A critical step to achieving proper pressure inside a pressure canner is allowing it to VENT. What does this mean?
Venting is also “exhausting” the canner, a process of letting steam (and air) come out of the canner through the vent pipe for a period of time before beginning the pressure processing time. Air trapped in a canner lowers the processing temperature and results in under processing of low-acid foods. To be safe, the USDA recommends that all types of pressure canners must be vented 10 minutes before they are pressurized.
It is STEAM, not water, that does the processing in a pressure canner. Low-acid foods (foods with a pH of 4.6 or higher such as all vegetables excluding tomatoes, meats, seafood, soups and sauces) are not acidic enough to destroy bacteria, their spores, and the toxins they produce or prevent the growth of botulinum bacteria in a vacuum. The heat-resistant spores produced by C. botulinum can only be destroyed with the correct combination of temperature, pressure, and tested time. Temperatures in the range of 240°F to 250°F (115°C to 121°C) are needed to kill spores (USDA 2015). Water can get no hotter than the boiling point (212ºF, 100ºC), but steam can. Steam trapped in the canner increases the atmospheric pressure inside the canner causing the boiling point of water to increase to 240ºF-250ºF, the temperature needed to destroy bacteria and C. botulinum, that would otherwise be free to grow in a vacuum sealed jar.
In order to reach the optimum temperature to destroy botulinum bacteria, air inside the canner must be exhausted to allow space for a pure steam environment to build. There is a vast amount of air in a canner due to the space between the water level and the lid as well as the air that escapes from inside the jars and from the water. The most “jar air” comes from those with raw-packed foods.
HOW TO VENT
The vent or petcock is a short hollow pipe that sticks up above the canner lid. When open, it allows air and steam to escape from the canner. When closed, it holds steam inside. To vent a canner, leave the vent port uncovered or manually open the petcock (some older models). After placing jars inside the warm canner and securing the canner lid, set the burner on high. Watch for steam to escape from the vent pipe. When a strong, visible, funnel-shaped steam cone emerges, set a timer for 10 minutes and let the canner continuously steam. After the 10 minutes, add the weight or counterweight to the vent or close the petcock to pressurize the canner.
WHAT HAPPENS DURING VENTING?
As the water boils inside the canner, the empty spaces become a mixture of steam and air. As steam increases, it pushes the air out creating a pure steam environment. USDA processing times are based upon a pure steam environment which makes venting so very important.
In addition to venting, remember to adjust for altitude. Most recipes list processing time based on altitudes near sea level. To ensure the health of those who enjoy your foods, always use a tested recipe and follow instructions. Remember to VENT for 10 minutes to ensure that any and all microorganisms are destroyed.
Source: National Center for Home Food Preservation and USDA Complete Guide to Home Canning (2015)