Tips & Techniques

The Need-to-Know Guide to Botulism, For Safe Canning & Preserving

March 18, 2016

“On a scale of one to botulism, how concerned should I be about the months-old garlic confit sitting in my pantry?”

Photo by James Ransom

It hadn’t even been five minutes into my conversation with Cathy Barrow, longtime Food52 contributor and author of the canning cookbook, Mrs. Wheelbarrow’s Practical Pantry, before I hijacked the interview to address the five-month-old ingredient my boyfriend was heading home to cook dinner with.

“I would say it has potential for making you sick,” Cathy said, diplomatically. I texted my boyfriend to abandon the Aglio e Olio—and find a Hazmat suit.

Okay, so I may have overreacted, but it’s easy to get nervous after reading the C.D.C.’s definition of botulism—"a rare but serious paralytic illness caused by a nerve toxin that is produced by the bacterium Clostridium botulinum and sometimes by strains of Clostridium butyricum and Clostridium baratii."

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There are five main kinds of botulism: wound botulism, infant botulism, adult intestinal toxemia, iatrogenic botulism, and—the subject of my concern—foodborne botulism, which is most frequently caused by the ingestion of canned and preserved foods when Clostridium botulinum is present, and can be fatal by way of muscle paralysis. According to the C.D.C:

All forms of botulism can be fatal and are considered medical emergencies. Foodborne botulism is a public health emergency because many people can be poisoned by eating a contaminated food.

The spores of Clostridium botulinum, according to the U.S. Food Safety and Inspection Services, are found everywhere (including on the surfaces of fruits and vegetables—that's where canned and preserved food comes into play). The spores themselves are "are generally harmless," but "the danger can occur once the spores begin to grow out into active bacteria and produce neurotoxins."

And contaminated food can look, smell, and taste normal. (Like I said: It's scary!)

But, as Cathy pointed out to me, botulism is also extremely rare. According to the C.D.C.’s website there are 145 cases of botulism reported in the United States each year (you may recall 2015’s church potluck disaster, which afflicted 28 people and killed one). And of those 145 cases, only 15% are foodborne and only 3 to 5% fatal (compared to botulism's 50% fatality rate fifty years ago), due to advancements in treatment and awareness.

That’s roughly four people who die of botulism per year—less than death by vending machines, which, by some counts, kill three times as many people annually. (If a vending machine is tipping over, get out of its way.) As Cathy joked, "If you want to kill your family, there are better ways [than botulism] to do it."

Still, as with any food safety concern, avoiding botulism requires diligence. But as Harold McGee, author of the blog Curious Cook and food chemistry expert, wrote to me:

Fear doesn't belong in the kitchen. Prudence and caution do, developed from a basic understanding of how disease microbes behave and can be controlled.

So how does a reasonable home cook prevent it?

Here are 4 things to keep in mind to give due caution to botulism:

Photo by James Ransom

1. Some foods are more likely to host the botulism-causing bacteria than others.

The bacteria that causes foodborne botulism, Clostridium botulinum (a group of pathogenic bacteria initially lumped together by their ability to produce the botulinum toxin), is unique in that it thrives in low oxygen conditions—specifically, it prefers an environment that’s low in acid at room temperature. This is what Cathy refers to as the “perfect storm situation.”

In other words, low-acid vegetables (a pH level higher than 4.6 is to be considered “low acid”) like green beans, corn, garlic, onions, and foods like beets, potatoes, and other tubers and root vegetables that grow underground are more likely to provide a happy home for botulism-causing spores.

“When people tell me, I just canned my grandmother’s special sauce—with onions and peppers, which are two things that are extremely low acid—it’s like they’ve created a botulism stew,” Cathy says, “They’re better off canning the peppers and tomatoes separately, then making the sauce later.” The more low-acid vegetables in one place, the more likely it is for the bacteria to be able to survive.

It’s also important to keep in mind that meat and fish have low acid levels as well—so in places where fermented seafood is common, like in Alaska, the same precautions for low-acid vegetables should be observed.

On the other side of things, raspberries, blueberries, and most fruits are high acid (a pH less than 4.6), and high in sugar, which further deters the growth of the botulism spores. “If you have jam in your fridge for 15 years,” Cathy said, “the worst thing you’re going to get is mold," which is more tolerant of high-sugar environments.

Cathy also pointed out that vinegar-pickled vegetables are also not likely to host the botulism bacterium. Because pickled vegetables are covered in an acidified brine, the process creates a high enough acidity to prevent the risk of botulism.

For more information on the pH levels of food, refer to this helpful guide from the C.D.C.

2. Boiling alone does not kill botulism bacteria (and neither does cooking).

The reason Clostridium botulinum is so hard to kill is because it forms heat- and chemical-resistant endospores (dormant, stripped-down structures) in unfavorable environments. Endospores allow bacteria to lay dormant for extended periods of time until conditions have improved. In short, if you're canning a low-acid food, simply boiling the jar using the traditional water bath method won't kill the bacteria: While water boils at 212° F, botulism bacterium only die at 241° F.

