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I have a simple question. If you taste or read about wine, you encounter all kinds of flavours: butter, cherry, thyme etc. etc. But the grapes have never been in touch with butter, for example. Where does the variety of tastes come from? How can the grapes develop so many different flavours that remind real world stuff?

Thanks!

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As for butter

Many white wines undergo a process that causes them to take on buttery flavors (which sometimes come across as buttered popcorn or even butterscotch). That process is called malo-lactic fermentation, and here's the quick take on how it works. Just after the grapes are made into wine, a special type of beneficial bacteria is added to the wine. That bacteria converts one of the acids in wine (malic) to another type of acid (lactic). The two acids feel different on the tongue. Malic acid is very sharp (it's the acid in a Granny Smith apple); lactic acid is fairly soft (it's the acid in milk). Most Chardonnay's go through malo-lactic fermentation and are therefore soft and buttery. (Taken from Why Do Wines Taste Buttery?).

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There are several categories of organic compounds that are responsible for aromas - esters, ketones, and aldehydes are the most notable.

Esters are formed when an alcohol and an acid combine; creating esters is a common laboratory experiment in high-school chemistry - I remember making peppermint and banana. This is the actual compound responsible for making peppermint smell like peppermint; the way we synthesized it in the lab isn't necessarily the way the peppermint plant makes it, but the result is identical.

A similar process happens during fermentation and aging of wine: the acids and alcohols present combine to form the compound that makes a cherry smell like a cherry, a thyme sprig smell like thyme, etc.

Just to be clear, most fermented beverages are going to have some of these aroma compounds as byproducts of fermentation. But grapes are notable in that they produce many more different acids than other fruits. So the range of possible aroma compounds is greater in wine than in, for example, cider - since apples don't have the variety of acids that grapes do, and can't make as many different esters.

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