Safe Pressure Canning
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It's canning season, time to store and 'put up' summers bounty. Most people are fine with boiling water bath canning and in most instances it works just swell. Pressure canners can be a tad intimidating for the unitiated, but they do serve a practical purpose.
Pressure canners are best for canning low acid foods like most fresh vegetables and many fruits. The lack of acid in certain food is condusive to deadly bacterial growth that hot water baths and vinegar alone do not allways kill. Pressure canning can kill these deadly bacteria. A pressure canner does not guarantee safe food unless other caveats are adhered to and it is used correctly.
It's not rocket science, it just requires you follow some simple specific guidelines to ensure the safety of food you preserve. The following are the primary steps you should follow when pressure canning.
Test Your Equipment
1 Test your dial gauge every year before using. They are sensitive and can easily be knocked out of kilter from being bumped around or dropped.
Check the manufacturers manual that came with your canner if you still have it, for specific instructions for your model. If you do not have it, major manufacturers generally have online versions you can reference.
Always use an approved recipe and make note of the processing time in minutes and the pounds of pressure required for your elevation.
2 Altitude. At sea level water will boil at 212 degrees. This is the highest temperature that liquid reaches under normal cooking conditiond, higher than that and it vaporizes and becomes steam. In a pressure canner, the steam is retained nad ubnder pressure which creates an elevated heat neccesary to destroy botulism spores. The pressure needed at sea level is 10 pounds however as the altitude increases, atmospheric pressure decreases throwing off the ratio. Greater pressure is needed at higher altitudes to create the same temperature it did at sea level. Increase pressure by one pound for each 2000 feet above sea level. Adjust pressure for altitude.
3 Vent pressure canners. When you close the lid on a pressure canner, air is trapped inside. This trapped air, in addition to sometimes harboring airborn bacteria will lowers the pressure canners temperature resulting in under processing. To vent a canner, leave the vent port uncovered or manually open petcocks to allow for proper venting. Heat the canner with lid locked into place until water boils and generates steam. When steam flows freely from the vent port or pet cock it is venting, 10 minutes is sufficient. After venting, close the vents or petcock, place the weight over the vent port and seal the pressure canner as per its design to start pressurizing the canner.
The reccomended venting tme is 10 minutes to help establish pressure and ensure there is no air remaining in the canner. Pressure canners are calibrated to account for water / steam pressure the presence of air pressure throws the equation off. Pressure within the canner should be water vapour. By venting the steam has forced all the air out.
Pressure canning actually uses temperature to eliminate bacteria and spores. The pressure achieves the temperature. Spring loaded pressure canners for this reason are not advisable for canning as the valves on these models automatically close stopping the venting process.
4 When using a pressure canner, never lower pressure too quickly. When processing time is completed, remove canner from heat and let stand until the pressure zeroes out. Patience is a virtue.
Up to date research based advice can be found at the National Center for Home Food Preservations website http://nchfp.uga.edu/