Low-acid foods have to be preserved at a higher temperature than high-acid foods. The low-acid environment welcomes the growth of bacteria like botulism, a form of food poisoning that can cause permanent nerve damage or even death.
Pressure canning exceeds the temperature of water bath canning, getting your product into the safety zone. The temperature must reach 240 degrees Fahrenheit, which can only be achieved through steam under pressure.
All vegetables (except for tomatoes which are botanically a fruit), meats, seafood and poultry must be preserved in a pressure canner.
For pressure canning you need:
Flats (also called snap lids)
Pressure canner with valves, seals and gauges
Rack (if you don’t have a rack you can use a folded towel in the bottom of the pot)
Like water bath canning, you can get all the gadgets if you want too, but these are the essentials.
One thing you will notice about pressure canning is that nearly all of the steps are identical to the method for water bath canning. Differences (in italics) are really only related to the equipment. So, once you have learned to use your pressure canner correctly, you will find it every bit as easy as water bath canning.
Sanitize your jars, lids and rings. If you have a dishwasher, you can wash them in the dishwasher – the heat from it is enough to sterilize everything. Otherwise, you need to boil them for at least 10 minutes, lifting them carefully in and out with the jar lifter. Leave the items in the dishwasher or the hot water until ready to use.
Prepare your canner. Place your rack or folded towel in the bottom of your canner add about 3 inches of water to the canner. Most p-canners have a line to which you fill the water. In pressure canning it is not necessary for the water to cover the lids. (Always check the instructions on your individual canner – it there is a discrepancy, go with the instructions that came with your product.) At this point, you can turn the burner on low to begin warming the water, but don’t bring it to a boil yet.
Fill your jars. Line up your jars on the counter near the stove. If the surface is not heat proof, place a towel on the counter first because the filled jars will be very hot. Using the funnel, ladle the prepared product into the jars, leaving the headspace recommended in your recipe.
Put on your lids. With a dry clean dishtowel, carefully wipe the lip of the filled jars, making sure to get any residue of food off. Place the flats on each jar, then finger tighten the rings – you don’t have to really torque on them.
Place your jars in the canner. Place the closed jars into the canner. Be careful not to let the jars touch because not only could they could break when they bump together in the boiling water, but in p-canning the steam must be able to completely circulate around the jars.
Build steam in the canner. Before putting the lid on the canner, check the vent pipe every single time to be sure it is clear. Place the lid firmly on the canner, latching it as per the specifics of your canner, and increase the heat to bring the water to a boil. At this point steam should be coming out the vent pipe. Reduce the heat until a moderate amount of steam is coming steadily out the pipe for 10 minutes. The purpose of this is to release the air and build up the steam inside the canner. If you don't give it the whole 10 minutes, your canner will not build pressure. As patience is not my strong point, I learned this from experience.
Close the vent. After exhausting the steam for 10 minutes, depending on your canner, either close the petcock or place the weighted regulator on the vent pipe. When I place the regulator on, I always put a dishtowel around my hand, cause, yeah, steam is HOT. It sometimes makes a loud high-pitched noise when you are putting the regulator on - I scared myself the first time and screamed, causing my child to think I'd gone and blown myself up. (teehee) Don't be alarmed by the various rattling, whistling and bubbling noises. P-canning is loud business.
Pressurize the canner. Turn up the heat on the burner and wait until the gauge has reached the desired pressure. (Pressure will differ based on altitudes and recipes). This usually takes 3-5 minutes. Note: if you lose pressure during processing you must re-start the processing time. Adjust the heat to maintain the pressure – this takes practice. Monitor your canner throughout the processing time to be sure the pressure is maintained. I have found that approximately #4 on the dial on my electric stove keeps my pressure between 10-12 pounds quite steadily.
Release the pressure. When your processing time is over it is time to release the pressure. It couldn’t be easier. Turn off the burner. Take the canner off the burner and put it on a heat-proof surface. Walk away. Allow the canner to return to room temperature and release pressure naturally. Don't try to do anything to cool it down faster - that is how people get hurt p-canning. Pressure is completely reduced when the air vent/cover lock and overpressure plug have dropped and no steam escapes when the pressure regulator is tilted. The gauge, if your canner has one, should be completely at zero. This can take 45 minutes to an hour and cannot be rushed!
Open the vent. When pressure is gone, open the petcock or remove the weighted regulator. If the regulator doesn't want to come off - there is likely still some pressure in the canner. Don't force it - walk away for another 15 minutes. Once the vent is open, leave the canner for another 2-5 minutes.
Remove the jars from the canner. Use potholders to protect your hands while you unlatch the lid of your p-canner. Very carefully remove the lid to the canner, facing it away from you so that you are not burned by the steam that will rush out. Using your jar lifter, carefully remove the jars from the canner, one by one. Then place the jar on your towel or heat-proof surface.
Allow 12-24 hours for the jars to cool and seal. Let the jars stand in a draft-free place without being moved or bumped, usually overnight. Jars that are sealed properly will bubble away on the counter for quite some time after they are removed from the p-canner. You will hear a musical “pop” as the jars seal in the cool air – that is the lid getting sucked down and forming a seal on the jar. When you are ready to store the jars, you can remove the rings and then test the seal by pushing down with your finger. If it pops back and forth it is not sealed. Put it in the refrigerator and use the unsealed product right away. Store your sealed jars in a cool, dark place.