Some chemicals change character as their quantity increases; that is, if you add more and more of something, you don’t just get a stronger version of the same thing; it transforms into the smell of something else. One chemical, o-amino-acetophenone, smells like malt in parts-per-billion concentrations, like tacos in parts per million, and enough like Concord grapes in parts per thousand that it actually is used in grape soda. This is an extreme example, to be sure, but consider a fruity ester, ethyl acetate. At low levels, it contributes a pleasant fruitiness to ales. However, when a certain
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