For low-acid foods that are more likely to host botulism, turn to pressure canning. Not to be confused with a warm water bath or pressure cooking, pressure canning is the only way to get the internal temperature of the canned food to 250° F, killing botulism. “Even if you put your can in a 350° F oven, you won’t be able to get the center of the jar cooked to a higher temperature,” Cathy said. In short, if a recipe instructs you to pressure can, do it—it's a step that can't be skipped or improvised.

Likewise, there’s no guarantee that even after you’ve opened your can and added its contents to a baked good or a sauce that cooking that dish will kill any strains of botulism. The only way to guard canned food against botulism is to pressure-can it for the correct amount of time—period.

3. Once you’ve canned properly, there are still precautions to take.

Even after you’ve killed all botulism-causing bacteria at the time of canning, once you've opened that can, the bacteria can find a way in. Because of this, Cathy avoids keeping canned foods, once opened, like canned tomatoes or soup, for more than three to four days, and uses clean utensils every time she serves herself.

And you're still feeling a little hesitant about the whole pressure-canning thing, there are plenty of foods (the high-acid and/or high-sugar kinds) that you can safely preserve with a simple boiling water bath. Here are a few of our favorites—from compotes to quick pickles and jam:

4. A fear of botulism should never keep you from canning.

With all of this in mind, Cathy—who cans everything from rhubarb chutney, to peach pie filling, to tomatoes (did we mention, she's written a book on it?)—has never met anyone who's suffered from botulism.

It's not that botulism isn't out there, but when all of the precautions are taken, canning is a safe and wonderful way to preserve your favorite foods year-round. As Cathy puts it, “Follow the rules, be smart, and stop worrying so much.

Do you have any questions about botulism? Are you a first-time canner? Tell us in the comments below!

See what other Food52 readers are saying.

  • juanita
  • robbieruth
  • Markk
  • Sarah Wilson
    Sarah Wilson
  • Coby Dahlstrom
    Coby Dahlstrom
I eat everything.


juanita December 5, 2022
In the article, it states that boiling alone does not kill botulism spores. Also, it is written botulism dies at temperatures of 241 degrees and boiling temperature is 212 degrees. While this is true of boiling. 212 is when boiling begins, but the temperature continues to rise and does not just remain at 212. 241 degrees is reachable on the stove. So, I don't understand why boiling won't kill botulism.
robbieruth August 22, 2022
I canned some green beans using a pressure cooker but, mistakenly, only processed them for 10 minutes (should have been 20) at 10 psi - should I or rather CAN I reprocess them? If not, will they be safe to eat as is?
Markk November 7, 2021
Hi, I canned cucumbers with a recipe that resulted in low vinegar and hi pH about 6.
It has been setting out in kitchen for months.
I decided perhaps I could open jars and add in more vinegar to kill off any botulism they may be growing.

Do you think adding in more vinegar after it has already been canned and sitting on shelf for 5 months will kill it off if it is in fact growing ?
Sarah W. September 15, 2019
In theory, would it be safe to pressure can a homemade spaghetti sauce following the guidelines of the ground beef safe pressure canning time (90min at 11lbs pressure) since it is the ingredient with longest pressure canning time in the the ingredients of the sauce...
Coby D. May 15, 2019
Hi there. I am pickling eggs for the first time and am using a typical dill recipe with turmeric added for color. one of the boiled eggs broke open (when I over-smashed them into the jar), and now the brine has smashed egg yolk mixed into it making it sort of milky in texture. do I need to worry about botulism? or will the brine be fine as is? thank you!
julie R. September 4, 2018
I have been canning various fruits and vegetables for 10 years and have never had a problem. But, this August I canned 5 quarts of tomato sauce and I could not remember if I added lemon juice to 2 of the jars. I only thought of this once the water bath canning method was finished. Is it possible to take these sealed jars and re-can them in a pressure canner to reduce any risk of botulism growth?
Sherry October 9, 2017
I canned tomatoes for the first time this year. I used the water bath method, but followed the instructions in my canning book. I also made sauce, with onions, peppers, etc., again, following all instructions. But now I am afraid to use them. Should I have a real fear of botulism or are my canned tomatoes and sauce ok to eat?
Tiff September 13, 2017
I had canned beets 3 weeks ago and saw a very slight white room at settled at the bottom of my canned jar, when I turned it upside down it melted away into the juice (which did not look court at all). This concerned me but I used hard water with my vinegar/sugar brine and I think it is mineral settlement. When we opened the had it did splash, but every her has done this so far with all my canned foods. It sleeper perfectly fine and the beets were firm and a perfect texture. I was so scared to eat then though because of that white film! This article made me feel sorry if okay with trying them so we decided to give it a go. They tasted fine and smelled fine and I followed a waterbath recipe that 300 other people have used before and reviewed on allrecipes. So hopefully we'll be okay tomorrow?! How fast does botulism take to act? That was a question I had from the article.
janet June 8, 2016
now i am greatly confused. i thought most consumer pressure canners only reached a temperature of 240? and i just read that you need 250 to properly kill the spores? is this correct?
Jennifer S. March 20, 2016
Please proofread and get the facts straight. It's not nice to scare people away from ever canning, let alone via misinformation.

