Yes, empty. They don't need to be closed. Full closure might lead to exploding flasks as the air inside heats. You can stack empty glass vessels in any arrangement, but it is better if the mouths point downwards so they drain.
Material to be sterilized should not touch the pressure cooker. The metal will carry off heat faster than it will pass through the glass from the steam, and the region of glass near where it touches the metal will not get hot enough to be sterilized.
Some people sterilize the lids at the same time and carefully assemble them after the cycle. This could be done with the lid gently set on the flask during heating, or side by side on the trivet. I would advise waiting until it's cooled to room temperature to open the pressure cooker and cap the flasks because if you cap them when warm they will suck in contaminated air when opened.
Pressure cookers don't have separate liquid and glass settings as do medical and scientific autoclaves. Liquids in a vessel may boil over during pressure cooking if the vessel is too full. Liquids should go into a deeper container loosely covered with a heat-resistant lid, to prevent both condensation dripping into the vessel and boilovers getting into the pressure cooking valves.
You can prevent boilovers during heating by raising the heat slowly. But boilovers will happen during cooling because heat dissipates from the pressure cooker to the outside faster than from the hot liquid to the steam inside the pressure cooker. Commercial autoclaves have very slow cool-down phases to prevent this. So there is no reason to heat a pressure cooker slowly because it doesn't prevent all boilovers.
I use pressure cookers to sterilze my glass hummingbird feeders and the nectar. I do this because it takes longer for the nectar to spoil. I can fit 2 x 2-cup feeders and one x 1-cup feeder, together with a 4-cup / 0.5 liter measuring cup of nectar into my 5 liter pressure cooker. But that is not enough nectar to fill those feeders, and the nearly full 4-cup measuring cup will boil over somewhat. The glass feeders wind up with dried sugar on the outside, which I need to rinse off carefully. It all fits into the 12-liter pressure cooker, but still I prefer to sterilize the bottles and the nectar separately.
Last edited by estación seca; 05-03-2023 at 05:14 PM..
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