Canning is a science. Do it right, and you feed your family for a year.

Clostridium botulinum spores are everywhere in soil and dust. They are harmless when you eat them; the danger is the toxin they produce when they grow in a sealed, oxygen-free jar of low-acid food. The toxin cannot be smelled or tasted, and surviving botulism poisoning requires weeks of antitoxin treatment.

Grandma's recipe worked because she was lucky, or because her recipe was actually a high-acid one that happened to be safe. Cases of home-canning botulism in the United States are still reported every year, almost always from untested recipes or from water-bath processing of foods that require a pressure canner.

There are exactly two USDA-approved methods for shelf-stable home canning. Which one you use is decided by the food's acidity, not by preference.

The dividing line between the two methods is pH 4.6. At or below pH 4.6, botulism spores cannot grow, so a boiling water bath is enough. Above pH 4.6, only the higher temperature of a pressure canner will destroy the spores.

• High-acid (water bath OK): fruit, most berries, pickles made with 5% vinegar, jams, jellies, and marmalades.
• Low-acid (pressure canner required): green beans, corn, potatoes, carrots, peas, beets, meat, poultry, fish, and combination dishes.
• Borderline, must be acidified: tomatoes and figs. Add bottled lemon juice or citric acid to every jar: 1 Tbsp lemon juice or 1/4 tsp citric acid per pint; 2 Tbsp or 1/2 tsp per quart. Fresh lemon juice is not consistent enough and is not approved.

• Canner that fits your method (water bath, atmospheric steam, or pressure).
• Canning jars designed for thermal processing. Reused commercial jars (mayo, pasta sauce) are not safe.
• New two-piece lids for every batch. Flat lids are single-use; bands are reusable.
• Jar lifter, canning funnel, headspace tool/bubble remover, magnetic lid wand.
• Timer and a way to hold a rolling boil or steady pressure.
• Bottled lemon juice or citric acid for tomatoes and figs.
• Canning salt (not iodized) for pickles.

1. Wash jars in hot soapy water; keep them hot until filled. Sterilize jars for products processed under 10 minutes.
2. Prepare the tested recipe. Do not scale or double the acid or thickener.
3. Fill jars, leaving the recipe's headspace (usually 1/4 in for jams, 1/2 in for pickles and fruit).
4. Slide a bubble remover down the jar. Wipe rims with a clean damp cloth.
5. Center the flat lid; screw the band on to fingertip tight only.
6. Lower jars onto the rack in boiling water. Water must cover jars by 1 inch.
7. Return to a full rolling boil, then start the timer. Adjust for altitude.
8. Turn off heat, remove the lid, wait 5 minutes, then lift jars straight up.
9. Cool undisturbed on a towel for 12 to 24 hours. Do not press the buttons while hot.
10. Check seals: the lid should be concave and not flex when pressed. Remove bands, wipe, label, and store.

1. Add 2 to 3 inches of hot water to the canner (or the amount your canner's manual specifies).
2. Load filled, lidded jars onto the rack.
3. Lock the lid. Heat on high until steam flows freely from the vent.
4. Vent for a full 10 minutes. This is not optional. Skipping the vent traps air and gives a false pressure reading.
5. Place the weight or close the petcock. Bring to the recipe's pressure (usually 10-11 psi, adjusted for altitude).
6. Start the timer once full pressure is reached. Hold pressure steady. If pressure drops, restart the entire timer once pressure is regained.
7. At the end, turn off the heat and let the canner depressurize naturally. Do not run cold water on it and do not force the lid.
8. Wait 10 minutes after the pressure gauge reads zero, then remove the weight and open the lid away from your face.
9. Lift jars, cool undisturbed 12 to 24 hours, check seals, remove bands, label, and store.

Water boils below 212°F at altitude, so tested recipes need more time in a water bath or more pressure in a pressure canner. Look up your elevation and adjust every batch.

Source: USDA Complete Guide to Home Canning, adapted from NCHFP. Confirm your recipe's specific chart, as some products have finer breakpoints.

• Remove the bands after cooling and cleaning. Storing with bands on can hide a broken seal and cause the ring to rust to the jar.
• Label with contents and date. Store in a cool, dark, dry place at 50 to 70°F.
• USDA recommends using home-canned goods within 12 to 18 months for best quality. Safely sealed jars keep longer, but color, flavor, and nutrition decline.
• Never store jars near a stove, in an attic, or in a garage that freezes or overheats.

Botulism toxin is odorless and colorless. You cannot smell your way to safety. Throw out any jar with any of these signs, without tasting, and dispose so that pets and wildlife cannot reach it.

• Broken or bulging lid, or a lid that flexes when pressed.
• Liquid seeping from under the seal, or a leaking jar.
• Cloudy or foamy brine (a few spice-related exceptions exist; know your recipe).
• Off odors when you open the jar.
• Mold on top, or growth inside the jar.
• Spurting liquid when you open the seal.
• Any color, texture, or smell that is not what you canned.

When in doubt, throw it out. USDA recommends detoxifying suspect jars by boiling the unopened container, contents, lid, and band submerged in water for 30 minutes before disposing in a sealed bag.

• Open-kettle canning. Filling hot jars, capping, and skipping processing. The jar may seal from thermal contraction, but the food was never brought to a safe kill temperature.
• Oven canning. Dry oven heat does not transfer to the jar contents the same way boiling water or steam pressure do. Not safe, and jars can shatter.
• Dishwasher canning. Same reason. Dishwashers do not reach or hold canning temperatures.
• Inversion sealing. Flipping filled jars upside down to "seal" from residual heat is not equivalent to processing.
• Paraffin wax on shelf-stored jam. USDA has advised against this since 1989. Freezer jam and refrigerator jam are the modern alternatives.
• Reusing single-use flat lids. The sealing compound compresses on first use and will not reliably seal again.
• Improvising a recipe. Changing quantities, swapping vinegar strengths, or adding extra vegetables shifts the acidity and heat penetration in ways you cannot see.

• USDA Complete Guide to Home Canning (2015 revision), hosted by NCHFP. Free PDF downloads for every guide.
• National Center for Home Food Preservation, the University of Georgia's Extension resource for tested recipes.
• USDA FSIS: Clostridium botulinum, the food-safety fact sheet on the organism and its toxin.
• Ball Blue Book Guide to Preserving, the current edition. Widely available in print and reviewed against USDA guidance.
• Your local Cooperative Extension office, for gauge testing and regional guidance.