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I have a jar with ground coffee stored in a cupboard. Every time I open that cupboard a specific smell gets out, one that can surely be associated with coffee but that is not the same one that you can smell when opening the jar.

That smell is something like the "bad version" of that good and invigorating smell of ground coffee. Is not pleasant, and more acute.

Why does that gas have a different smell? Is it due to carbon dioxide and/or the polyphenols?

  • Sounds like you need to clean your cupboard and store your coffee for shorter periods of time. Can you explain more of what the "bad" smell is like? – hoc_age Mar 3 '15 at 22:26
  • The smell is exclusively due to the coffee. When the jar is not inside, there is no smell at all. That smell is something like the "bad version" of the one you get after opening the jar with the ground coffee. It is difficult to describe it. – Nicolás Mar 4 '15 at 1:54
  • Does the jar itself smell bad? Does the coffee taste bad (versus just bad smell)? As for your particular guesses: carbon dioxide is odorless, and polyphenols are probably too diverse to generalise. However, fresh coffee should smell like fresh coffee; off-smells must be coming from some chemical changes occurring as coffee gets older. Does my answer help? – hoc_age Mar 4 '15 at 13:34
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My guess, based on what you've said, is that the two processes involved (that are giving you the off-smells) are oxidation and rancidification.

Rancidification ("going rancid") is a process by which fats (oils) break down into... other chemicals you don't want (and smell bad). The smell could be described as rank or rotten.

Oxidation is another class of chemical reaction that could also produce off-smells and tastes. This could be caused by contact with air (specifically, for example, oxygen) or other factors.

More broadly, autolysis describes other factors of, for example, plant material breaking down by itself over time (e.g., self-degredation by a cell's own enzymes). There are likely other factors like this, if you store coffee (especially pre-ground!) for any length of time.

These descriptions are admittedly non-technical. If you're looking for more, ping me in comments.

See also this question and its answers: When does coffee go off? It discusses oxidation and other factors.

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