If you've had a glass of wine with friends, odds are someone has described at as oaky. And perhaps you've heard a white wine described as having notes of green pepper.

None of that happens because someone tossed a bell pepper into the wine as it was being fermented. Rather, it happens because of unique flavor, aroma and color compounds that make us just .1 percent of wine. The other roughly 99.9 percent is basically the same from wine to wine: water and ethanol, plus trace amounts of acids and sugars.

And perhaps more than with any other drink, in wine, these tiny characteristics are what separates an average wine from one that is truly great.

"We expect milk to be just like the milk we tried a week ago," says Gavin Sacks, an associate professor at Cornell University and an expert on wine cultivation. "There is an expectation that wine is going to be different."

But don't think that the differences you're tasting are really being sensed by your tongue. Sacks says with wine, and in general, most of what we call taste is actually coming from our nose.

When we swallow, a puff of air comes back up, directing compounds into our nose, helping us perceive the differences in "taste" to whatever we're eating or, in the case of wine, drinking. Hold you nose the next time you take a drink of wine and the uniqueness of the wine will disappear, leaving you with merely the sweet of the sugars, the sour of the acids and the bitter from the alcohol, Sacks adds.

But, fortunately, when we're drinking wine, we usually don't hold our nose. So we're rewarded with complex flavors that can be imparted in various ways. Notes of oak or vanilla typically come from the toasted oak barrel where many wines are aged. Other flavors come from the particular type of grape used to make the wine.

Take a riesling grape, for example.

"[It] tends to be acidic, but often smells floral," Sacks says. "Those floral aromas are derived from compounds that are derived from grapes. Those compounds are called monoterpenes."

Interestingly, those monoterpenes are what you smell when you eat Skittles.

And then there's the syrah grape. Syrahs are typically noted for their tastes of black pepper, which comes from compounds called rotundones. 

"It's present in the skins," Sacks says. "It's partially extracted during fermentation."

Those green pepper notes, mentioned earlier, come from a compound called methoxypyrazine.  Methoxypyrazines are actually present in green peppers, as well as wine.

"Nature tends to just keep using the same compounds over and over, rather than inventing new ones," Sacks says.

The compounds are present in the grapes, but decrease as the grape ripens. So a winemaker who wants his wine to taste like green pepper would harvest the grapes a little early. They can also decrease the amount of methoxypyrazine in the grapes by exposing them to more sunlight.

To be sure, though, wine isn't all chemistry. As Sacks points out, we wouldn't have more than 100,000 different wines if it weren't for the way subtle changes can totally change the flavor profile in ways that may be completely unexpected. 

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