When canning at home, the most important thing to remember is that products containing vegetables, poultry, meats, fish, or seafood must be canned in a pressure canner. Fruit, pickles, jams, jellies, pickles, sauerkraut, marmalades, and fruit butters can safely be canned by boiling water bath or steam methods(1).
Low-acid foods such as meats and vegetables should be processed in a pressure canner unless they have had the proper amount of acid added. The reason for this is that water bath and steam canners can't reach a high enough temperature to kill all the naturally-occurring bacteria that grow in enclosed, moist, low-acid environment like a canning jar. Clostridium botulinum, the bacteria that causes botulism poisoning, is one of the worst. Often, people assume that because water is boiling, it must be hot enough to kill all potential "germs." However, depending on elevation, water boiled in a regular pot only reaches 208 degrees F to 212 degrees F, temperatures that are too low to do the job properly. Pressure canners, on the other hand, attain a temperature of 240 degrees F, which is high enough to preserve the food safely(2). Oven and microwave canning are never recommended.
In the past, many considered tomatoes acidic enough to can safely using water bath or steam canning, but in recent years the popularity of lower-acid tomato varieties have led many home canners to adopt the pressure canning method just to be safe. Remember, just because a product contains mostly tomatoes does not mean it is safe to process by boiling water bath or steam. Mixing acid and low-acid foods will change the pH level of the product enough to require pressure canning(3). Figs are another "borderline" fruit that may be best preserved using pressure canning(4).
Many state university extension services do not recommend steam canning because of the difficulty of determining the exact heat attained (though it usually is higher than that of the boiling water bath) and the proper time needed to process foods(5). Yet, many home canners swear by steam canners because they're quick and easy to use. It takes quite a while for a large kettle of water to come to a boil, but the steam canner uses much less water and is ready sooner. Full water bath canners are extremely heavy, while steam canners are lightweight and easy to handle. Tomatoes take about 45 minutes to process in a steam canner, and fruits such as pears, peaches, apples, or grapes about 20 to 30 minutes. If you choose to use a steam canner, follow the instructions very carefully and remember that steam canners are never the right choice for canning vegetables or meats(6).
Regular pressure cookers are not recommended for canning because their thinner walls and smaller size do not allow for correct build-up and reducing time of the pressure, thus failing to destroy all harmful organisms. Use pressure canners that are specifically intended for that purpose.
Do not allow the pressure to drop during canning. If the pressure drops below the recommended level, increase the heat to bring the canner back to the proper pressure and start the timing process all over again from the beginning. Even if your product is a little overcooked, that's far better than spoiled.
If you live at an altitude higher than 1,000 feet above sea level, read your canner's instructions on increasing time or pressure level.
Be sure to vent the canner with steam exiting for the specified length of time to prevent air becoming trapped in the closed canner, as trapped air lowers the pressure and results in under-processing.
Use quart or pint-sized Mason jars, not old mayonnaise or pickle jars for processing. Mason glass is thicker and tempered to withstand the needed heat and pressure without cracking. Wide-mouth or regular openings are fine. Use new lids and clean rings that are round, not warped or rusted. Half-gallon jars are not recommended for home canning.
Be as clean and careful as possible in preparing your jars and your food. Wipe the jar rims with a clean cloth or paper towel before putting lids on. Follow all instructions carefully.