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Most instructions for pressure canners mention, that you should let the steam vent for a while, before placing your weight on it.

What is the purpose of this? Why not start with the weight on? And how can I determine how long exactly I need to let it vent?

Example: enter image description here

Source: https://www.thechoppingblock.com/blog/under-pressure-pressure-canning-for-advanced-preserving

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  • This is kind of dangerous because it's possible that hot water may shoot out of it. I wouldn't recommend removing the weight when the content is boiling. Also this guide ask you to close the lid before heating. When you stop the cooking, the cooker cools quicker and pressure drops quicker, then pressure could build up inside the jar (see the bubbling after the jar is out). This isn't entirely safe either (the author has stronger jar than lid so it's OK so make sure you don't have the opposite combination).
    – user3528438
    Nov 10, 2019 at 20:40
  • Im not sure I understand what you mean? According to this guide you close the lid, start heating, while the valve is free, wait until steam starts exiting the valve, then let it steam for 10 minutes and only then put the weight or regulator on. What I don't understand is why not start heating with the regulator or weight on? Why is this venting at the beginning necessary?
    – user1721135
    Nov 10, 2019 at 21:41
  • i'm saying it's not only unnecessary, but dangerous.
    – user3528438
    Nov 10, 2019 at 22:30
  • So what is the alternative, start with the weight on?
    – user1721135
    Nov 11, 2019 at 9:17
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    @user3528438 whatever your intuitions about the danger of steam, your recommendation means that the food was not canned properly and is unsafe to eat. Please read my answer below and especially visit the source I quote. The step of venting cannot be left out.
    – rumtscho ♦
    Nov 11, 2019 at 10:25

2 Answers 2

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In canning, you have to be very sure that you have produced the exact conditions needed for the process to work properly. If you deviate a bit, it can happen that bacterial spores survive. These "exact conditions" mean not only "keeping temperature X for time Y". You have to make sure that proper heat exchange happens where it needs to happen.

In the case of the venting step, it is needed to ensure that your canner is voided of air entirely. When the water starts to boil, it turns to a gas and this gas mixes with air. If you do it in a sealed container with a small exit (like a canner), the steam can push out the air, but it needs some time to do so. This is why recipes are designed with a vent time at the beginning. Air pockets would interfere with the heating process. Only after you have achieved the canner-full-of-steam-without-air condition, can you start building the pressure.

You can see a quote for it here:

Then allow the steam to escape for an additional 10 minutes to vent the canner. This step removes air from inside the canner so the temperature is the same throughout the canner

Which brings us to your other question, how to know when to stop. The simple answer is: do exactly as your recipe says, and don't deviate. If it says "10 minutes", put a timer and let it go for 10 minutes. OK, you do have some wiggle room in the "longer" duration, if it takes you 3 minutes to manage to put the weight on, your food is still safe despite it having been vented for 13 minutes. But don't go below what the recipe tells you. And also note how sensitive the recipes are to change - canning the same recipe in a different size of jar already changes the safe times. So, really, don't try to second guess your recipe, do as you are told.

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  • I only have a very short manual for my pressure canner and it is in chinese. So I already need to approximate. I guess the venting time also depends on how much water you put in, what you can, what size your canner is and as you say what size of jars you use. Is there any way to approximate or calculate? 1/3 water + 15min venting should be OK? I am only worried, that if I overdo the venting I will lose too much water.
    – user1721135
    Nov 12, 2019 at 11:16
  • "If steam does not come through the open vent in a steady stream at the end of 20 minutes, keep heating the canner until it does" so a steady stream of steam would indicate the absence of air and I am ready to go? I am canning grilled paprika btw, not seafood.
    – user1721135
    Nov 12, 2019 at 11:17
  • The exact amount of water you use is not critical, the limits come from 1) it should not be so much that your jars reach it, and 2) it should not be so little that it all evaporates into steam for the duration of the canning operation. Canners are typically designed such that you simply fill it to some point closely below the level on which you place the jars, but do look at generic descriptions of the pressure canning process, there is one on the site I cited. As for the time, you should not be approximating or calculating it, it has to come from your recipe.
    – rumtscho ♦
    Nov 12, 2019 at 11:23
  • What do you mean by "your jars reach it". If you double stack the jars the bottom ones will be covered? I read somewhere that the water level including jars, should not be more than 2/3 of the canner. Is this a safe rule to follow?
    – user1721135
    Nov 12, 2019 at 12:56
  • You certainly don't want to cover your jars. My pressure canner's user manual specifies "3 quarts" of water, and that's below the rack. Now that I searched on the Interent, it seems that the water reaching the jars is not forbidden per se, but 2/3 still sounds awfully full to me, I would be afraid that something is going wrong if the bottom jars are covered or even substantially submerged. This deserves its own question ("how high can the water level in the pressure canner be?"), would you like to ask it yourself?
    – rumtscho ♦
    Nov 12, 2019 at 15:46
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Have a look at this: Air and Steam Mixture

Mere pressure at up to 30 PSI ('15 PSI' on the gauge is really ~30 PSI absolute at sea level) will do nothing to pasteurize your food. It's high heat that does so.

Following the venting procedures on a Presto left approximately a 20% air mixture in with the steam, according to my own measurements and calculations. At that amount of air, the '10 PSI' on the gauge in reality equates to about 109 degrees Celsius (228.2 Fahrenheit) and not the theoretical 113C / 235F that is often claimed for '10 PSI'; at '15 PSI' it's 115C / 239F; not the theoretical 121C / 250F.

And that's after letting it vent for 10 minutes.

If you don't vent it, you're increasing the air portion in the canner even further and thereby lowering the temperature from what has been shown to successfully pasteurize the food.

The recipes have been tested using actual canners, so I am not too concerned that my temperatures are lower than the theoretical ones.

But you may barely get above water canning temperatures if you don't vent: air mixture drastically lowers the temperature at a given pressure.

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