Safe Home Canning: Sorting the Evidence From the Noise

Key takeaways

• Low-acid foods: vegetables, meat, poultry and seafood must be processed in a pressure canner
• High-acid foods: most fruits, jams, pickles and properly acidified tomatoes can use a boiling-water canner
• Most useful single change: Use up-to-date tested recipes from the USDA or your local Cooperative Extension office.
• Commonest mistake: Oven canning, open-kettle canning or canning in a slow cooker or multicooker without a tested method.
• See a doctor if any of the signs under "When to speak to a doctor" below apply to you.

What it is

Preserving food in jars at home using tested methods that destroy germs and seal the jar. It sits within Food Safety. What follows are the published numbers, not the ones that get passed around.

The numbers, in one place

What helps

What follows is the practical part: the things guidance actually asks of you.

• Use up-to-date tested recipes from the USDA or your local Cooperative Extension office
• Have the dial gauge on your pressure canner tested every year
• Use canning jars and new lids, and follow the recipe's headspace and processing time exactly
• Check seals after jars cool and refrigerate or reprocess any that did not seal

What to avoid

The common mistakes, including the ones that sound sensible.

• Oven canning, open-kettle canning or canning in a slow cooker or multicooker without a tested method
• Using a boiling-water canner for vegetables, meat or soups
• Eating from jars that are bulging, leaking, spurting liquid when opened, or that smell bad; when in doubt, throw it out
• Inventing or changing recipes, such as adding more vegetables, onions or flour

In depth

Home canning works by heating filled jars enough to destroy germs and then forming a vacuum seal that keeps new germs out. The main danger it must guard against is Clostridium botulinum, a bacterium whose spores are common in soil and on fresh produce. The spores are harmless in ordinary conditions, but inside a sealed jar of low-acid food, with no oxygen and at room temperature, they can grow and produce botulinum toxin, one of the most potent toxins known.

Acidity is what decides the method. Botulism spores cannot grow in acidic foods, so fruits, jams, jellies, pickles and acidified tomatoes can be safely processed in a boiling-water canner at 212°F (100°C). Vegetables, meats, poultry, seafood and soups are low in acid, and boiling water does not destroy the spores. For these foods, a pressure canner is needed, because steam under pressure reaches about 240°F (116°C), hot enough to kill spores in the tested time.

Tested recipes are central to safety. Processing times are determined in laboratories by measuring how heat moves through a specific jar size filled with a specific food at a specific density. Changing the recipe, for example by adding more onions or peppers to salsa, thickening with flour, or using larger jars, changes how heat penetrates and how acidic the food is, and can leave the center of the jar underprocessed even if the lid seals perfectly.

Signs and symptoms

• Double or blurred vision and drooping eyelids
• Slurred speech and difficulty swallowing
• A thick-feeling tongue and dry mouth
• Facial weakness and muscle weakness that moves down the body
• Trouble breathing, which can lead to respiratory failure
• Symptoms usually beginning 12 to 36 hours after eating, though this can vary

Causes and risk factors

• Canning low-acid foods without a pressure canner
• Using recipes that are untested, outdated or altered
• Not adjusting time or pressure for altitude
• Inaccurate dial gauges on pressure canners
• Not acidifying tomatoes with lemon juice or citric acid
• Storing low-acid foods in oil or garlic-in-oil at room temperature

How it is checked

Botulism is diagnosed mainly from its distinctive symptoms and a history of eating a risky food, such as home-canned vegetables. Doctors also rule out conditions that can look similar, such as stroke, Guillain-Barré syndrome or myasthenia gravis, with a careful neurological examination and sometimes additional tests.

Because treatment needs to start quickly, doctors do not wait for lab confirmation. They contact the state health department, which works with the CDC to release botulinum antitoxin. Samples of blood, stool or the suspected food may be sent to public health laboratories to confirm the diagnosis afterward, so keep any remaining food or jars for investigators.

Treatment and support options

Learn from tested sources

The USDA Complete Guide to Home Canning and materials from Cooperative Extension offices provide tested recipes and step-by-step instructions. Many Extension offices offer classes and will test the dial gauge on your pressure canner. Recipes from older cookbooks, family traditions, blogs and social media may not have been tested and can be unsafe, even if they have been used for years without problems.

Use the right equipment

A pressure canner is a heavy pot with a locking lid and either a dial gauge or a weighted gauge, designed to hold enough jars for the processing times to work. It is different from many electric pressure cookers, which have not been tested for home canning. Use canning jars made for the purpose, new flat lids each time, and screw bands in good condition.

Follow the process precisely

Measure headspace, remove air bubbles and wipe jar rims before applying lids. Vent the pressure canner for the time stated before pressurizing, hold the correct pressure for the full processing time, and let it depressurize naturally. After cooling for 12 to 24 hours, remove bands and check that each lid is concave and does not flex. Refrigerate or reprocess unsealed jars.

