Usually instructions I've seen for making broth suggest removing the fat - often more easily accomplished after you've chilled your broth - all the fat rises to the top and solidifies, and is easily plucked away from the delicious, nutritious gelatin.

Whether that's due to rancidity or not, I can't say for sure. But I wouldn't imagine that low a temperature would make the fat rancid, even after 24 hours.

This question brings up several points for me.

I cannot speak from experience as I have never made stock for more than 5 hours as I have only made stock in a pan on a stove.

I do not know if fat becomes rancid when making stock, I expect no one has ever tested it.

Heating food in water with the heat remaining below 100 degrees Celsius usually greatly reduces oxidation of fats. Fats oxidise much faster when exposed to dry heat.

I wonder if you have a particularly sensitive nose for rancid compounds?

My understanding is that the very long cooking times for stock (up to 24 hours) is intended to dissolve the minerals such as calcium out of the bones. I do not rely on stock for my calcium.

If you are only interested in extracting gelatin from the bones and connective tissue a shorter cooking time of more like 5-7 hours should be sufficient. I have even made ham hock stew in only two hours and extracted enough gelatin that the stew will set solid when chilled in the fridge.

When I make bone broth I usually simmer it 24-36 hours, never had an issue. The tallow that floats to the top is great. I only make beef bone broth though, every weekend.

Something just dawned on me. Did you put veggie aromatics in your broth? I ask because if you put those in early during a long cooking process, they will get very bitter and taste rancid. I am pretty sure that is what happened in my case. This time around, I just have the bones in there. Any aromatics can be added during a quick boil before using.

I made a 24 hr bone broth a couple of weeks ago and it was awful! I have no idea why..but I plan on looking into it. I cooked it in a crock pot.

I don't think the fat goes rancid, but depending on the bones you use, it might indeed stink. In my experience meaty marrow bones = stinky broth. The best tasting beef broth IMO (though perhaps not the most nutritious) is made from the neck or tail--lots of cartilage and fat but not much bone or marrow. Just made some the other day in my big crock pot with oxtail pieces. Cooked about 20 hours. Result smelled awesome.

I only use bones, vinegar, and salt in my bone broth, and never have had a problem with the fat going rancid. In fact, I use the raised beef fat from the first pot for cooking. However, there is a smell that comes with cooking bones or rendering fat that is... distinct. Some folk find it mighty unpleasant, though it doesn't really bother me. My mate says it smells "rancid' to her -- but we figured out quickly that she has that same reaction even when I'm rendering lard or crisping chicken skins, so it's the long cooking of the fat at low temps that brings it out, I guess.

I use a stovetop for my stock, and a larte, stainless, French-style heavy stock pot. I usually keep the same large beef bones going for several days (though I get the most tallow on the first couple of days). I cook for 24-36 hours, strain, put the bones back in the stockpot, add more water and vinegar, and start 'em all over again. Never had an issue with rancidity in the tallow I collect.

The funny smell is likely due to compounds in the organ meats breaking down after a long simmer. Better to leave the liver out of the broth as it occasionally gives off a truly funky smell to the entire pot. Can also become bitter.

As a student in the culenary arts i can speak from an educated position on this. First of all you need to be skimming your stock regularly while cooking. Second the industry standard is 4-6 hours min. Heers another thing are you cooling youre stock rapidly? If not your stock is now a giant petrie dish breeding germs you need to put your pot in a sink of ice water and rapid cool it. Between certain temps germs can grow if you put ur pot in an ice bath and fridge it right away this wont happen. (i dnt remember the exact temps between 70 and something). This will stop any rancidity. Also germs cant breed between the 2 temps so while at a simmer germs wont invade they will however soon as you change the temp dramatically .

Sometimes, when my broth is heading towards 30+ hours of simmering (with lid), it gets all rainbow coloury. Looks almost like i've put some dish wash in there (without the foam though). Does anyone know why this is and/or do you recognize it?

How long can bone broth be refrigerate before going bad?