How Does a Pressure Canner Work?
A pressure canner seals in steam so pressure builds and the boiling point rises to about 240F, hot enough to destroy the spores that survive boiling. That is why low-acid foods such as vegetables, meats and beans must be pressure-canned using tested processes, not water-bath canned.
How a pressure canner makes low-acid food safe
Steam exhausts to remove air
Sealed steam raises pressure
Boiling point rises above 212°F
Held for the tested time, food is safe
A pressure canner preserves low-acid food by reaching a temperature that boiling water alone cannot. Understanding why that matters is the key to using one safely.
How it reaches canning temperature
Sealed under a locked lid, water inside the canner turns to steam that cannot escape, so pressure builds. As pressure rises, so does the boiling point: at about 10 to 15 psi the interior reaches roughly 240°F (116°C), well above the 212°F of an open pot. Held at that temperature for the tested time, the heat penetrates the jars and makes low-acid food safe to store.
The canning cycle, step by step
| Stage | What happens | Why it matters |
|---|---|---|
| Vent (exhaust) | Steam flows freely for the time your manual states | Removes air so the gauge reads true pressure |
| Pressurize | The vent is closed or weighted; pressure climbs | Raises the internal temperature above boiling |
| Process | Pressure is held steady for the tested time | Heat penetrates the jars to make food safe |
| Cool down | Heat off; pressure falls on its own | Forcing it risks seal failure and injury |
Pressure canner vs pressure cooker
They look similar but are not interchangeable for canning. A pressure canner is large enough to hold jars with the right headspace and is designed and tested to reach and hold canning pressure; a pressure cooker or multicooker is built to cook food fast, not to process jars to a validated safety standard. Do not use an ordinary pressure cooker or an electric multicooker for pressure canning low-acid foods unless the manufacturer specifically publishes tested canning processes for that model.
- National Center for Home Food Preservation (NCHFP) — tested home-preservation methods and the USDA Complete Guide to Home Canning
- USDA Food Safety and Inspection Service (FSIS)
- U.S. Food & Drug Administration (FDA) — Food
- FoodSafety.gov
Frequently asked questions
Why can't I water-bath can vegetables?
Boiling water only reaches 212F, which does not destroy Clostridium botulinum spores in low-acid foods. Only the higher temperature reached under pressure makes them safe, so vegetables, meats and beans must be pressure-canned using tested recipes.