Safe home canning: how to avoid botulism

Home canning is back in fashion, and canned food is an important part of any pantry. But it carries one risk few people know well: botulism. In both Europe and the US, most foodborne botulism cases come from home-canned food, usually low-acid vegetables processed the wrong way.

The good news: you can bring the risk close to zero by following a few rules.

What botulism is

Botulism is poisoning by a toxin made by the bacterium Clostridium botulinum. Its spores are everywhere in soil and on vegetables. In a sealed jar with no air, low acid and room temperature, spores that survived processing can grow and produce the toxin.

The toxin can’t be seen, smelled or tasted. A tiny amount can cause paralysis.

High-acid and low-acid foods

This is the key rule:

Type	Examples	How to process
High-acid	Fruit, jams, jellies, fruit in syrup, vinegar pickles, sauerkraut	Boiling water bath, per a tested recipe
Tomatoes	Juice, sauce, crushed tomatoes	Water bath with added acid (lemon juice or citric acid)
Low-acid	Vegetables (beans, peas, corn, peppers), mushrooms, meat, fish, soups	Pressure canner only

Why? Boiling water reaches 212 °F (100 °C). Botulinum spores survive that. They’re destroyed at 240–250 °F (116–121 °C), which only a pressure canner reaches. In high-acid foods, spores can’t grow, so a water bath is enough.

The safety rules

1. Use tested recipes, not guesswork. Acid levels and processing times matter.
2. For vegetables, meat and mushrooms, use a pressure canner, or choose another method: freezing, drying or vinegar pickling.
3. Clean jars without chips, new lids each season.
4. Don’t shorten processing times. At high altitude water boils below 212 °F and times increase.
5. Store jars somewhere cool and dark, ideally below 60 °F (15 °C).
6. Date every jar and use within 12 months.
7. Garlic, chilies and herbs in oil are a separate risk: refrigerate and use within a few days, or freeze.

When to throw it out without tasting

• The lid is bulging or no longer sucked down
• Liquid spurts or foams when opened
• An odd, sour or rancid smell
• Cloudy liquid, mold or rising bubbles
• The jar is leaking or the lid is rusted

Throw the closed jar away in a bag. Don’t feed it to animals. Wash your hands and clean any surfaces it touched thoroughly.

Important: a jar can contain the toxin with none of these signs. That’s why, for home-canned low-acid food you’re not certain about, the safest rule is to boil it for 10 minutes before eating.

Symptoms of botulism

Usually 12–36 hours after eating, but sometimes after a few hours or several days:

• double or blurred vision;
• drooping eyelids;
• slurred speech, trouble swallowing, dry mouth;
• muscle weakness moving down from the head to the body and limbs;
• difficulty breathing.

Call the emergency number immediately. Botulism is treated in hospital with antitoxin, and some people need a ventilator. Tell doctors what you ate and keep the jar for testing.

Store-bought cans

Commercially canned food is processed at high temperatures and is very safe. The rules are simple: don’t buy or use bulging, deeply rusted or leaking cans. For how long cans last past the date, see our guide to canned food after the expiration date.

For the rest of your food supply, see pantry rotation and dehydrated and freeze-dried food.