of alcohol.
The second group is known as ‘contributory compounds’ and this is another set of 16 compounds present in most wines, but at relatively low levels. Some of them are even present below the detection threshold for most people, but they have synergistic effects with other compounds even if we can’t smell them. These include volatile phenols (guaiacol, eugenol, isoeugenol, 2,6-dimethoxyphenol, 4-allyl-2,6-dimethoxyphenol), ethyl esters, fatty acids, acetates of higher alcohols, ethyl esters of branched fatty acids, aliphatic aldehydes, branched aldehydes and vanillin and its derivatives.
Finally, in the third group, we have impact compounds, responsible for giving characteristic aromas to certain wines, even when they are present at very low concentrations. They can even be the molecules that give varietal identity to wines. For example, Sauvignon Blanc is very interesting as a grape variety because much of its characteristic aroma is believed to come from a small number of impact compounds, chiefly methoxypyrazines (of which the most significant is 2-methoxy-3-isobutylpyrazine) and three thiols (4-mercapto-4-methylpentan-2-one [4MMP], 3-mercaptohexan-1-ol [3MH], and 3-mercaptohexyl acetate [3MHA]).
Another group of importance is the monoterpenes, such as linalool, which is important in many white wines, such as Muscat. This has floral, citric, table grape aromas. Then there’s rose-cis oxide, which gives Gewürztraminer its sweet flowery, rose-petal aroma. A newly discovered one is rotundone, which gives pepperiness to Syrah, at incredibly tiny concentrations.
But to make things even more complex, we have a concept called the nonvolatile wine matrix. This is where wine possesses constituents that don’t have any aromatic characteristics of their own, but they strongly influence the way that the various aromatic molecules present in wine are perceived.
In an intriguing study Ferreira’s group took a range of red and white wines. They removed the aromatic fraction from each of the wines, but saved it. They then took the dearomatized wine matrix of each wine and added back different aromatic fractions. Bizarrely, when a trained sensory panel examined these reconstituted wines they found that red wine matrix was combined with white wine volatiles, the panels described the resulting wine as smelling like a red wine. A similar effect occurred when red wine volatiles were added to a white wine matrix, and white, yellow, and tropical fruits all start appearing in judges’ tasting notes. The remarkable thing about this study is that it demonstrates that the nonvolatile matrix is having an important effect in actually modifying the perception of the volatile components of wine.
It’s interesting to be able to isolate the different molecules from wine, and describe the smell of each. But this sort of reductionistic approach is unlikely to help us understand the aroma of wine. It seems that it’s based on a complex interplay between the different olfactory molecules, all working in concert with each other, but also with other components of the wine, too. And then there’s the variability that comes from us, too, with our varied olfactory systems, and also our varied experience. All in all, it’s a fascinating subject – calling for lots of practice.