I'm not a wine critic or a winemaker. As a chemical engineering student, most of my time is spent in a materials science lab, where I work with surfactants and strange forms of silica. Even back when I did study yeast, I was more interested in genomic analysis than the bouquet of their by-products. In my spare moments, though, I can inevitably be found with my nose buried in a wine glass or a book about wine.
I first became interested in wine after sampling a few bottles with friends on my 21st birthday. I was, and still am intrigued by this ancient liquid's unpredictability. It's nearly impossible to know what aromas and flavors the next glass holds, no matter how much you have read about the producer, region, vintage, and cuvée. A bottle of wine can transform dramatically over years, and a glass of wine can change with equal guile over minutes. It's a scientist's nightmare, a substance that cannot be categorized or classified with any certainty, and yet there is information in each glass - wines can speak unerringly of terroir, vintage, and varietal. Scent, and its inseparable counterpart, taste, is the most primal and intuitive of senses, and the most strongly linked to memory. A single sip can transport you to a memorable dinner several years ago. Yet romantic and ephemeral as wine can be, it is ultimately a real substance, made up of identifiable molecular species. What can science tell us about tasting wine?
First, let's examine our sense of smell. The human nose is like a vast array of unique keyholes, and each scent molecule is a key. When the key matches with the keyhole (in reality a complex protein receptor), a nerve sends a signal to your brain that says "cloves" or "green pepper," identifying the scent. As the analogy implies, our sense of smell is incredibly accurate at the molecular level. A classic example is the pair of molecules S-carvone and R-carvone, which are perfect mirror images of each other, yet the first smells of caraway and the second of spearmint. There are exceptions - we can be fooled by similarly-shaped molecules, and we sometimes register several molecules in combination as a single scent - but for the most part, smell is highly specific.
In other words, when you smell apricot in the glass, it's probably because there is a substance in apricot that is also in the wine. There are hundreds, perhaps thousands, of different substances that can be produced by our complex little fungiform friends known as wine yeasts, and scientists have not yet isolated and identified all of them, so the next time you smell something strange in a wine, like black pepper or capsicum, know that it's not just your imagination.
Conversely, don't turn your nose up at a fellow taster who seems to smell something that no one else does. We've all been at tastings where everyone likes a certain wine except one person, who claims it smells corked, or oxidized. That taster is probably right. Everyone has different thresholds for different scent molecules, and these can vary widely. Below this threshold you cannot detect the substance. In the key-and-lock analogy, some keyholes are rustier than others, preventing the keys from getting in easily, and your friend may have different set of rusty keyholes than you do. When only one person in a group perceives the wine as being corked, reason suggests the TCA level is just below everyone else's threshold. With practice, you will develop a good idea of how your thresholds for different substances compare to your friends'.
Threshold variation is largely genetic, and highly variable. Combine this with simple thermodynamics, and we can understand that swirling the wine and swishing it around in one's mouth during tasting change the flavor profile you perceive. Let's back up: When you swirl wine, you increase the population of aroma molecules in the air above the liquid surface. Some aromas you could not perceive before are now above your threshold. Other aromas that you detected before swirling have become more intense, or have become masked by secondary scents. Similarly, when you aerate the wine in your mouth, you warm it and form a temporary emulsion with air – again, increasing the gas-phase concentration of aroma molecules. You perceive a different "picture" of the wine than from the glass.
What does this science mean? First, wine is complicated. What you taste depends as much on yourself as on the wine. Second, if you do the simple swirl, sniff, and swallow that so many "wine educators" recommend (see the Wine Spectator's how-to videos), you only see one side of the wine. It's like looking at a sculpture from the front, without seeing its other sides. To see the full 360 degrees of the wine, you need to sniff softly, sniff deeply, sniff when the wine is still, sniff after swirling, aerate the wine in your mouth, swish it around, then make sure you exhale after spitting or swallowing to observe the retronasal finish. The dainty taster who delicately sniffs Lafite from four inches above the glass might as well peer at a Picasso through a pinhole.
Far from demystifying wine, science provides remarkable insight as to the depth and extent of wine's complexities. It reassures us that our often rhapsodic tasting notes have validity. It tells us to have confidence in what we smell, even if no one else smells it. It disabuses us of the notion that we must be delicate and proper when tasting wine. Thermodynamics can tell us why wine changes in the glass, if not precisely how. Gas chromatography can identify hundreds of aroma compounds that appear regularly in wine, but cannot match them precisely to the organoleptic phenomena of smell. Wine lovers can learn something from science, true, but perhaps scientists could also learn a thing or two from wine.