Pressure cooked stock out overnight, still safe?

No, it is not safe. As soon as a pressure cooker loses pressure it is not hermetically sealed. Stock, in particular, is often used as a culture medium in petri dishes to GROW bacteria. They love the stuff.

Pressure cooking or pressure cookers do not confer magical powers to food - once the pressure is gone all the regular food handling rules apply:

1. Always refrigerate perishable food within 2 hours (1 hour when the temperature is above 90 °F).
2. When in doubt, throw it out.

Here's a little story about Bending the Rules on Bacteria and Food Safety, wherein a cookbook author feeds lethal week-old stock to his family - and the expert opinion on why this is NOT ok.

No, it's not safe, even withe a closed pressure cooker air must enter in order to equalize pressure.

This goes for everything you cook: The temperature danger zone goes from 41f to 149f and you need to reach the bottom bound within these margins: 149f to 70f in 2 hours max 70f to 41f in 4 hours max After that you have to put the food in the fridge or the freezer.

What this does is to minimize the reproduction of microorganism such as fungus and bacteria, since you don't give them enough time on their "comfort zone".

I've read a number of opinions since posting this and I have to say that I think the top answer is conservative to the point of absurdity.

According to a strict reading of the FDA food safety rules, yes this stock should be considered a lethal poison. However there are a number of mitigating factors here, which I believe render this food completely safe. The FDA rules will keep you safe, but they are written to keep you safe under any circumstances.

To be clear, after cooking there were zero bacteria in this stock or on the stockpot. There were zero botulism spores in the stock or stockpot. Pressure cooking on high for an hour has completely eradicated everything. So the only potential source of contamination would have been from airborne bacteria or bacterial spores happening to float through the extremely small aperture of the vent after de-pressurization.

There is probably a tiny bit of ongoing air exchange due to convection between the heated air inside and cool air outside the pot. All told though, this is clearly going to be a tiny amount of air overall. Heck, let's say it's a few liters, which I think is generous.

So, what we're terrified of here is that there was enough airborne E. coli or botulism or something evil in a few liters of kitchen air to colonize a stock to the point of being unsafe (even with reheating) in maybe 5-8 hours. I have to believe that if that was even remotely possible, we'd all be dying left and right simply from breathing our kitchen air for more than a few minutes at a time.

Do what you feel is right, but in my opinion saying to throw this out is 100% about fear of legal liability and 0% about science or sense. Yes, I ate the stock. It was delicious. YMMV.

I often make a pressure cooked stock, turn off the gas, leave the pot alone (allowing the pressure to dissipate naturally), leave the lid undisturbed, and return to it the next day to strain, package and use or freeze. Depending on your altitude, the temperature inside your pressure cooker can reach 250 degrees F. This is well beyond the temperature to kill off any potentially harmful bacteria. As you state, if you don't open the lid, nothing can enter. Bacteria don't crawl or fly around, so it is hard to imagine something getting in through the pressure release...unless you dumped some tainted liquid on the pressure cooker...then....maybe... Draw your own conclusions, for me it is a routine practice.