Many years ago, while instructing me in the use of our pressure cooker, my father told me to always let steam escape from the top stack for a couple of minutes before putting the weighted cap on it. This, he said, would purge air from the cooker and thereby prevent oxygen from attacking the food at the elevated pressure and temperature inside.

This makes a lot of sense to me, but I never see it mentioned in pressure cooking books or instructions. What's the deal?

  • What does "attacking the food" mean?
    – Catija
    Jan 5, 2018 at 20:53
  • Oxygen is a highly reactive element that, well, oxidizes things.
    – Rob Lewis
    Jan 5, 2018 at 21:06
  • 2
    But why is that a concern? If you were slow cooking the same thing in the oven or a slow cooker... you'd still have the same result? Why is this a special concern in a pressure cooker? With the shorter cooking time, I would think it'd be less of an issue?
    – Catija
    Jan 5, 2018 at 21:24
  • Yah, this advice sounds like nonsense to me. Also, it isn't even possible on many designs of pressure cooker. I suspect that this is a case of "no, only you".
    – FuzzyChef
    Jan 6, 2018 at 0:11
  • Why is that a concern? Oxygen is the active ingredient in some kinds of bleach. It turns apples and potatoes brown. I have heard that it can destroy nutrients (heard of antioxidants?). Like any other reagent, it is more active at higher temperatures (like in a pressure cooker).
    – Rob Lewis
    Jan 6, 2018 at 0:52

1 Answer 1


In pressure canning, the USDA says:

Air trapped in a pressure canner lowers the temperature obtained for a given pressure (for example, 10 or 15 pounds pressure) and results in underprocessing. To be safe, USDA recommends that all pressure canners must be vented 10 minutes before they are pressurized.

Which might be where your dad borrowed that idea from. The effect in that case is more from the air taking up space that should be filled with steam, and preventing it from being filled with steam (rather, being filled with a steam/air mixture), making the transfer of heat from steam to jars less effective.

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