My friend Paul writes:

Here's a question that hopefully won't tax your superior barista knowledge banks too greatly. A co-worker of mine once mentioned that occasionally brewing coffee smells like tuna fish to him. I thought that was odd until I noticed it too. Now it doesn't happen all the time, but sometimes when I smell brewing coffee there is a distinct odor of tuna. I've done a google search and found other people who have had this experience, but I've found no satisfactory explanation.

My guess is that somehow it's related to the chemical trimethylamine, which is what gives fish a "fishy" odor. The chemical has an extremely low odor threshold, so it can be detected by scent even in very low concentrations. But what could be producing the trimethylamine? Do you have any idea what could be causing this?

I have no idea, actually. I checked my copy of Illy and Viani's Espresso Coffee, a collection of scientific papers about coffee, and couldn't find anything on it. A table reviewing odor compounds found in ground coffee doesn't include trimethylamine. The compound with the closest description is probably methanethiol, which leaves a "putrid, cabbage like" sense impression.

Another possibility is that the coffee with that smell was stored improperly and picked up off flavors from something else. Any other possibilities?

[Cross-posted at Eternal Recurrence.]