If you live at more than 1000 feet above sea level, then the processing times and pressures specified in most canning recipes can no longer be used as written. At high altitudes, you will need to adjust those numbers in order to can food safely. Fortunately, the adjustments are simple.
Why High-Altitude Canning Is Different
Both types of canning—boiling water bath canning, and pressure canning—are affected by altitude. Boiling water bath canning relies on a combination of high acidity in the food and the heat of the boiling water to safely preserve the food. However, water boils at different temperatures depending on the altitude. This is because the higher the altitude, the lower the atmospheric pressure, and hence the lower the boiling temperature.
Understanding this requires some understanding of the physics of how liquids boil. The familiar action we called boiling is really the process by which dissolved gases in water or another liquid are being surrendered to the outside air. At lower temperatures, water can hold more dissolved gases. At higher temperatures, though, water loses its capacity to hold dissolved gases. At sea level, 212 degrees F is the point where water begins to frantically surrender all its gases in the dramatic bubbling action of boiling. At sea level, the atmospheric pressure is such that it can accept the gases being emitted by the boiling liquid at this temperature. However, at higher altitudes, the atmospheric pressure lessens and the liquid is able to surrender its gases (boil) at a lower temperature.
Up to 1000 feet (305 meters) above sea level, water boils at 212 degrees F (100 degrees C). But at 2500 feet (762 meters, water boils at just 207.1 degrees F (97.3 degrees C). Since the temperature of the water is part of the safety factor in water-bath canning, that temperature difference is significant. Foods are not sterilized as effectively since the boiling water is not as hot.
Pressure canning relies on temperatures higher than that of boiling water to safely preserve low-acid foods (such as unpickled green beans). Therefore, pressure canning is also affected by the lighter atmospheric pressure found at higher altitudes.
Adjusting for High-Altitude Canning
To adjust recipes for high-altitude canning, start with these two basic concepts:
- For boiling water bath canning, higher altitudes require you to add processing time.
- For pressure canning, high altitudes require that you increase the pressure.
Altitude Adjustments When Boiling Water Bath Canning
How you adjust the processing time with boiling water canning will depend on the altitude, and on the length of processing time called for in the recipe.
If the recipe calls for processing times of more than 20 minutes:
- At 1,001 to 3,000 feet (305 to 914 meters) above sea level: increase processing time by 5 minutes.
- At 3,001 to 6,000 feet (914 to 1,829 meters) above sea level: increase processing time by 10 minutes.
- Above 6,000 feet (1,829 meters) increase processing time by 15 minutes.
For example, if the recipe calls for processing jars of tomatoes in a boiling water bath for 35 minutes and you live at 5,000 feet above sea level, you'll need to process them for 45 minutes instead.
If the recipe calls for processing time of less than 20 minutes:
- At 1,001 to 6,000 feet (305 to 1,829 meters) above sea level: increase processing time by 5 minutes.
- Above 6,000 feet (1,829 meters) above sea level: increase processing time by 10 minutes.
Altitude Adjustments When Pressure Canning
Most pressure canning recipes call for processing at a pressure of 10 psig (pounds-per-square-inch gauge) relative to the surrounding environment. At sea level, this is the pressure necessary to create a boiling temperature of 240 degrees F, the temperature that kills botulism bacteria. If you're using a pressure canner with a deadweight gauge, the kind that shows 5-10-15 psig, increase the pressure to the 15 psig setting if you are more than 1000 feet above sea level.
For pressure canners with dial gauges, adjust the pressure in increments as follows:
- 1001 to 3000 feet (305 to 914 meters): increase pressure by 2 psig
- 3001 to 5000 feet (914 to 1,524 meters): increase pressure by 3 psig
- 5001 to 7000 feet (1524 to 2134 meters): increase pressure by 4 psig
- Above 7000 feet (2,134 meters): increase pressure by 5 psig
For example, if a recipe calls for processing your jars of food in a pressure canner for 20 minutes at 10 psig and you are at 3500 feet above sea level, you will still use the 20 minute processing time but you will increase the pressure to 13 psig.
Tips for Canning at Higher Altitudes
There are a couple of other things to keep in mind about canning at high altitudes. These are less about safety than your valuable time.
- Jellies will reach the gelling stage quicker at high altitudes, and a candy thermometer will not give you an accurate reading on when they are ready. At sea level, a reading of 220 degrees F (104.4 degrees C) is a fairly reliable temperature for testing the gel point. But above 1000 feet, 220 degrees results in something more like a paste than a jelly.
- A canning session can take longer at higher altitudes. Because water takes longer to boil, this means it will take your boiling water bath or pressure canner longer to reach readiness. Keep that in mind when you're planning an afternoon of high altitude home canning.