Botulism in Home Canning — How Common Is It?

A while back I was researching topics related to home canning. At some point I found myself on a forum frequented by European home canning enthusiasts. According to one of them, the obsession of American home canners with botulism must mean it’s a terrible problem “over there.” That got me wondering, just how common does botulism occur as a result of home canning?

Botulism Defined

All my adult life I’ve heard more warnings about botulism poisoning as a result of home canning than I’ve heard about actual incidents of botulism. And the few cases I did learn about left me with more questions than answers. To start with, what exactly is botulism poisoning?

Botulism toxin attacks the nervous system. Symptoms normally appear between 18 and 36 hours after eating toxin-contaminated food. Double vision is often an early sign, followed by weakness in the muscles controlling the mouth and throat. Difficult breathing and eventual complete muscle paralysis can lead to death.

The good news is an antitoxin is readily available, and early treatment can result in full recovery. Even without treatment recovery is possible, but often results in permanent damage.

The scary part is that you can’t see, taste, or smell the toxin. Yet just a single taste on the tip of your tongue can send you to the ER.

The Source of Botulism

Botulism typically comes from Clostridium botulinum, bacteria, which occur in soil all over the world. These bacteria are either actively growing (vegetative cells) or forming dormant spores.

The seed-like spores can remain dormant in soil, or in water, for many years. Harmless spores, in fact, are on the surfaces of most fresh produce.

Under the certain conditions, however, spores become activated and grow into vegetative cells. And it’s these spores that produce the toxin that results in botulism poisoning.

How Spores Activate

The dormant spores can begin to grow and make toxin under the following conditions:

• An environment of low-oxygen or no oxygen (anaerobic). Fresh produce is open to air and thus not a problem. Anaerobic conditions, however, can occur in rotting foods or foods that are vacuum sealed.
• Low-acid foods. Most vegetables and meats are low in acid, while fruits are generally high in acid.
• Processed foods that are low in sugar. Jelly and jam preserve well because they are high in sugar.
• Processed foods that are low in salt. Using salt to enhance flavor isn’t enough to prevent spore growth. But food preparations, such as corned venison, that entail copious amounts of salt generally do not support spore growth.
• A temperature range between 40° and 120°F. Most refrigerators run at 38°F or less. Freezers run at 32°F. Home canning heats foods at higher temperatures than 120°.
• Foods with low water activity.

Water Activity

Water activity, simply put, is a measure of free water, meaning that no other molecules bind the water, thus making it available to support microbial growth. Measuring requires an expensive meter that registers water activity on a scale from 0.0 to 1.0, where 1.0 represents pure water.

A water activity level of 0.85 or less means insufficient water is available to support the growth of harmful bacteria. Jams, for instance, have a water activity level ranging from 0.75 to 0.80, thanks to all that sugar.

Dehydrated foods have a water activity level of 0.60 to 0.75. That equates to about 10% moisture, which is plenty low enough to prevent botulism growth. Even dried low acid foods, properly stored, won’t absorb enough moisture from the air to support spore growth.

Fresh fruits and vegetables, on the other hand, have a water activity of 0.98 to 1.00. That leaves home canners with either acid or high heat to deactive botulism spores.

Botulism in Home Canning

The CDC tracks botulism occurrences in several categories, and foodborne botulism is just one of them. Overall, instances of foodborne botulism are rare. And, despite all the hoopla, cases resulting from home canned foods are rarer yet.

From 2017 to 2021, CDC reported 20 known cases of botulism from home canned foods. Dating back to 1990, add 170 documented cases. That adds up to 190 total over a span of 32 years, or an average of less than 6 victims per year.

The CDC does not say how many home canning cases result in death. But they do indicate that foodborne botulism (from all sources, including commercially prepared foods) is fatal in fewer than 5% of cases. That computes to less than 1 fatality every 3 years from home canning botulism.

To put things into perspective, from 2015 to 2024, an average of 20 Americans per year died from lightning strikes. And, according to CDC, almost 90% of all lightning strike victims survive. That means, on average, 200 Americans per year are struck by lightning, compared to 6 per year stricken with botulism poisoning from home canned foods.

Unfortunately, the CDC does not typically track the reasons for botulism in home canned foods. But in the few cases where we know the reason, it comes down to improper canning procedures.

Botulism is Easy to Avoid

The reason botulism from home canned food is so rare is that it can’t happen at all if no bacteria or spores are in the jars to begin with. And preventing C. botulinum from getting into canning jars is easy.

For starters, can only fresh, sound produce and meat. Damaged or rotting foods may result in an absence of oxygen, and thus could activate botulism spores.

Once you select fresh, sound foods for canning, make sure they are thoroughly clean. That should eliminate most, if not all, C. botulinum bacteria that might be present.

Even if some bacteria sneak into a canning jar, they are easy to destroy. If you are canning foods with a pH less than 4.6 — mostly fruits and pickled items — acid will destroy the spores. These foods therefore may be processed in a water bath or steam canner, both of which heat to the boiling temperature of 212°F.

For low acid vegetables and meats, higher heat is needed to destroy any existing spores. The only way to produce heat that’s high enough at home is through the use of a pressure canner, which reaches a temperature of at least 240°F.

Either way, the amount of time required for processing depends on the type of food, the size of the jars, and how the food is packed into the jars. For instance, the food might be cold packed or hot packed; it might be densely packed or loosely packed. But, by following current canning recommendations, you can’t go wrong.

Note that tomatoes — which are technically fruits — present a special case, because some are high in acid and some are low in acid. Details and current recommendations are available from the reliable sources below.

If you are still concerned about botulism after carefully following the recommended procedures, heat your canned foods before serving them. Should any spores still have been present in the jars, and should they have activated during storage, simply warming the food to an internal temperature of 185°F for 5 minutes will destroy any existing toxin. And, hey, we heat most vegetables and meats before serving them, anyway.

Refrigerate leftovers, including unused portions in an open jar, within 2 hours. Make that 1 hour if the ambient temperature is greater than 90°F.

See? Easy!

Reliable Sources

Don’t rely on internet forums and videos for instructions on home canning, especially if you are just learning to can. Although some are reliable, others can be completely off base. Until you have enough canning experience to know what’s right and what’s not, stick with reliable sources. They include:

Ball Complete Book of Home Preserving and related material from the same source.

USDA Complete Guide to Home Canning and related material from the same source.

Home canning publications put out by your state’s Extension office.