The Science of Smell

Dear Caveman:
When I read your wine descriptions, I find intriguing expressions such as: “Smells like a Mediterranean-style vegetarian pizza. Sun-dried tomatoes, black olives and herbs, with a touch of cherry vanilla." Where do these aromas come from? If they are actually the by-product of fermentation of crushed grapes in an oak barrel, then aren’t these conclusions about “sun-dried tomatoes, black olives and herbs, with a touch of cherry vanilla” socially constructed and ultimately subjective?
Ricardo

The short answer is that while the naming of these aromas may be subjective – in that each of us has our own “aroma and taste memory” and thus associates certain aromas with different things – there is a scientific explanation as to how a wine made only with grapes can evoke such un-grapey smells.

The sources of many of these aromas are volatile aromatic compounds. Some come from the grapes themselves or are by-products of the fermentation of the grape’s juice. But the ones that Ricardo was referring to are results of the aging of the wine, both in an oak barrel and in a bottle. This is still an area of wine that is not completely understood, but research is happening on a number of different fronts, so here is the science.

Aroma vs. bouquet

Émile Peynaud, a French oenologist considered by many to be the father of modern winemaking, drew a distinction between aroma and bouquet. For Peynaud, “aroma” is used to describe what we smell in a young wine – those grapey and fruitier aromas that result from the pressing of the grapes and the fermentation of the grapes’ sugars. If you have ever been in a room where a wine is fermenting, you will never forget the smell – ripe, juicy fruit mixed with a blend of alcohol and yeast. Open any bottle of Beaujolais Nouveau and you will have a good idea of what I am talking about.

Peynaud said “bouquet” is the result of aging a wine, which is where many of these non-grapey aromas can result. While the subtle chemical interactions are not completely understood, these new odours result from the interaction between those primary aromatic compounds and outside influences like oak barrels and oxygen.

It starts with the grape

So everything starts with those primary aromas, and thus the grapes you bring in from the field. The Chinon referred to by Ricardo evoked sun-dried tomatoes, black olives and herbs. I took a look at some other reviews I had done of wines made with the same cabernet franc grape, and I found one that described the wine (Saint-Nicolas-de-Bourgeuil 2005, Les Mauguerets-La Contrie) as smelling of “green peppers and violets … (and) a mix of raspberry and charcoal.” Why the difference?

Two University of British Columbia researchers, Steven Lund and Joerg Bohlmann, recently published a study that examined how a number of different factors affect primary aromas. They refer to the assortment of chemical compounds that cause aromas as volatile organic compounds (VOCs). How and where a grape is grown will ultimately affect the degree to which these grapes will show such things as varietal character as well as “the dozens to hundreds of chemical compounds that have yet to be discovered and characterized.”

So the same grape grown in different soils, in different years, will smell different. And the amount and proportion of these compounds to one another will ultimately affect the bouquet as a wine ages.

The molecules of scent

Lund and Bohlmann have broken down a wine’s aromatics into component compounds. So if your Gewürztraminer smells of flowers, it is in part due to “monoterpene compounds, chiefly geraniol and citronellol.” And if your Sauvignon Blanc tastes slightly grassy, the compound is part of the “methoxypyrazine family, specifically 2-methoxy-3-isobutylpyrazine,” which develops during the green stages of the grape but gets metabolized as the grape ripens. If your Sancerre tastes like freshly cut grass and your white Bordeaux doesn’t, although they are both made with Sauvignon Blanc, that is because the Bordeaux is often riper, so it will have less 2-methoxy-3-isobutylpyrazine.

How about black pepper in Shiraz? Rotundone is the chemical compound responsible for that one. A study done by the Australian Wine Research Institute of different vineyards in Australia showed that shiraz grapes showed different levels of this compound depending on the clones used, soil types and climate.

And what about the sun-dried tomatoes in my Chinon? According to Jamie Goode, “cis-3-hexenol is the prime culprit,” although cabernet franc also has the same leafy 2-methoxy-3-isobutylpyrazine as the Sauvignon Blanc I mentioned.

Bouquet

So as a wine matures and its bouquet develops, much like great cuisine, its odour becomes the sum of its component parts. Many of these VOCs are in a sense dormant when the wine is young. As VOCs react with one another, as well as with oxygen and alcohol, they will begin to show themselves.

A good example of this is oak. In a young, freshly bottled wine, the oak is often very pronounced, and in fact the wine has a distinct smell of wood. But as the wine ages, the vanillin – which is an oak-derived VOC and part of the family of aldehydes – will react with oxygen to give an odour of vanilla. Oak barrels are also a source of many of the cooking spices we find in wines, like cinnamon, coriander and nutmeg. Every barrel, depending on the wood source, will offer a different aromatic cocktail to the wine.

So this is why a grape can ultimately smell like “sun-dried tomatoes, black olives and herbs, with a touch of cherry vanilla.” But why doesn’t everyone smell that? Well, there is the cultural factor. If you have never smelled a sun-dried tomato, the name you give to such an aroma might differ. But there is also sensitivity: All noses are not created equal. Many VOCs are in such small concentrations that people with highly sensitive noses might pick up on elements that others miss.

But for the majority of us, it is simply a question of working our aromatic memories. Because wines can be complex, they often do not make us think of one particular aroma. This is why I sometimes will mention situations, like walking through a cool forest in the fall after a rain. How do you develop this memory? The first step is to load your memory with as many smells as possible.

So, as the proverb goes, stop and smell the roses – or the pizza, in my case.