How quickly can chicken stock turn sour? So, I made a pot of chicken stock (which I've never done before) and it seemed to be going great! I roasted a whole young chicken, pulled the meat off, and tossed those bones into a pot with a couple more skin-on, bone-in chicken breasts, some chicken thighs, and some onions, garlic, and shallot.

I wasn't following a particular recipe, but used Chef John's recipes for the basic technique. I left it simmering until around 16 hours because I got distracted, and have read that letting a stock simmer too long can result in bitterness, but that didn't happen.

I forgot to let the stock sit and cool for an hour before straining as the recipe says, but I can't imagine how that would negatively impact the stock...

I tasted it when I strained it, hot, and it tasted fine! I decided to transfer the now-strained stock to a clean pot and reduce it so it would fit in my storage containers. But when I went back and tasted it after it had returned to a simmer, it had a very noticeable sour taste. 

I went online and tried to see what could make a chicken stock turn sour literally as it's cooking, but I can't find anything. The only information I've been able to find doesn't really apply here: I didn't even let the stock cool before straining it and returning it to the heat. I really can't imagine any lacto-friends gaining a foothold in the maybe 30 minutes max the stock was off the heat.

Literally everything I've found has said that chicken stock will turn bitter if overcooked. Can it turn sour instead of bitter? What on earth would make it do that?

I'll use this stuff for hot and sour soup I guess, but I'd like to know what I can do to avoid this in the future. Is it most likely because of how long it cooked? Or is there a reason you need to cool the stock before straining it?

UPDATE: I poured a couple tablespoons in a ramekin to cool ASAP just to see what if anything would change and.... the sourness is gone? Or greatly reduced? It still smells good, still tastes chicken-y, but that sourness is still present in the larger, still-warm container.
Boiling stock for a long time will not make it sour.

In my experience, high protein food will not go sour when it goes bad. It goes putrid. Sourness requires sugars. Either way, I agree that your stock would not have been given sufficient opportunity to go off.

I am forced to blame experimental error. Perhaps your utensils were not as clean as you thought. Maybe you just ate a spoonful of honey and your tongue was uncalibrated.