The processing time (for water bath canning), pressure requirement (for pressure canning) and blanching time (for freezing and drying some foods) all need to be increased at higher elevations to destroy heat-resistant bacteria and to ensure home-preserved food products are safe to enjoy.
Because water and other liquids boil at a lower temperature as elevation increases (for example, at sea level water boils at 212 degrees F, while at 5,000 feet water boils at 203 degrees F) it is critical to know your elevation to make accurate food preservation adjustments.
2. Use tested recipes and follow all high elevation adjustments (sometimes referred to as altitude) provided--including pressure adjustments and/or blanching and processing times.
For more information on high elevation cooking adjustments read CSU Extension's High Altitude Food Preparation Guide.