Anyone who has been enjoying fine wines for at least a little while is probably familiar with the oft-repeated mantra: “Swirl, Sniff, Sip, Swallow” (although “Spit” may replace “Swallow” in a serious wine tasting). The most challenging of these commands is usually the directive to sniff, which tends to perplex recent arrivals to the world of wine. “It just smells like wine,” they say, or, if pressed, they might come up with a lame, “Um, it smells kind of flowery, maybe?” Sooner or later, if they stick with it, they too will learn the lingo and be able to toss around terms like “stone fruits,” or “wet stones”, or “herbaceous” to describe the fragrance of a given wine.
Of course what you describe as “melon” may be my “passion fruit;” your “blackberry” may be what I think of as “cassis,” and we may never agree on the esoteric “gooseberry” or “lychee”. Well, don’t feel bad. The science of wine aromas is immensely complicated, and even the experts are only just beginning to truly understand it. At Cornell, Virginia Tech, Oregon State and other universities across the country (and the world), scientists are devoting themselves to sorting out some of the mystery of why wine smells the way it does.
One reason the subject is so complex is because the aromatic components found in wine are influenced by any number of different things, including environmental factors in the vineyard such as climate and soil, by the type of grape, and by the condition of the fruit (under-ripe, mature, or beginning to rot). In the winery, aromas are influenced by conditions during fermentation (pH, temperature, nutrients in the juice, and various microflora), and during maturation. The type of yeasts used will contribute specific aromas, sometimes depending on whether they are commercial yeasts or occur naturally on the grapes (many producers are convinced that “natural,” or “indigenous” yeasts enhance aromatic complexity).
The aroma components of wines include hundreds of individual constituents representing a number of chemical functional groups. A multitude of ethyl esters is involved in compounds that contribute to fruity aromas in wine (acetate esters, for example, are responsible for tropical fruit and banana-like notes in certain wines). Higher alcohol compounds such as isobutanol are unpleasant on their own, but can provide complexity to wines if their concentration is low enough.
What about that herbaceous character found in some red wines? It’s due to a group of nitrogen containing compounds called pyrazines, which are present in green plant tissues including grapes. Pyrazines are what give a vegetal aroma (often compared to the smell of bell peppers) to Cabernet and Sauvignon Blanc. Rates of leaf photosynthesis and associated hormonal changes in the plant play a role in green-fruit aromas and flavors. Leaves in the fermenter can be a source of herbal character, as can stem contact during fermentation (which is why many premium producers use post-destemming sorting of some red fruit varieties such as Cabernet). Levels of moisture in the soil, and crop load can affect levels of pyrazines and related methoxypypyrazines. (Methoxypyrazines not only contribute to odor but also impact palate balance).
For many of us, the most luscious and seductive wine aromas are the intricate fragrances associated with Muscat and related aromatic wines such as Riesling, Gewurztraminer and Viognier. The principal components responsible for these various aromas are terpenes. Innumerable terpenes have been identified, some of which contribute floral attributes, spicy characteristics, citrus components, or herbal notes to a wine. Like methoxypyrazines, terpene concentration is influenced by factors such as grape variety, terroir, viticultural technique and winemaking process.
We haven’t even discussed the truly unpleasant smells that sometimes affect wine, such as rotten eggs (hydrogen sulfide), stinky water (methyl mercaptan), garlic and rubber (diethyl disulfide). But for non-scientists such as myself there comes a point when too much thinking about the chemistry of wine has to give way to actually pouring it into a glass, giving it a swirl to help volatilize the fragrance, and raising glass to nose. The way a wine tastes is, of course, the critical factor, but since aroma is the gateway to the most complete sensory acceptance and enjoyment of wine, it can be enlightening to give the wine a good, thoughtful sniff before taking that first sip.
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