The single most important fact about home canning is the pH 4.6 line. Foods below pH 4.6 (high-acid: tomatoes with added acid, most fruits, pickles, jams) can be safely processed in a boiling water bath at 212 F. Foods above pH 4.6 (low-acid: nearly all vegetables, meats, seafood, beans, soups, mixed recipes) must be processed in a pressure canner at 240 F to 250 F to destroy Clostridium botulinum spores. Get this wrong and you make a product capable of fatal poisoning. Get it right and home canning is one of the safest, most reliable ways to preserve food for a year or longer. This guide breaks down the two methods, what each can and cannot do, and the specific rules that keep home canning safe.
The acidity rule, explained simply
Bacterial spores that cause botulism cannot germinate in acidic environments. The cutoff is pH 4.6. Below that, the food is too acidic for the spores to grow and produce toxin, even in a sealed jar at room temperature. Above 4.6, the spores can germinate, multiply, and produce toxin in the oxygen-free environment inside a sealed jar over weeks or months of storage.
Water bath canning processes the food at 212 F (boiling). That temperature kills yeasts, molds, and most bacteria, and creates a vacuum seal. It does not kill botulism spores. The acidity of the food does the long-term protection work.
Pressure canning brings the food to 240 F (at 10 PSI sea level) or 250 F (at 15 PSI). That temperature does kill botulism spores within the processing time. Acidity is not the protection mechanism (though acidity still extends quality). The heat does the work.
This is why the method has to match the food. Putting low-acid vegetables in a water bath canner gives you boiled vegetables sealed in jars with live botulism spores waiting to germinate. Putting high-acid fruits in a pressure canner is wasteful but not dangerous.
What goes in a water bath canner
These foods are reliably safe with water bath processing because their natural or adjusted pH is below 4.6:
- Most fruits and fruit juices (peaches, pears, apples, berries, cherries, plums)
- Tomatoes only with added acid (lemon juice or citric acid, at the USDA-specified ratio)
- Jams, jellies, and preserves (high sugar plus fruit acid)
- Pickles and relishes (vinegar at 5 percent acidity, used at correct ratios)
- Pickled vegetables (pickled green beans, pickled carrots, pickled okra, when made to tested recipes)
- Fruit butters (apple butter, pear butter)
- Salsas made to tested USDA recipes (not improvised)
- Chutneys with adequate vinegar
The water bath canner itself is simple: a large pot deep enough to cover jars with 1 to 2 inches of water, plus a rack to keep jars off the bottom. Total processing times typically run 5 to 45 minutes depending on jar size and contents.
What requires a pressure canner
These foods cannot be safely water bath canned. They must be pressure canned to be shelf-stable at room temperature:
- All plain vegetables (corn, green beans, carrots, beets, asparagus, peas, potatoes, squash)
- All meats (beef, pork, poultry, game)
- Seafood (fish, shellfish)
- Dried beans, lentils, peas
- Soups, stocks, broths, chili
- Combination dishes (spaghetti sauce with meat, stew)
- Pumpkin and winter squash (USDA does not recommend canning pureed pumpkin at all, only cubed)
- Mushrooms
Pressure canners come in two types: weighted-gauge (Presto, All American) and dial-gauge (older Presto models). Weighted-gauge canners are easier to use and do not require annual calibration. Dial gauges must be tested for accuracy each season at a county extension office.
Processing times for low-acid foods typically run 20 to 90 minutes at 10 to 11 PSI. Times increase at higher altitudes.
The tomato edge case
Tomatoes sit right at the pH 4.6 line. Some varieties test below it; modern hybrid tomatoes often test above. The USDA conservative ruling: all tomatoes intended for water bath canning need added acid, specifically 2 tablespoons of bottled lemon juice or half a teaspoon of citric acid per quart, or twice that for pints (which seems backwards but is correct because pints process less time per volume).
Tomatoes can also be pressure canned without added acid, which keeps the flavor more like fresh tomato. The choice is a flavor preference, not a safety one, as long as you follow the rule consistently.
