Time in the canner, by altitude
Updated
The published table
Processing time is altitude-dependent for the same reason the set temperature is: water boils cooler higher up, so a given number of minutes delivers less heat. The table already does that arithmetic. Reading a sea-level time at elevation means underprocessing, which is the failure that does not announce itself.
| Altitude | Process time | Jar size |
|---|---|---|
| 0 to 1,000 ft | 5 minutes | Half-pints or pints |
| 1,001 to 6,000 ft | 10 minutes | Half-pints or pints |
| Above 6,000 ft | 15 minutes | Half-pints or pints |
That is the recommended process time for jelly without added pectin, hot pack, in a boiling-water canner. Boise is near 2,700 feet, so the middle row is the one that applies locally and the five-minute figure people repeat from recipes is wrong here by double.
The steps
None of these are difficult and two of them are the ones people improvise. The timer starts at a specific moment rather than when the jars go in, and the jars come out to a specific place rather than onto a cold surface.
- Have the canner water hot and deep enough to cover the jars by an inch or two before the jars go in.
- Lower the filled, lidded jars in on a rack.
- Bring back to a full boil, and start timing from the boil, not from when the jars went in.
- Hold the boil for the time your altitude requires.
- Remove and set the jars somewhere they will not be moved.
The trade hiding in the time
Longer in the canner buys safety and costs set, and the two pull against each other. This is worth understanding rather than splitting the difference, because only one side of that trade is negotiable. Process for the published time at your altitude and solve the set elsewhere, in the concentration rather than in the canner. A firmer set is always available upstream; a safe one is not.
A longer process is more heat into a product that has already been cooked, and extension guidance notes that the extra time can produce weak gels in fruits low in pectin. That is on sterile jars, and the trade you make either way.