Most of us have eaten a grape-- usually a table grape from the grocery store. If you're lucky enough to have visited a vineyard or an enterprising tasting room around harvest, you may have even had the chance to taste a wine grape. Neither of these experiences, however, really elucidates much as to how a wine made from the same grape will actually smell and taste. If an alien came to earth and was given each, I doubt it would deduce that one was derived from the other. Why is this?
First of all, let's talk grape berry structure. The three main components of a grape are its skin, its pulp, and its seeds. Different compounds are found at different concentrations in each of these parts of the grape.
What even is a grape made of? Seventy-five to eighty-five percent of a grape is water. Fifteen to twenty-five percent is sugar. Acid makes up half to one percent, and pectin accounts for about a quarter of a percent. Add these numbers up, and you have very little left to work with in order to distinguish one type of grape from another. Luckily, the compounds that account for the vast differences in the wines made from these grapes require very small concentrations for the human nose to detect them once they are released from their bound forms.
These aroma and flavor compounds fall into a camp known as secondary metabolites. These largest group of these are phenolic compounds. Phenolics are a diverse group, including tannins, which contribute structure and mouthfeel, anthocyanins, which give wine its color, and flavonols, which work in concert with tannins and anthocyanins to boost their effects. These are generally found in all parts of the berry. The role these compounds play in terms of flavor is that they are responsible for the bitterness and astringency of a wine. In total, these make up about 0.65% of a grape.
And finally, the really fun stuff-- the compounds that really differentiate one grape variety from another...
Terpenes are responsible for floral aromas in wines, most notably in Muscat varieties and their cousins. They have fancy names such as geraniol (which smells like geraniums), and linalool (which smells like roses). It doesn’t take a very high concentration of these compounds to tickle your nose. In fact, many ripe grape aromas are dominated by terpenes. They come in two forms-- free, which is just like it sounds, and-- glycosylated, which is a fancy way of saying "attached to a sugar molecule." When attached to a sugar, they don't have an aroma. When free, they do.
Pyrazines are the compounds culprits for vegetable-like aromas in wine, and their detection thresholds tend to be very low. They can be a good thing, such as in herbaceous New Zealand-style Sauvignon Blanc, or a bad thing, such as in Cabernet Sauvignon, which if picked too early, can be overwhelmed by green bell pepper odors.
Volatile esters tend to be accountable for the various fruity aromas you get when sticking your nose in the glass. They can smell like specific fruits like strawberries, peaches or even bananas, or can just contribute to a generally “fruity” bouquet.
Thiols are sulfur-based compounds that span a huge range of aromas from cooked cabbage and garlic to passionfruit and grapefruit. As you can imagine, they can be either a pleasant or unpleasant addition to the drinking experience.
Norisoprenoids are compounds that are derived from carotenoids, which play a part in photosynthesis. The two best known compounds in wine that are associated with this class are beta-damascenone, which is the primary aroma in roses, and also present in fruits like raspberries, and TDN, a diesel-type aroma, which is not a norisoprenoid per se, but tends to increase as carotenoids break down.
So now that I’ve overwhelmed you with wine science, why don’t you go fill a glass with your preferred alcoholic blend of esters and monoterpenes, with judicious splashes of pyrazines and thiols. I’ll meet you back here in a few weeks to talk through the role of yeast in wine aroma and flavor.
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