Quick Answer
Adjusting for elevation is crucial in canning because higher elevations result in lower atmospheric pressure, which can affect the boiling point of water and potentially lead to undercooked or spoiled food. This can cause botulism, a deadly foodborne illness. Proper adjustment ensures safe and healthy canned goods.
Understanding Atmospheric Pressure
Atmospheric pressure decreases by about 1 inch of mercury (inHg) for every 1,000 feet (305 meters) of elevation gain. This reduction affects the boiling point of water, as it takes less energy to vaporize water at lower pressures. For example, at 5,000 feet, water boils at 198°F (92°C), which is below the standard canning temperature of 212°F (100°C).
Canning at Elevated Elevations
To ensure safe canning at high elevations, canners must adjust the processing time and temperature. The USDA recommends using a pressure canner at elevations above 1,000 feet, and adjusting the processing time as follows: add 1 minute for every 1,000 feet of elevation gain. For example, if a recipe calls for 30 minutes at sea level, it would require 40 minutes at 5,000 feet. Additionally, canners should check the pressure gauge regularly to ensure it reaches the recommended 10-15 PSIG.
Calculating Pressure Loss
Canners can estimate the pressure loss at their elevation by using the following formula: pressure loss (PSIG) = (elevation in feet / 1,000) x 0.002. For instance, at 5,000 feet, the pressure loss would be approximately 10 PSIG. This calculation helps canners determine the minimum pressure required to achieve the desired processing temperature.
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