Why does crockpot caramelized onions usually work, but occasionally get weird chemical smells?

Question

I've been making Crock-Pot caramelized onions for years, to freeze it in smaller chunks for later use. Three times out of four, the onions caramelize down in the Crock-Pot beautifully.

But about one time out of four, they start emitting this really bizarre strange chemical smell, and are inedible. When this happens I just have to throw out the whole batch. Onions aren't too expensive, but if I could figure out how to avoid this loss of onions 25% of the time, it would be useful.

I don't get it, I think I do the same thing each time. I peel and slice up the onions into quarters, stuff a crock pot full of them, pour some oil over the top sprinkle a bit of salt. Then put the lid on and plug it in on my porch to caramelize over the course of several hours.

The times it works, it emits this very intense onion smell which then becomes an intense caramelized onion smell. The times it doesn't work it's this bizarre chemical smell I can't really describe more specifically than that. Has anyone else had this problem and figured out what it was?

Answer 1

My first thought is that one of the onions was bad. You'd probably notice a bad bit, but maybe not cutting into quarters when I show con caramelised onions, I slice them into rings which means I'll feel any slimy bits, discard them, and wash anything that was near them.

I don't add salt or much oil, but I can't see how they'd make a difference, unless you never use that oil for anything else and it's rancid.

It's also worth making sure you haven't accidentally got onion (or anything else) on the outside of the inner pot. Dry burnt onion smells pretty awful, but the contents of the pot would be fine.

When you say they're inedible, if you've tasted them, can you describe the taste? Or is the smell too off-putting, even after cooling?