Altitude adjustments
Water boils at lower temperatures at higher altitudes. The processing times in USDA charts assume sea level. Adjustments:
Water bath canning:
- 1,001 to 3,000 feet: add 5 minutes
- 3,001 to 6,000 feet: add 10 minutes
- 6,001 to 8,000 feet: add 15 minutes
- 8,001 to 10,000 feet: add 20 minutes
Pressure canning (using a weighted-gauge canner):
- 0 to 1,000 feet: 10 PSI
- Above 1,000 feet: 15 PSI
Pressure canning (using a dial-gauge canner):
- 0 to 2,000 feet: 11 PSI
- 2,001 to 4,000 feet: 12 PSI
- 4,001 to 6,000 feet: 13 PSI
- 6,001 to 8,000 feet: 14 PSI
Get your exact altitude before processing the first batch each season.
Common mistakes that cause unsafe jars
Reducing processing time because the jar looks done. Processing time is set by heat penetration through the coldest part of the jar contents, not by visual cues. A jar that looks fully heated may still have a cool center.
Reusing lids. Two-piece canning lids have a single-use sealing compound. The rim band can be reused indefinitely, but the flat lid with the rubber seal must be new each time.
Inverting jars to “seal” them. This was a 1970s practice that the USDA has explicitly disavowed. Inversion creates a seal that does not have the safety of a true processed seal. Always process in the canner.
Doubling a recipe. Doubling the recipe does not mean doubling the processing time. It usually means longer than double, and the safe way to handle it is to process the food in standard batches rather than scale up a single batch.
Using a digital instant-pot for canning. Despite some manufacturer claims, the USDA has not approved any multicooker (including newer pressure-rated models) for canning. The temperature data is not equivalent to a tested pressure canner. Use a dedicated canning vessel.
When to call the extension office
Every state has a Cooperative Extension Service with a food preservation specialist. They will test your pressure canner gauge, answer specific recipe questions, and provide the latest USDA bulletins free. The National Center for Home Food Preservation at nchfp.uga.edu is the central archive for tested recipes.
If a recipe you find online conflicts with a USDA or Ball Blue Book recipe, follow the USDA or Ball version. Pinterest and Facebook canning recipes are a documented source of botulism outbreaks. Stick to tested sources for the actual processing parameters.
For more on the equipment side, see our methodology page for how we evaluate canning gear. A home canning setup that works for a decade typically pays for itself in produce-glut summers within two seasons.
Frequently asked questions
Can I water bath can green beans or carrots if I add vinegar?+
Only if the final pH is verifiably below 4.6, which most home cooks cannot confirm without a calibrated pH meter. The safer route for low-acid vegetables is pressure canning. Pickled green beans (heavily brined with vinegar at 5 percent acidity in the finished jar) can be water bath canned because the pickling brine pushes pH below 4.6. Plain canned beans cannot.
Why is botulism such a concern with home canning?+
Clostridium botulinum spores survive boiling water (212 F) and germinate in an anaerobic environment like a sealed jar with low-acid food at room temperature. The toxin they produce is tasteless, odorless, and can be fatal in microgram amounts. Pressure canning at 240 F (10 to 15 PSI) is the only home method that reliably destroys the spores. Water bath canning relies on acidity (pH below 4.6) to prevent germination instead of killing the spores.
Are old family canning recipes safe to use today?+
Often not. USDA processing times were updated in 1988 and again in 2009 based on improved food safety data. Many recipes from before 1988 use shorter processing times that fail to inactivate harmful organisms reliably. Open kettle canning (just pouring hot food into jars and letting them seal as they cool) was once common and is now considered unsafe for any food except jams and jellies with high sugar content. Use modern USDA or Ball Blue Book guidelines.
What happens if my jars do not seal properly?+
Unsealed jars must be refrigerated and used within a week, or reprocessed within 24 hours with a new lid (not a reused one). A failed seal usually shows up as a lid that flexes when pressed in the center. Properly sealed lids are concave and do not move. Reprocessing requires emptying the jar, reheating the contents to a boil, putting them in a clean hot jar with a fresh lid, and running the full processing time over again.
How long does home canned food actually last?+
USDA guidance says best quality within one year, though properly processed and sealed canned goods often remain safe for two to three years if stored in a cool, dark, dry place. Quality degradation (color, texture, flavor) happens before safety degradation. Throw away any jar with a broken seal, bulging lid, off odor, mold, cloudy liquid (in clear products), or active bubbling, regardless of how recently it was canned.