I have canned jams, pickles and applesauce since childhood, safely. But I am a scientist and I take canning safety very seriously. Some common mistakes I've seen include: "I don't like so much sugar - I cut it by half" and "using the sanitary cycle in my dishwasher is as good as boiling my jars". Both of these can get you very sick, and your jam probably won't gel with so little sugar. Another assumption you should be wary of is that all tomatoes are high-acid. Some varieties are low acid, and may need pressure canning.

You might be better off just warning your reader that not following instructions to pressure can is a potentially fatal mistake, and link to an authority on the subject. You could even link to where to buy a pressure canner, as it's a specialty item. Food safety deserves a more accurate presentation, one with checked facts and less panic.
Leslie S. March 20, 2016
Hi Jennifer, I completely agree! This piece isn't meant to scare people, but is instead meant to bring awareness to botulism in general. And while it mentions pressure canning, it definitely doesn't contain information on how to pressure can—just the temperature it should reach—so I highly suggest readers interested in canning refer to the USDA's site to understand how to correctly—and safely—can! Here's the guide:
Winifred R. April 4, 2016
You are obviously a long time canner or professor of food science. I, too , am a scientist (Ph.D. Marine Science) and longtime canner. I agree that checking in with USDA is the way to go for food preservation on safety issues. I do jams and pickles and very limited amounts of other canning because I don't have a pressure canner (tomato sauce or whole tomatoes by USDA recipe). The reason we can do these is that the sugar, salt and vinegar in the recipes is an environment that
Winifred R. April 4, 2016
doesn't allow bacteria to thrive in general.
Steve K. March 18, 2016
There is only one true and authoritative guide to safe canning, recipes and techniques, and that is the National Canning and Home Food Preservation site, NCHFP through the university of KY. It is the acknowledged authority on safe food preservation. Some of the preserving and canning blogs make me cringe form misinformation. Do yourself a favor and check the NCHFP site.
Leslie S. March 20, 2016
I absolutely agree—this piece is meant to provide people with general information on botulism. For more detailed information, readers should absolutely refer to the CDC's site and to the USDA's guide to canning. I've copied both below here for your reference:
Danielle February 22, 2016
Hi Leslie- Cool article in general. In regards to C. botulinum, you have your pH numbers mixed up. C. botulinum does grow in low acid environments. I know this can be confusing, but since acid is on the low end of the pH scale, 0-7, and base is on the high end of the scale, 7-14, low acid means foods that are 4.6 or higher (more basic). Therefore, foods like tomatoes, raspberries, and things pickled in vinegar and are below pH 4.6 (higher acid) inhibit the growth of C. bot spores. Foods like you garlic in oil, rice, fish, spinach, etc do not have enough acid to inhibit growth and need another method of prevention like high pressure cooking to kill the botulism spores. In your article you refer to acid and pH as the same thing, when they are on a inverse scale- the more acid the lower the pH number, the less acid, the higher the pH number. Hope that helps make some sense.
Leslie S. March 21, 2016
Hi Danielle thank you for your concern and explanation! You're correct that I misreported the pH in one sentence, and as a result we took the article down immediately to adjust and correct it to republish it. Thank you again for calling this to our attention since it's, of course, hugely important.
Bryan N. February 22, 2016
Tomatoes are not low acid, which is why most canned tomato recipes call for added acid (lemon juice or citric acid).
Winifred R. April 4, 2016
According to the USDA tomatoes are inconsistent in their acidity. Varieties can be lower acid, and that accounts for the more recent recommendation for added lemon juice. Back in the day (say 30 years ago) they were considered enough by themselves but newer varieties and different growing conditions have meant changes in this. Use the lemon juice for safety. BTW they still taste good.
shebakes February 22, 2016
Someone really needs to proofread before publishing, especially something as important as this. Generally "acidic" refers to anything with a ph below 7. But for purposes of canning and botulism, "low-acid" means anything with a ph above 4.6 - which generally includes all fresh vegetables *except* (most) tomatoes, which are very acidic. Most fruits, including berries, are *not* "low-acid." They have a ph lower than 4.6, as the chart you reference shows. This article is a mess, and you should really pull it and refer people to a competent and complete food safety site instead.
Leslie S. March 21, 2016
Hi shebakes thank you for pointing this out! I absolutely got this confused in the sentence about fruit jams, but it's since been corrected and republished. Thank you again for calling this to our attention!
sigmini February 22, 2016
Does this mean homemade, not pasteurized sauerkraut is generally safe?
702551 March 19, 2016
Are you talking about the stuff that has been made for centuries? The basic category of pickles? Long before the technique of pasteurization was invented?
PG T. February 22, 2016
"The more acidic vegetables in one place, the more likely it is for the bacterium to be able to survive." ------I> Don't you mean "The more LOW acidic vegetables in one place..." This is a bad article/spot for this kind of typo. You are making me nervous.
Leslie S. March 21, 2016
Hi PG Tipsy, the article was immediately taken down after I realized my error but has since been republished with corrections. Thank you for your concerns!
Emily S. February 22, 2016
This article really gets me.