Water Bath vs Pressure Canning, Which Foods Need Which
Foods with a pH of 4.6 or lower, such as fruits, jams, and pickles, can be canned in a boiling-water canner. Foods with a pH above 4.6, including all fresh vegetables except most tomatoes, meats, poultry, and seafood, need a pressure canner. The National Center for Home Food Preservation (NCHFP), which publishes material from the USDA Complete Guide to Home Canning, and the Centers for Disease Control and Prevention (CDC, page updated April 2024) set out this dividing line and the habits that keep home-canned food safe.
1. The organism you are canning against
The concern is Clostridium botulinum, the bacterium behind botulism. NCHFP explains that its spores are on most fresh food surfaces and survive harmlessly in soil and water. Under the right conditions, they become active cells that can produce a deadly toxin within 3 to 4 days. Those conditions are:
- a moist, low-acid food
- a temperature between 40°F and 120°F
- less than 2 percent oxygen
A sealed jar of low-acid food can meet all three conditions, which is why the canning method has to destroy the spores. CDC adds that home-canned vegetables, which are low-acid foods, are the most common cause of botulism outbreaks in the United States.
2. The dividing line: pH 4.6
NCHFP says the choice between a pressure canner and a boiling-water canner depends on the acidity of the food, measured as pH. The lower the pH, the more acidic the food.
| Low-acid foods | Acid foods | |
|---|---|---|
| pH | Higher than 4.6 | 4.6 or lower |
| Examples (NCHFP) | Red meats, seafood, poultry, milk, and all fresh vegetables except most tomatoes | Fruits, pickles, sauerkraut, jams, jellies, marmalades, fruit butters |
| Method | Pressure canner | Boiling-water canner |
CDC's list of low-acid foods adds figs and some tomatoes, and it states the rule plainly: pressure canning is the only recommended method for low-acid foods. A boiling-water canner, CDC says, will not protect against botulism for these foods.
Mixtures follow the low-acid side. NCHFP notes that most mixtures of low-acid and acid foods, like a vegetable soup with tomatoes, have pH values above 4.6 unless the recipe adds enough lemon juice, citric acid, or vinegar to make them acid foods.
3. Why boiling water is not enough for low-acid foods
The answer is temperature. NCHFP explains that botulinum spores are very hard to destroy at boiling-water temperatures. Low-acid foods should be processed at 240°F to 250°F, which is reachable in a pressure canner operated at 10 to 15 pounds per square inch of gauge pressure.
The time difference shows how big the gap is. According to NCHFP:
| Situation | Processing time |
|---|---|
| Low-acid foods in a pressure canner at 240°F to 250°F | 20 to 100 minutes |
| Low-acid foods in a boiling-water canner | 7 to 11 hours |
| Acid foods in a boiling-water canner | 5 to 85 minutes |
The exact time depends on the food, how it is packed, and the jar size.
4. The tomato and fig exception
Tomatoes are usually thought of as acidic, but NCHFP says some are now known to have pH values slightly above 4.6, and so do figs. To can them as acid foods in a boiling-water canner, they must be acidified to pH 4.6 or lower with lemon juice or citric acid. Properly acidified, they can be processed safely in boiling water. Use a tested recipe that tells you exactly how much acid to add per jar.
5. Equipment and recipe rules from CDC
CDC's home canning guidance adds several specifics:
- Use only tested recipes. CDC says to use recipes and cookbooks that follow the USDA Complete Guide to Home Canning, and not other recipes, even from a trusted friend or family member.
- Use a real pressure canner. CDC recommends one that holds at least four 1-quart jars sitting upright on the rack.
- Skip multi-cookers for canning. CDC says not to use an electric multi-cooker, even if it has a "canning" or "steam canning" button.
- Check the gauge. An inaccurate gauge means the wrong pressure. CDC notes that many county extension offices will check gauges.
- Vent and cool correctly. Vent the canner before pressurizing and follow recommended cooling steps.
- Update old recipes to current processing times and pressures.
Elevation matters too. NCHFP says that at 1,000 feet or more, sea-level times may lead to spoilage because water boils at a lower temperature. Use the time or pressure listed for your elevation.
6. Storing and opening home-canned food
From CDC:
- Can only what you will use in 1 year, label and date the jars, and store them in a clean, cool, dark place. For best quality, keep them between 50°F and 70°F, and never above 95°F or near hot pipes, a range, a furnace, an uninsulated attic, or direct sunlight.
- Boil before eating home-canned tomatoes, foods containing them, and all low-acid home-canned foods. At altitudes below 1,000 feet, boil for 10 minutes, and add 1 minute for each additional 1,000 feet.
- Throw out any container that is leaking, bulging, swollen, cracked, or abnormal, that spurts liquid or foam when opened, or whose food is discolored, moldy, or smells bad.
- If you do not know whether safe canning guidelines were followed, CDC's advice is to throw it out.
7. Key takeaways
- pH 4.6 is the line: foods above it are low-acid and need a pressure canner; foods at or below it can go in a boiling-water canner.
- All fresh vegetables except most tomatoes, plus meats, poultry, seafood, and milk, are low-acid.
- Pressure canners reach 240°F to 250°F, which boiling water cannot, and botulinum spores need that heat.
- Tomatoes and figs must be acidified with lemon juice or citric acid to use the boiling-water method.
- Use only USDA-tested recipes, a gauge-checked pressure canner, and boil low-acid home-canned food before eating.