Water boils at 212 °F at sea level, but at lower temperatures as you climb: about 1 °F less for every 500 ft. Food in a boiling water bath at altitude therefore heats more slowly and less, and the processing times in canning recipes, which are written for sea level, are no longer enough to destroy molds, yeasts and bacteria. The same applies to pressure canners: at altitude, the gauge reads the pressure above the lower outside pressure, so you must raise it to reach 240 °F inside.
Water-bath canning
For high-acid foods (fruit, jams, pickles, acidified tomatoes), the USDA adds processing time:
| Altitude | Add to the recipe time |
|---|---|
| 0 to 1,000 ft | nothing |
| 1,001 to 3,000 ft | 5 minutes |
| 3,001 to 6,000 ft | 10 minutes |
| 6,001 to 8,000 ft | 15 minutes |
| 8,001 to 10,000 ft | 20 minutes |
Some recipes give their own altitude table; when they do, use it instead.
Pressure canning
Low-acid foods (vegetables, meat, beans, soups) must be pressure canned. Keep the recipe time and adjust the pressure:
| Altitude | Dial gauge | Weighted gauge |
|---|---|---|
| 0 to 1,000 ft | 11 psi | 10 psi |
| 1,001 to 2,000 ft | 11 psi | 15 psi |
| 2,001 to 4,000 ft | 12 psi | 15 psi |
| 4,001 to 6,000 ft | 13 psi | 15 psi |
| 6,001 to 8,000 ft | 14 psi | 15 psi |
| 8,001 to 10,000 ft | 15 psi | 15 psi |
A weighted gauge only offers 5, 10 or 15 psi, so above 1,000 ft you jump to 15.
Example
In Denver, at about 5,300 ft, a jam recipe with 10 minutes of water-bath processing needs 20 minutes. A dial-gauge canner should be run at 13 psi and a weighted-gauge canner at 15 psi. Water boils at about 201 °F.
Find your altitude
Search your town on a topographic map or a mapping app, or ask your county extension office. Use the altitude of your kitchen, not the nearest city.
Safety
- Use tested recipes from the USDA, the National Center for Home Food Preservation or extension services.
- Have dial gauges tested every year; a gauge off by more than 2 psi should be replaced.
- Never water-bath can low-acid foods: botulism spores survive boiling.
- Discard jars with bulging lids, leaks, spurting liquid or an off smell, without tasting.