Why do rotten onions smell good?

August 7, 2007 7:34 PM

Why do rotten onions sometimes smell good?

Every so often I'll cut open an onion (usually ones I buy at an outdoor market) and the flesh inside will be soft and brown -- clearly rotten. This is usually just the first few inner layers.

But the weird, weird thing is, the rotten part smells very sweet and fragrant. Has anyone else experienced this, and can anyone explain what might be going on, on a biochemistry level, that transforms the smell of onions (which I love, but is nowhere close to being sweet) to something that smells like flowers or candy?

posted by Deathalicious to food & drink (6 answers total)

PS: I always throw away the affected parts when this happens, but I've always wondered whether the rotten part is safe to eat, and what it tastes like.
posted by Deathalicious at 7:36 PM on August 7, 2007

I'd guess (IANABotanist) that the rotting parts are decomposing into simple sugars. Sometimes even compost has a sweet (although pungent) smell.
posted by zackola at 8:14 PM on August 7, 2007

maybe it's the sweet smelling bacteria that's causing the onion to rot. i know certain species of pseudomonas cause onion rot and many pseudomonads smell good.
posted by brandz at 8:15 PM on August 7, 2007

Onions definitely contain sugars, which is why they are able to undergo the maillard reaction. It's not fermentation, however, if this is to be believed and onions lack the lactic acid necessary for anaerobic fermentation. So there's have to be yeast present, which seems doubtful in an onion core.

Googling for sweet smell and rot turns up "mucor rot," which is identified as happening based on a wound on the outside of fruits, not at a core. However it clearly indicates that decomposition can result in a sweet scent.
posted by phearlez at 9:27 AM on August 8, 2007

I may have posted too soon. Here's an entry on the kind of rot and damage that can occur in an onion and there IS yeast identified. So it could well be fermentation that's going on there.

The page also usefully mentions how hard it can be to identify this kind of rot but since it happens from the neck in you can check the stalk for firmness, which I had never known to do.
posted by phearlez at 9:30 AM on August 8, 2007

Yay! Best answers for everyone.

With a name like "rot", it has to be good. I'm not putting that in my mouth ever.
posted by Deathalicious at 7:39 PM on August 8, 2007