Vivid Ready

The only method that makes low-acid food shelf stable. It is temperature that does the work, not pressure, and that distinction explains the ten-minute venting step almost everyone skips.

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

Canner types

Type	How it works	Verdict
Weighted gauge	A weight rocks or jiggles on the vent at a fixed pressure	Recommended for UK use. Cannot drift out of calibration
Dial gauge	A pressure dial you read and adjust to	Accurate, but must be tested annually
Pressure cooker	Domestic cooker	Usually not suitable. See below

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

DANGER

Vent the canner for a full 10 minutes of continuous, steady steam before applying the weight.

This is the most commonly skipped step in pressure canning and it silently invalidates the entire process.

The canner starts full of air. As the water boils, steam gradually pushes that air out through the vent. Until it has all gone, the canner contains a mixture, and a mixture at 10 psi is cooler than pure steam at 10 psi. The gauge cannot tell the difference; it reads total pressure regardless of what the gas is.

The result is food processed at, say, 108°C while you believe it was at 116°C. The jars seal. They look perfect. They are underprocessed low-acid food, which is the precise definition of a botulism risk.

How to do it: heat until steam comes from the open vent in a strong, continuous funnel, not intermittent puffs. Only when it is continuous do you start the 10 minutes. Then vent for the whole 10 minutes without interruption, and only then apply the weight or close the petcock.

Skipping venting is not a shortcut with a small quality penalty. It is the difference between a safe jar and an unsafe one, and it leaves no evidence.

Holding pressure

Start timing when the correct pressure is reached, not when the weight first moves.

Adjust for altitude. Below 1,000 ft, weighted gauge canners use 10 lb and dial gauges 11 lb. Above that, both increase, and the tested tables give the figures. Almost all British households are below 1,000 ft, but check if you are in upland Wales, the Pennines or the Highlands.

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.