Pressure Canner vs Water Bath Canner: Which One Each Food Needs

Foods at pH 4.6 or lower are acid foods, and foods above 4.6 are low-acid foods. The USDA's Complete Guide to Home Canning, which the National Center for Home Food Preservation (NCHFP) publishes online, draws the canner line at that number.

Acid foods include fruit, pickles, sauerkraut, jams, jellies, marmalades and fruit butters. Low-acid foods include red meat, seafood, poultry, milk and every fresh vegetable except most tomatoes. A mixture of the two, such as a vegetable soup with some fruit in it, usually lands above 4.6 unless a tested recipe adds enough lemon juice, citric acid or vinegar to pull it under.

The concern is Clostridium botulinum. Its spores live in soil and water and ride on most fresh food, where they do no harm because they only grow without air. Sealing a jar takes the oxygen down, and NCHFP lists less than 2 percent oxygen among the conditions the spores need, along with moist, low-acid food between 40 and 120°F. Under those conditions the bacteria multiply and can make the toxin that causes botulism, and NCHFP says that can happen within 3 to 4 days of growth.

The spores are hard to kill at boiling-water temperatures, and the hotter the canner, the more easily they die. Water boils at 212°F at sea level. NCHFP sterilizes low-acid food at 240 to 250°F, which a pressure canner reaches at 10 to 15 PSI. At those temperatures the time needed runs from 20 to 100 minutes depending on the food, how it is packed and the jar size. In a boiling-water canner the same job would take 7 to 11 hours by NCHFP's figures, which is why low-acid food goes in the pressure canner.

Acid stops those bacteria from growing, or kills them faster when heated, so acid foods can be sterilized at 212°F. That is why jam and pickles can go in a boiling-water canner.

A boiling-water canner is a large, deep pot with a rack and a lid. NCHFP describes them as aluminum or porcelain-covered steel, deep enough that at least 1 inch of briskly boiling water covers the jar tops during processing. These are the steps as NCHFP lays them out, and the processing time itself always comes from the tested recipe.

A pressure canner seals with a locking lid and traps steam, which raises the temperature well past boiling. Most home models hold seven quart jars or eight to nine pints. NCHFP flags two serious errors: pressure that is not raised for elevation, and air left trapped inside the canner.

A pressure canner can process some acid foods too, if the recipe was tested for one. NCHFP still recommends boiling water for acid foods because it is faster: roughly 25 to 60 minutes in total against 55 to 100 minutes in a pressure canner. The table lists the stages NCHFP counts.

Water boils at a lower temperature the higher you live. NCHFP warns that using a sea-level time at 1,000 feet or above can leave food under-processed and lead to spoilage. Boiling-water recipes make up for it with more time, and pressure recipes make up for it with more pressure.

The adjustment differs by food, jar size and pack style, so there is no single add-on that works for everything. The USDA guide gives a table for each product. To see how far one recipe moves, here is the guide's own example: peaches, hot pack, boiling-water canner.

A pressure canner is built to hold jars, usually seven quarts or eight to nine pints, unlike an ordinary pressure cooker. NCHFP does not recommend small pressure saucepans for canning. Test a dial gauge every year, and replace one that is off by more than 2 pounds.

Electric multi-cookers, including Instant Pot-style models, need their own mention. University of Wyoming Extension says neither the USDA nor any university has shown that these appliances can safely process low-acid food, and its educators do not endorse USDA canning processes in them. They are fine for cooking food in the pot, but a stovetop pressure canner is what low-acid canning requires. Wyoming also notes that the Ball FreshTECH system is not a multi-cooker and is tested only for the high-acid foods in its manual.

NCHFP's equipment page contains two statements about atmospheric steam canners. The list of methods not recommended says they are not currently recommended. An update on the same page says research with the University of Wisconsin supports them for acid or properly acidified foods (pH 4.6 or lower) under specific controls. NCHFP's steam-canner note adds that they are not pressurized vessels and never suit low-acid food, that processing time including the altitude adjustment must be 45 minutes or less, and that the canner must not be opened during the process. If you own one, follow the University of Wisconsin directions NCHFP links to. NCHFP also advises against open-kettle canning and against processing filled jars in an oven, a microwave or a dishwasher.

A common and dangerous mistake is putting a low-acid food in a boiling-water canner because the recipe looked simple. Pickled green beans are an acid food because a tested pickle recipe brings them under pH 4.6. Plain green beans with a splash of vinegar do not qualify, and only the tested recipe can tell you how much acid is enough. The food safety page explains what counts as tested here.

The other common errors are skipping the 10-minute vent, force-cooling the canner, and letting the boil or the pressure lapse without restarting the clock. NCHFP also warns against "dry canning" vegetables without the covering liquid the research used, because that can leave food under-processed and, for vegetables, adds a botulism risk.