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Pressure canning
Pressure canning — kept by heat and pressure. How the method works, the ratios and temperatures that make it safe, the steps in order, what it costs in nutrients, and the classic Nordic practice.
How it works
Clostridium botulinum spores survive boiling. They are inactivated reliably only at around 116 °C, and water in an open pan cannot get there — the temperature is pinned at 100 °C no matter how hard it boils. A pressure canner raises the boiling point by holding steam under pressure: about 116 °C at 10 psi on a weighted gauge, or 11 psi on a dial gauge, at sea level. Held for the tested time for that food, in that jar size, that combination gives the safety margin. This is why processing times in a pressure-canning guide are specific to the food and the jar and are not interchangeable.
How long it keeps
A sealed jar of low-acid food keeps a year at best quality, stored dark and cool. Boil the contents for 10 minutes before tasting if you have any doubt at all.
The numbers
| Figure | Value |
|---|---|
| Target temperature | about 116 °C |
| Weighted gauge at sea level | 10 psi |
| Dial gauge at sea level | 11 psi |
| Venting before pressurising | 10 minutes of steady steam |
| Before tasting home-canned low-acid food | boil it for 10 minutes |
| Foods with no tested home process | puréed pumpkin or winter squash, dairy, eggs, flour-thickened soups, butter |
How to do it
- Use a canner built for pressure canning. An ordinary pressure cooker and an electric multicooker are not substitutes unless the manufacturer specifically validates the appliance for canning.
- Have dial gauges tested for accuracy; a gauge reading low is a batch processed cold.
- Fill jars to the tested headspace, wipe rims, fit new lids.
- Vent steam steadily for a full 10 minutes before the weight goes on — trapped air lowers the real temperature inside.
- Hold the pressure steady for the whole tested time. If it drops, bring it back up and start the timing again from zero.
- Let the canner depressurise on its own. Forcing it cools the jars too fast and pulls liquid out of them.
What makes it safe
- Low-acid food in a boiling-water bath is the classic route to home botulism. There is no version of this that is fine "because it was boiled a long time".
- Do not invent times. Use a published process for that exact food and jar size.
- Do not can dairy, eggs, thickened soups, purées of pumpkin or winter squash, or butter. There is no tested home process for them.
- Any jar that is bulging, leaking, foamy or off-smelling goes out unopened. Do not taste to check.
- Boiling home-canned low-acid food for 10 minutes destroys botulinum toxin if it is present. It is a cheap habit.
Where it goes wrong
- Assuming an electric multicooker with a "canning" button is a canner. Most are not validated for it.
- Skipping the vent, which is the single commonest error and leaves air pockets below temperature.
- Timing from when the burner went on rather than from when the pressure was reached.
- Stacking jars without the rack, so the bottom layer sits on the metal.
What it costs in nutrients
| Nutrient | Survives the method |
|---|---|
| Vitamin C | Low |
| B vitamins | Low |
| Vitamin A and carotenoids | High |
| Minerals and fibre | High |
This is the hottest treatment on the page and it costs the most vitamin C and thiamin. Minerals, fibre and the carotenoids are essentially unaffected, and the softening of plant cell walls makes some of them easier to absorb. Judge a pressure-canned vegetable on convenience and safety, not on vitamin content.
The Nordic tradition
Pressure canners are common in North American kitchens and uncommon in Baltic and Nordic ones, which shapes what households here actually do with low-acid produce: they freeze it, ferment it, or pickle it in vinegar. All three are good answers. If you do not own a pressure canner, the practical rule is simple — green beans, peas, carrots, beetroot without vinegar, mushrooms, pumpkin, meat, fish and stock go in the freezer or the crock, not into a boiling pan of jars.
Which foods suit it
| Food | Verdict | Why |
|---|---|---|
| Beetroot | Good | The only safe canning route for plain beetroot without vinegar. |
| Green beans | Good | The only safe canning route. Follow a published process for the jar size exactly. |
| Tomato | OK | Necessary once the jar has enough low-acid company — meat, plenty of onion, peppers — that it is no longer a tomato product. |
| Carrot | OK | The only safe canning route for plain carrots. Texture suffers. |
| Onion | OK | Possible for plain onions; rarely worth the jar. |
| Pumpkin and winter squash | OK | Cubes only, packed in water, never mashed or puréed. |
| Peas | OK | The only safe canning route, and the texture pays for it. |
| Wild mushrooms | OK | The only safe canning route for plain mushrooms in water, and the texture suffers. |
| Baltic herring | OK | There are tested processes for canning fish, and they are unforgiving. Follow one exactly or do not do it. |
| Cabbage | Poor | Technically possible for low-acid vegetables, but cabbage comes out grey and limp. Ferment or freeze instead. |
| Garlic | Poor | Turns to a soft grey paste. There is no reason to do it. |
| Cucumber | No | Heat that high destroys the texture entirely, and the acid routes make it unnecessary. |
| Apple | No | Unnecessary. Apple is high-acid and boiling water is enough. |
| Plum | No | Unnecessary for a high-acid fruit. |
| Rhubarb | No | Unnecessary for a fruit this acid. |
| Strawberry | No | Unnecessary for a high-acid fruit. |
| Raspberry | No | Unnecessary for a high-acid fruit. |
| Blackcurrant | No | Unnecessary for a fruit this acid. |
| Lingonberry | No | Unnecessary for a fruit this acid. |
| Sea buckthorn | No | Unnecessary for a fruit this acid. |
| Bilberry and blueberry | No | Unnecessary for an acid fruit. |
| Herbs | No | No reason to. |
Safety rules, temperatures, blanching times, headspace and the pH 4.6 line: USDA Complete Guide to Home Canning and the National Center for Home Food Preservation guidance (US Government / land-grant publications, public domain). Acidity classes: the FDA's published approximate-pH tables. Jam composition: Directive 2001/113/EC. Fish freezing before raw or lightly cured use: Regulation (EC) No 853/2004, Annex III. Checked 2026-09-06.
Sources for this method
Frequently asked questions
Why can I not just boil the jars for longer?
Because boiling water cannot exceed 100 °C, and the spores in question survive that indefinitely. Time does not substitute for temperature here. Only pressure raises the temperature to about 116 °C.
Can I pressure can in an electric multicooker?
Only if its manufacturer has specifically validated that model for pressure canning. Most have not, and the temperature they actually hold has not been verified against the tested processes.
Can I can pumpkin purée?
No. There is no tested home process for puréed or mashed pumpkin or winter squash, because the purée is too dense for heat to penetrate predictably. Cubed pumpkin can be pressure canned; purée is frozen instead.
What if the pressure dropped during processing?
Bring it back up and start the full processing time over from zero. A partial process is not a process.
The other methods
All methods compared
The ten hurdles, and the foods each one suits.
Freezing
The default method in a northern kitchen: fast, forgiving, and the one that keeps the most of what was in the food. It pauses spoilage rather than ending it.
Drying
The oldest method, and the only one that needs no cold, no jars and no electricity once it is done. It works by taking the water away rather than by making the food hostile.
Fermenting
The method that makes its own preservative. Salt buys time for lactic acid bacteria, and the acid they produce is what keeps the jar.
A pantry you can see
TempoLife tracks the food you logged fresh and the jars you put down in autumn, so the batch numbers on the lids match the calendar.