This page assumes you have already read water bath or pressure and established that the food in front of you is low-acid and therefore needs this method. If you have not, go back: the decision matters more than the technique.
What follows is how to actually run a canner.
Pressure is not the point
The most useful thing to understand, and the thing that makes every other rule on this page obvious.
Pressure canning is not about pressure. It is about temperature. Water boils at 100°C at atmospheric pressure, and no amount of extra time at 100°C destroys C. botulinum spores. Raise the pressure and you raise the boiling point: at 10 to 11 psi, steam reaches roughly 116°C, and at that temperature the spores are destroyed in the times the tested tables give.
Pressure is simply the means of getting to the temperature. That is why:
- Trapped air ruins the process. A canner containing steam and air at 10 psi is cooler than a canner containing pure steam at 10 psi, because the gauge is reading the combined pressure of both. You believe you are at 116°C and you are not. The food is underprocessed and nothing about it looks wrong.
- The tables assume pure steam. Every USDA and NCHFP process time was measured in a fully vented canner. Skip the venting and the numbers no longer describe what is happening inside your pot.
- Altitude changes the pressure required, because the atmosphere is pushing down less to start with.
The canner
A pressure cooker is not a pressure canner. Some modern electric multi-cookers advertise a canning function; most cannot hold a verified pressure long enough or evenly enough, and many are not tall enough for jars. Unless the manufacturer explicitly states the unit is tested for pressure canning low-acid foods and gives validated processing instructions, treat it as a cooker.
Weighted gauge is the better choice in the UK, for a practical reason: dial gauges must be tested annually for accuracy, and in the United States county extension offices do this free. There is no equivalent service in Britain. A weight is a physical object that cannot go out of calibration, which removes the problem rather than requiring you to solve it.
If you do use a dial gauge, it should be tested each year and replaced if it reads more than 1 lb high or low at 5, 10 or 15 psi.
The sequence
- Prepare the food and jars as for any canning: hot jars, correct headspace, air bubbles removed, rims wiped clean, lids fingertip tight. The detail on water bath canning applies identically here.
- Put 5 to 8 cm of hot water in the canner, or whatever depth the manufacturer specifies. Unlike a water bath, jars are not submerged.
- Load the jars on the rack, upright, not touching.
- Fit and lock the lid.
- Heat until steam flows steadily from the vent.
- Vent for 10 minutes. See below. This is the step.
- Apply the weight, or close the petcock.
- Bring up to pressure, and start timing only when the correct pressure is reached.
- Hold pressure for the full tested time.
- Turn off the heat and let it depressurise on its own.
- Wait 10 minutes after zero, then remove the weight, then open the lid away from you.
- Lift jars out, cool 12 to 24 hours undisturbed, check seals.
Venting, and why ten minutes
Holding pressure
Start timing when the correct pressure is reached, not when the weight first moves.
Adjust for altitude, because thinner air means a given pressure reading corresponds to a lower temperature, and 116°C is the whole point of the method.
The weighted gauge jumps straight to 15 lb above 1,000 ft because it has no 11, 12 or 13 lb setting to move to. That is a limitation of the hardware rather than a judgement about the food, and going up rather than down is the safe direction.
Almost every British household is in the top row, but check if you are in upland Wales, the Pennines, the Peak District or the Highlands. The process time does not change with altitude in pressure canning; only the pressure does. That is the opposite of water bath canning, where the pressure cannot change so the time does, and it is worth keeping the two straight.
Hold the pressure steady. A weighted gauge should rock or jiggle at the rate the manufacturer specifies. Adjust the heat to maintain it.
Do not walk away. A pressure canner needs watching for the whole process, both to maintain pressure and because it is a pressure vessel with a fire under it. This is not a background task.
Coming down
The cooling phase is where impatience causes both accidents and failed batches.
- Turn off the heat and leave the canner completely alone. Do not move it, do not run water over it, do not lift the weight, do not open the petcock.
- Let the pressure fall to zero on its own. This takes 30 to 60 minutes on a full canner and cannot be hurried.
- Wait a further 10 minutes after it reaches zero, then remove the weight.
- Wait a couple more minutes, then unlock and open the lid, tilting it away from your face so the escaping steam goes away from you.
- Lift the jars straight out and set them on a towel or board with gaps between them.
What goes wrong
| Problem | Cause |
|---|---|
| Liquid lost from jars | Cooled too fast, pressure fluctuated, or jars overfilled |
| Seals failed | Residue on rims, bands overtightened, or force-cooling |
| Food underprocessed but looks fine | Venting skipped, gauge inaccurate, or pressure dropped mid-process |
| Canner will not reach pressure | Lid seal perished, vent blocked, or not enough water |
| Weight not rocking | Heat too low, or vent partly blocked |
| Jars cracked | Thermal shock, or jars touching each other or the canner wall |
| Scorched base | Insufficient water in the canner |
| Dark deposits inside the canner | Hard water. Harmless. A splash of vinegar in the water prevents it |
Annual maintenance: check the lid gasket for perishing and replace it when it hardens; check the vent pipe is clear by holding it up to the light; check the overpressure plug is soft and intact; and have a dial gauge tested if you have one.
A pressure canner is a pressure vessel. The safety devices, the overpressure plug and the locking mechanism, are what stand between a normal afternoon and a serious accident. Do not modify, bypass or ignore them, and do not use a canner with a damaged or missing safety plug.