A steam canner. 
A water bath canner with a rack to hold jars.
A pressure canner with a gauge to tell the inside pressure.
Canning is the process of sterilizing and sealing food into air-tight glass jars. Canned goods can last for years at room temperature as long as the seal is intact. Foods that have been canned can retain most of the nutrients and flavors of the fresh food, and, for some foods, canning actually helps release nutrients and flavors that aren’t present in the raw food. I’m going to explain differences among the three types of canning to help you decide when to use each method.
The whole point of canning is to heat up food to kill off any wee beasties that might spoil the food. The most concerning wee beastie is Clostridium botulinum which produces botulism toxin, a powerful neurotoxin. C. botulinum mostly hangs around in a spore form which can only be killed with heat over 240F, but can start metabolizing and producing the toxin in low-acid environments. Additionally, heating up the food also heats up the air in the jar, and as the jar cools, the rubber seal helps form a negative pressure space that seals the lid onto the jar.
There’s three types of canning: steam, water and pressure. Each method has a different way to heat up the contents of the jars and kill off wee beasties.
Steam canning uses a little bit of water which is vigorously boiled to envelope the jars in hot steam to raise the temperature of the jar contents. I personally don’t use a steam canner. I realize that the thermo-conductive potential of hot gas is the same as hot liquid, but something about it gives me the willies… I can’t believe that a hot gas is enough to sterilize my food. Additionally, most canning books that I have tested their recipes in water bath or pressure canners. And, by “testing” recipes, I’m not talking about tasting only for flavor, but also to ensure the food is sterile and sealed. Untested recipes could mean unsealed jars or not sterile foods. I’m not willing to take the risk of converting recipes to steam canning from other canning methods.
Water bath canning uses boiling water at 220F (or thereabouts, depending on your altitude) to sterilize the jar contents. However, some wee beasties still thrive at temperatures above 220F, but, lucky for us, they can’t survive in acidic, sweet or salty environments. So, if a food is sweet, salty or sour (e.g. jams, pickles, naturally acidic fruit) then you only have to heat it up to 220 to kill off most of the wee beasties, then the sweet, salt or acid keeps the other beasties from metabolizing and spoiling your food. A water bath canner is nothing fancy, just a deep pot with a lid.
Unfortunately, if your food isn’t sweet, salty or sour, then heating it up 220F only kills off some of the wee beasties, leaving the hearty high-temperature beasties to thrive in your foods. This is how you kill people with botulism… Foods that aren’t sweet, salty or acidic, like vegetables and meats, don’t have a second method for retarding spoilage, so you have to get the food REALLY REALLY hot to kill off the wee beasties. The only way to get stuff in jars hotter than 220F is through increasing the pressure – hence the third method of canning is pressure canning. Because of the amount of heat and pressure necessary, low-acid vegetables like beans (green or shelled), potatoes, corn, peas, mushrooms, and all meats, eggs or fish MUST be pressure canned to kill off the wee beasties. A pressure canner is a special pot with a tight lid, an air-tight seal and a valve or weight to adjust the pressure.
Water Bath or Pressure Can? Technically, any food will be sterilized using pressure canning. But with sweet, salty or acidic foods, pressure canning is overkill. The amount of heat and pressure applied to the food to get it sterile will break down tender fruits, turn pickles into mush, and make jams hard and unspreadable. If your food is really sweet, salty or acidic, then water bath can it – it’s gentler on the food. There are some foods that are marginal in their sweetness, saltyness or acidity. For example, tomatoes are acidic, so whole tomatoes can be water bath canned, but when you cook tomatoes (heat reduces acid) with onions (low acid), and bell peppers (low acid) to make tomato sauce, you may reduce the acid enough to require pressure canning. Some fruits are acidic when unripe and less acidic (more sweet) when ripe (e.g. apples, pears) so the ripeness of the fruit may necessitate pressure canning. The solution is to test the acidity using pH strips or a pH meter and either add lemon juice or vinegar to bring the acid up and water bath can, or to pressure can the low-acidity food. According to the USDA Complete Guide to Home Canning, (Guide 01, page 4) the cutoff point is 4.6 pH. Lower than that (more acidic) and your food is fine to water-bath can. More than that (less acidic) then your food must be pressure canned. (Note most foods fall into the range of 3.0 to 7.0 pH, and few foods are alkali or basic and have pH between 7.0 and 14.0.) Additionally, any food with meat, regardless of how acidic, MUST be pressure canned. So, if you add italian sausage to your tomato sauce, it MUST be pressure canned.
In summary, avoid steam canning unless you have specific recipes that are tested using a steam canner. Follow the flowchart to determine if you water bath or pressure can the food.
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