How wine aroma happens is both very simple and very, very complicated. The simple version: molecules capable of leaving from the surface of the liquid (or carried up to the liquid in the tiny gas bubbles that make sparkling wine sparkle) are carried through the air up to our nostrils where those molecules meet sensory receptors that, when bound to the right kind of molecule, trigger a “smells like” response in our brain. Hence why we swirl and sniff: swirling encourages aromatic molecules to leave the wine; sniffing encourages those molecules to travel into our noses. As you’d expect, the complicated version elaborates on what kinds of molecules are capable of leaving the wine — of moving from the aqueous phase to the gaseous phase, we’d say — and what has to happen between an aroma molecule and a smell receptor for a message to be sent, and how activating a receptor turns into a perception of “smells like” in our conscious minds. But it’s actually even more complicated than that, because molecules that aren’t aromatic and that don’t ever leave the wine for the air influence what we smell, too.
A new study of the aromas of two different Australian shiraz (shirazs just doesn’t look right to me) is a good example of what it takes to make up a wine’s nose.
Wine — talking all wines collectively, not any one in particular — involves at least 800 or so different aroma compounds. The strategies used to figure out that sort of thing, and to analyze the aroma composition of specific wines, are all fundamentally based on separating out all of the many different molecules a wine contains. A common way of doing this is with gas chromatography which, to put it simply, separates molecules by the differences in how they interact with specific solvents. How long it takes a particular molecule to let go of the solvent — its “retention time” — is unique, so seeing a molecule’s retention time is as good as knowing the molecule’s name…at least when someone else has already done the meticulous work of correlating the two. Of those 800-some-odd wine aroma molecules, we can actually only name something like 10-20%. But we know the rest exist, because we can see their retention-time fingerprint pop up on the chromatography results. Even better than gas chromatography, for wine aroma purposes, is gas chromatography-olfactometry, which takes the apparatus for chromatography and adds a smelling port so that scientists can sniff the separated-out molecules as they come up in turn. In the case of the Australian shiraz, the gas chromatography came up with about 100 “odorants,” but the consensus among sniffers was that only about half of those actually smelled like anything. Of those, for only 27 or 28 were their concentrations in the wines high enough to theoretically be detectable (they exceeded their odor threshold). It took 44, added to a wine-alcohol-acid-sugar base, to make something that credibly mimicked the original wine.
Then the researchers asked their trained sensory panel to do something interesting: sniff the aroma + base wine-like synthetic and the same mix minus one of several key aroma compounds with the goal of identifying which molecules contributed to which perceived smells. The details get long-winded, but the final message stands out. Removing non-aromatic constituents changed aroma perceptions — sometimes more intense, sometimes less, depending on the aroma and the other molecules involved — even when the key aroma compounds themselves were left untouched. And some very obviously smelly compounds present in the wines in quantities far above their odor thresholds had a much smaller impact on wine aroma then their high concentrations would make you think.
In other words, wine aroma isn’t as simple as just pairing up odiferous molecules and their corresponding smells. And we can’t yet predict how or why wine will smell the way it does from first principles. Synthetic wine, then — or at least synthetic wine that replicates real wine — is going to take some time, and a lot more sniffing.