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Difference between a Boiling Water Bath (BWB) and Pressure Canning

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When should I use a boiling water bath (BWB) canner and when should I use a pressure canner and why?

The basic difference is that a water bath canner simmers jars at the boiling point of water (212° F at sea level), while pressure canners elevate the processing temperature up to 240° F.

212° F is sufficient for eliminating common bacteria, microorganisms, spores, etc. that might be in high acid foods, which generally include fruits and tomatoes. The "critters" that can survive in low acid foods are more numerous than in high acid foods and they will not be eliminated at 212° F. The safest way to know whether what you are canning is high acid is to test it with a pH meter before processing. The magic number is 4.6. Generally, foods below 4.6 may be processed in a waterbath canner. Anything above 4.6 must have its acidity increased by adding lemon juice or processed in a pressure canner. Notes; 1) the increase in acidity decreases the pH, 2) lemon juice may be suggested for some recipes to help with the colorfastness.

Headspace:

When processing in either a water bath canner or a pressure canner, the headspace will be prescribed in the canning and processing instructions for the canning recipe, taking the processing method and processing temperature and the associated expansion of the food during processing into consideration. Don't eyeball or mess with the headspace. Use a measuring device such as a short, sterilized plastic ruler. Keeping in mind that heat expands matter and cooling contracts matter, as the food is processed in a water bath or pressure canner, it expands within the jar and pushes the air within the headspace up and out of the jar. If the headspace is lower than the required level or there are a lot of air bubbles or air pockets within the jar, there will be too much air in the jar and not enough will be forced out during processing. If the headspace is higher than the required level, the contents of the jar may expand over the rim of the jar and overflow during processing. This MAY cause food particles to lodge between the rim of the jar and the seal of the lid, which will hinder the jar from sealing as it cools. Note: jars seal during cooling when the contents of the jar cool and contract.

In addition, it's important not to process canned foods using the incorrect process and associated processing time because the food may over-expand or under-expand. Some foods can be processed in either a water bath canner or a pressure canner, using less processing time in the pressure canner. (We are not sure this saves time as the pressure canner must cool before being reopened, but make certain to not process food in a pressure canner using the processing time recommended for a water bath canner.)

Remember that when you use the hot pack method for canning, the food is hot and has already expanded to some extent. After processing, cooling and sealing, the headspace in the jar might be greater than before processing and the liquid level of the food might be below the food, especially with spinach and other greens. This is nothing to worry about; although, the food above the level of the liquid may discolor.

Processing:

The principle of the water bath canner is to submerse canning jars in simmering water. The jars should remain submersed at all times, as the air must escape into the water and not be permitted to return to the jar. The principle of processing in a pressure canner is not solely based on the temperature of the water, but on the change in the atmospheric pressure within the pressure canner, which should remain constant along with the temperature. Theoretically, jars in a pressure canner would not need to touch the water. Practically, there has to be enough water inside the canner to ensure it doesn't all evaporate as vented steam during the pressure canning process.

None of the lids and caps of the jars should be submersed in a pressure canner. As you heat the pressure canner, the water heats and expands the water and air inside the canner. The vent and petcock permit the canner to maintain the elevated atmospheric pressure without over-expanding to the point that would cause it to explode, which sometimes happened on old, poorly vented pressure canners. As the atmospheric pressure elevates, the air inside is heated beyond the normal boiling point of water and the elevated temperature kills any potential botulism or microorganisms. Another key point is to ensure the pressure canner returns to its normal atmospheric pressure reading of zero (which varies based on elevation) naturally, without opening the vent or petcock, after processing to bring the atmospheric pressure down slowly, which seals the jars inside as the air compresses.

The following chart is a general guide for which foods should be processed using which method. Refer to the recipe's canning instructions for exact guidelines.

Food Water Bath Canner Pressure Canner
Fruit Jam, Jelly, Preserves X
Tomatoes with pH 4.6 or below X
Vegetables X
Anything containing Meat X
Anything containing Fish X
Pickles made with vinegar X

Also see our articles on Using a Boiling Water Bath (BWB) Canner for Home Canning and Using a Pressure Canner for Home Canning and the  Food and Drug Administration (FDA) Publication, Approximate pH of Food and Food Products.

No other canning methods should be employed as they are dangerous; open kettle canning, oven canning, microwave canning, dishwasher canning and steam canning.

 

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