Emergency treatment for botulism

Botulism is a medical emergency treated in the hospital with antitoxin, which stops the toxin from causing further damage but cannot reverse damage already done. Some people need a ventilator to help them breathe and may need weeks or months of care and rehabilitation to recover. Early treatment improves outcomes, which is why symptoms need an immediate 911 call.

Prevention

• Check your altitude and use the matching time or pressure in every recipe
• Store sealed jars in a cool, dark, dry place and use them within about a year for best quality
• Label each jar with its contents and canning date
• Inspect every jar before opening and discard any with signs of spoilage without tasting
• Refrigerate homemade flavored oils and use them within a few days

Day to day

Home canning can be a rewarding way to preserve a harvest and save money, and following tested methods makes it safe. Many experienced canners keep a notebook with recipes, dates and processing details for each batch. Before canning season, check your equipment, replace worn gaskets, have your dial gauge tested and refresh your supply of new lids, so that you are not tempted to cut corners when produce is ripe.

If you are unsure whether a jar was processed correctly, it is safest not to eat it. Contaminated jars should be discarded carefully, because botulinum toxin can be absorbed through the skin and eyes: wear gloves, place jars in a sealed bag, and clean surfaces with a bleach solution as described in USDA guidance.

For specific groups

Infants

Babies can develop infant botulism by swallowing spores, which grow in their immature gut. This is why honey should not be given to children under 12 months, including in home-canned foods or baked goods.

People at high altitude

Water boils at lower temperatures at higher elevations, so processing times for boiling-water canning and pressures for pressure canning must be increased above 1,000 feet. Tested recipes include altitude adjustments.

Community and church meals

Outbreaks have occurred at shared meals where home-canned foods were served to many people. Use only commercially canned or properly processed low-acid foods when cooking for groups.

Questions to ask your doctor

• What symptoms after eating home-canned food mean I should call 911?
• Are there conditions that make me more vulnerable to botulism?
• What should I do if someone ate from a jar that I now think was unsafe?
• How long can botulism symptoms take to appear?
• Is it safe to give my baby homemade baby food that was canned at home?

More questions, answered

Can I can vegetables in a boiling-water canner if I process them longer?

No. Boiling water cannot get hot enough to destroy botulism spores in low-acid foods, no matter how long you process. Vegetables, meat, poultry, seafood and soups must be processed in a pressure canner using a tested recipe.

Can I use my electric pressure cooker or multicooker for canning?

Most electric pressure cookers have not been tested for safe home canning, and their temperatures and heat-up and cool-down times may not match what tested recipes require. Use a stovetop pressure canner unless a specific appliance has been tested and approved for canning.

Is it safe to reuse jars and lids?

Canning jars can be reused if they have no cracks or chips. Flat lids are designed for one use, because the sealing compound may not seal reliably a second time. Screw bands can be reused if they are not rusted or bent.

Why do tomatoes need added acid?

Many tomato varieties are close to the line between high- and low-acid foods. Adding bottled lemon juice or citric acid ensures they are acidic enough for safe boiling-water processing. Bottled lemon juice is used because its acidity is consistent, unlike fresh lemons.

If a jar sealed, does that mean it is safe?

Not necessarily. A seal only shows that a vacuum formed. If the food was underprocessed, botulism spores can still survive and grow inside a sealed jar. Safety depends on using a tested recipe and the correct method, time and pressure.

Are freezer jams and refrigerator pickles safe?

Yes, when stored as directed. They are not processed for shelf storage, so they must be kept in the freezer or refrigerator and used within the time the recipe gives.

Related topics

• What the Guidance Really Says About Cooking Methods (Healthy Recipes)
• Food and Water Safety Abroad: Sorting the Evidence From the Noise (Travel Health)
• What the Guidance Really Says About Fruit and Vegetables (Nutrition)
• Foods to Avoid in Pregnancy: What the Evidence Actually Says (Pregnancy and Postpartum)

Where to go next

• Safe Home Canning: Just the Figures
• Making Sense of Safe Home Canning
• Safe Home Canning — the reference entry
• Food Safety: The Complete Guide
• The Evidence on The Four Steps to Food Safety, Without the Hype
• Safe Internal Cooking Temperatures: What Published Guidance Supports

When to speak to a doctor

Call Poison Control at 1-800-222-1222 or your doctor if you are worried about food from a jar that may not have been canned safely. Call 911 right away for double or blurred vision, drooping eyelids, slurred speech, trouble swallowing or breathing, or muscle weakness after eating home-canned food.

Where this comes from

• USDA — Complete Guide to Home Canning
• CDC — Preventing Botulism

How this page was put together

Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarized or generalized beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.

Medical disclaimer

This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to your doctor, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 911.