Home canning for beginners: the USDA way to can safely

Two methods: water bath or pressure canner

Everything in home canning comes back to acid. The USDA Complete Guide to Home Canning says acid foods have a pH of 4.6 or lower. Fruit, jams, jellies and pickles fall on that side, and so do tomatoes once you add lemon juice, citric acid or vinegar as a tested recipe directs. Acid foods can be processed in a boiling-water canner.

Low-acid foods are the rest: the USDA guide lists red meats, seafood, poultry, milk and all fresh vegetables except for most tomatoes. Those need far more heat. The USDA says all low-acid foods should be sterilized at 240° to 250°F, which only a pressure canner operated at 10 to 15 pounds of pressure can reach.

CDC puts it plainly: pressure canning is the only recommended method for canning low-acid foods, and you should not use a boiling-water canner for them, because it will not protect against botulism. If you want to can green beans, corn, soup or meat, you need a pressure canner, and a pressure canner can also do water-bath jobs.

What you need to start

The USDA guide says regular and wide-mouth Mason-type, threaded home-canning jars with self-sealing lids are the best choice. With careful use the jars can be reused many times, needing only new lids each time. The flat lid is the part you replace for every batch; the screw bands can be reused once they're washed and dried.

For low-acid foods, CDC says to use a recommended pressure canner that holds at least four 1-quart jars upright on the rack, and to make sure its gauge is accurate. Many county extension offices will check gauges for you.

The rest is small kit: a jar lifter for hot jars, a wide funnel, a tool to measure headspace and release air bubbles, and a clean towel to set hot jars on. Our canning supplies page lists a starter set.

• Mason-type canning jars (regular or wide mouth)
• New self-sealing lids, plus screw bands
• A boiling-water canner with a rack, a pressure canner, or both
• Jar lifter, funnel and headspace tool
• A tested recipe with current times

Follow a tested recipe, exactly

Canning is chemistry, so the recipe matters more than in ordinary cooking. The USDA guide says to use only tested recipes formulated to produce the proper acidity. That means a source such as the USDA guide itself or the National Center for Home Food Preservation, not an old family card or a recipe you've changed. CDC adds that if your recipe is old, you should use current processing times and pressures.

Altitude changes the times. The USDA guide warns that using sea-level process times may cause spoilage if you live at 1,000 feet or more, so follow the altitude adjustments in the recipe. Water boils at a lower temperature as you go up.

Once the jars come out, let them cool. The USDA guide says to test the seals after cooling jars for 12 to 24 hours, then remove the screw bands, which aren't needed on stored jars.

Methods not to use

Some popular shortcuts don't kill everything they need to. The USDA guide says open-kettle canning and processing filled jars in conventional ovens, microwave ovens or dishwashers are not recommended, because they don't prevent all risks of spoilage.

CDC also says not to use an electric multi-cooker appliance for canning, even one with a canning or steam canning button on the front, and not to use a boiling-water canner for low-acid foods.

• Open-kettle canning (hot food in jars, no processing)
• Oven, microwave or dishwasher canning
• Electric multi-cookers, even with a canning button
• A boiling-water canner for vegetables, meat, poultry or seafood

Storing and eating home-canned food

CDC says to can only the amount of food you will use in a year, unless directions for a specific food say otherwise, and to label and date your jars. Store them in a clean, cool, dark place: between 50°F and 70°F for best quality, and for safety never above 95°F, near hot pipes, a range or a furnace, in an uninsulated attic or in direct sunlight.

Before eating, CDC says to boil home-canned tomatoes, foods containing them and all low-acid home-canned foods in a saucepan. Below 1,000 feet, boil for 10 minutes, and add 1 minute for each additional 1,000 feet. Refrigerate any canned or pickled food after you open it.

Throw out any jar that is leaking, bulging or swollen, looks damaged or cracked, spurts liquid or foam when opened, or holds food that is discolored, moldy or smells bad. CDC's rule: do not eat food if you don't know whether safe canning guidelines were followed.

Why the rules are strict: botulism

The reason for all of this is a toxin. CDC says home-canned vegetables, which are low-acid foods, are the most common cause of botulism outbreaks in the United States. Botulism spores grow and make toxin in low-oxygen, low-acid conditions, which is exactly what the inside of a sealed jar of underprocessed vegetables provides.

CDC says you cannot see, smell or taste the toxin that causes botulism, and that even a small taste of food containing it can be deadly. That's why the safeguards are in the method rather than in a check at the end: the right canner for the food, a tested recipe, the right time and pressure, and boiling low-acid foods before you eat them.