Pressure Canning: The Only Safe Way to Can Low-Acid Food

Beans, corn, pumpkin and meat cannot be safely bottled the way fruit is. Here is why, and what that means if you own a Fowlers unit.

Why a water bath will not do

Spoilage yeasts and moulds die easily. Clostridium botulinum does not. It survives as a spore that is harmless on fresh food, because it only grows without air, and a sealed jar of low-acid food is exactly the airless environment it wants. What it produces there is one of the most poisonous substances known, and it is invisible, odourless and tasteless.

Boiling water reaches 100 degrees Celsius, or 212 Fahrenheit. That is not hot enough. The spores need about 116 degrees Celsius, or 240 Fahrenheit, and the only way a home kitchen reaches that temperature is under pressure. That single fact is the whole reason pressure canners exist.

So the dividing line is acidity, not the food group:

• High acid, pH 4.6 or below. Fruit, jam, properly acidified tomatoes, pickles in vinegar. A water bath is enough, because the acid stops the spores growing.
• Low acid, above pH 4.6. Beans, corn, carrots, potatoes, pumpkin, mushrooms, all meat, poultry, fish, and soups or sauces that mix them. These need a pressure canner. There is no substitute, no workaround, and no amount of extra boiling that gets you there.

Plain tomatoes sit on the line and are the classic trap. Modern varieties are often not acidic enough on their own, which is why every current recipe tells you to add lemon juice or citric acid. Acidified, they can go in a water bath. Unacidified, they cannot.

The practical position in Australia

Pressure canners are uncommon here and most are imported, so they cost considerably more than they do in the United States, and the tested recipes you will find are almost all American. That is not a reason to improvise. It is a reason to be clear-eyed about the choice in front of you.

• Freezing is the honest answer for most households. Beans, corn, peas and pumpkin all freeze extremely well after blanching, keep their texture better than canning does, and carry none of this risk.
• Pickle or acidify instead. Bringing a low-acid vegetable below pH 4.6 with vinegar moves it into water bath territory legitimately, which is what a tested pickle recipe is doing.
• If you do buy a pressure canner, use current tested recipes and follow them exactly, including the altitude rule. The science does not change at the equator.

What a pressure canner is, and is not

A pressure canner is a heavy pot with a locking lid, a vent, a safety release and a gauge, sized to hold jars upright with a rack underneath. It is not the same thing as a pressure cooker. A small electric multicooker with a pressure setting has not been tested for canning and cannot hold a verified temperature for the full processing time, so it is not a substitute however tempting it looks.

There are two kinds of gauge and the difference matters at altitude:

• Weighted gauge. A weight rocks or jiggles on the vent. You hear it working. It cannot be adjusted finely, so above 1,000 feet you simply move to the higher weight.
• Dial gauge. A needle shows the pressure, which lets you set it precisely, but the gauge drifts. Have it tested once a year, because a dial reading 11 when the canner is at 9 is a jar of food that was never safely processed.

The altitude rule people get backwards

In a water bath, altitude means you add time. In a pressure canner, the time never changes and you add pressure. Getting this the wrong way round is the most common mistake in home canning, and it produces jars that look perfect and are not safe.

Water boils cooler as you climb, so the same pressure reading yields a lower temperature. Raising the pressure puts the temperature back where it needs to be.

If you do not know your elevation, look it up once and write it inside a cupboard door. It does not change.

The method, step by step

1. Start from a current tested recipe. Not a family notebook, not a video, not this page. The recipe gives you the jar size, the pack style and the processing time, and those three things together are what makes the food safe.
2. Prepare jars and lids. Clean, warm, no chips on the rim. Jars do not need sterilising for a pressure process, because the process itself far exceeds sterilising temperature.
3. Fill, leaving the headspace the recipe states. Too little headspace and the jar vents and fails to seal. Too much and the air does not clear.
4. Release trapped air by running a plastic tool down the inside of the jar, then wipe the rim clean, fit the lid and tighten the band fingertip tight. Not hard, because air has to escape past it.
5. Two to three inches of water in the canner, or whatever the manufacturer states. This is not a water bath and the jars are not covered.
6. Vent for a full ten minutes. Lock the lid, heat until steam runs in a steady jet from the vent, and let it run for ten minutes before you close it. Air left inside insulates and lowers the real temperature, so this step is doing safety work even though it feels like waiting.
7. Bring it to pressure and start timing only when the correct pressure is reached. Not before.
8. Hold it steady. If the pressure drops below the target at any point, bring it back up and start the full time again from zero. This is the rule people most want to bend and it is not bendable.
9. Let it cool on its own. Turn the heat off and leave it. Do not run it under water, do not lift the weight, do not force the lid. Forced cooling siphons liquid out of the jars and warps lids, and the pressure inside is still dangerous.
10. Rest the jars twelve to twenty four hours, then check the seals, remove the bands, label with what and when, and store somewhere cool and dark.

When to throw a jar out

You cannot see, smell or taste botulinum toxin, so the rule is about the jar and not the contents. Discard, without tasting, if any of these are true:

• The lid is bulging, the seal has failed, or the jar spurts when opened
• It smells wrong, looks cloudy or has anything growing in it
• You cannot remember whether it was processed correctly, or how long ago
• It was processed by a method you now know was wrong

Do not feed a doubtful jar to animals and do not open it near food you intend to eat. A suspect jar is worth exactly nothing, and the food inside cost you a few dollars.

See also: The four ways to preserve, Passata and bottling for the water bath method, and Pickling for the acidify-instead route.

Processing times are deliberately not listed here. They depend on the food, the jar size and the pack, and they are revised. Use a current tested source for the time, and this page for the method. Altitude figures follow Penn State Extension, "Time, Temperature and Pressure in Canning Foods".