Snippets from Hidden Scents – this one is about the volatility of aroma compounds:
Were they to be artificially isolated, or even left on their own, their state might be subject to change depending on the conditions. Normal amounts of oxygen in the air are enough to change a sensitive odor molecule into something very different; such reactions are a major consideration in food design and artificial flavoring. Terpenes, which comprise an important element of citrus-smell, oxidize over time, changing the relative proportion of the overall mixture, and moving from “citrus” to “turpentine.” Moving in the opposite direction, butyric acid, which smells like the rancid butter from which its name is rendered, can react with ethanol to become ethyl butyrate, which smells of pineapple. Ethyl butyrate is then used as a primary ingredient in artificial orange juice flavor. Only with careful consideration can an odor be named with any degree of accuracy. The name type is just as important: Is this a reference to a specific chemical within the orange profile, or to the entire smell-of-orange as it occurs in the pureness of its essential oil?
In practical terms, when your OJ goes bad, it’s not your imagination, it really does smell like turpentine. And when you ask for pineapple juice at the bar, and they pull it out in that big metal can that was opened yesterday, and you all of the sudden smell rancid butter, it’s not the bartender. And to use molecules as a basis for organizing smells is not as useful as it seems.