What Every Home Canner Needs to Know…

home canner tomatoes

In Europe of the early modern period, one now common item came to be regarded first with suspicion and then with outright fear. A silent killer, it appeared to afflict only the wealthy classes, leaving its victims writhing in cramping constipation, succumbing to the malaise of intense nausea, and enduring the twin torments of diminished appetite and increased irritability – these latter two in potentially equal parts. The terms ‘deadly,’ ‘poisonous,’ and ‘toxic’ were liberally applied to the cause of the malady and, for over two centuries, it was looked upon with a mixture of fear and dread.

But what was the nature of this slayer? Plague, pestilence, or pandemic? Nothing so dramatic.  The killer was in fact a member of the Solanaceae family, Solanum lycoersicum to be precise. Otherwise known as the humble tomato.

Although part of the nightshade family of plants which includes belladonna, aka ‘deadly nightshade’, the savage reputation of the fruit was unfounded. The reason for its apparent toxicity was less to do with the tomato and more about its conveyance to the table. Contemporary tablewear for wealthy diners was made from pewter, the lead component of which leached out in the presence of the tomatoes’ acidity and resulted in lead poisoning. At the time, of course, the link between pewter and lead poisoning was unknown and thus tomatoes were unjustly demonized as toxic. 

Fortunately for us now the reputation of the ‘love apple’ has been largely restored thanks to its generous antioxidant content (lycopene) and high levels of potassium, folate, and vitamin C. Ubiquitous in most cuisines, in juiced form tomatoes also play well with counterparts such as lemon and lime, vegetables like olives and celery, and the all important vodka with a dash of Worcestershire sauce. In other words, a classic Bloody Mary over a weekend brunch. In spite of their (night)shady past, tomatoes enjoy a seat of honor at our 21st century table.

But in one way their reputation remains slightly tarnished with tomatoes casting a lingering deadly shadow: botulism.

According to literature published by the Department of Health and Social Services in Alaska, foodborne botulism is caused by Clostridium botulinum, a naturally occurring toxin which ‘acts at cholinergic neuromuscular junctions by inhibiting the release of acetylcholine from presynaptic motor neurons.’(1) With an incubation period of 18-36 hours following exposure, the bacterium which exists as either spores or vegetative cells produces toxins, the effects of which are considered irreversible. Botulism as a disease is ‘characterized by neuromuscular paralysis that begins with the cranial nerves and progresses to peripheral and respiratory musculature.’(2) The effected systems of the body fall into three categories: gastrointestinal/urinary, including vomiting, nausea, and diarrhea; neurologic, including dysphagia, unreactive pupils, and blurred vision; and muscular, including generalized fatigue and respiratory muscle paralysis. According to the Centers for Disease Control and Prevention (CDC), those at greatest risk of contracting the disease are intravenous drug users, those who drink a prison hooch known as ‘pruno,’ and home-canners.(3)

Wait? What could drug addicts, prison distillers, and Martha Stewart-style home canning fans possibly share that puts them in contact with the same toxin?

While intravenous drug users risk exposure to the bacteria through puncture wounds, prison drinkers and home-canners alike may unwittingly ingest bacterial spores which, when lodged in the intestines, create toxicity. A diagnosis of botulism is reached once certain other conditions such as Guillain-Barré syndrome, stroke, myasthenia gravis, and opioid overdose are ruled out, and a brain scan, lumbar puncture, and nerve/muscle function tests are used to confirm the diagnosis. The good news is that the timely administration of an antitoxin will prevent further damage; the bad news is that injuries already caused cannot be reversed.

But how does the apparently wholesome pastime of home canning vegetables from the garden or local farmers market put us at risk of an impairing disease?

Just as large scale commercial manufacturers must follow Good Manufacturing Practices (GMPs) and Standard Operating Procedures (SOPs), making safety precautions a priority in the home kitchen is equally important. To wit, the CDC’s evaluation of risk states the following: ‘Home-canned vegetables are the most common cause of botulism outbreaks in the United States. From 1996 to 2014, there were 210 outbreaks of foodborne botulism reported to CDC. Of the 145 outbreaks that were caused by home-prepared foods, 43 outbreaks, or 30%, were from home-canned vegetables. These outbreaks often occurred because home canners did not follow canning instructions, did not use pressure canners, ignored signs of food spoilage, or didn’t know they could get botulism from improperly preserving vegetables.’(4)

And even a slight contact with the toxin – one which is undetectable by smell, sight, or taste – can be lethal.

The foods most likely to present a danger are low-acid (that means a pH of more than 4.6) canning favorites such as corn, beets, asparagus, meats, seafood, and – of course – tomatoes. The bacterium thrives in moist environments at a temperature between 40º and 120º with less than 2% oxygen. So, per the guidelines established by the USDA’s National Institute of Food and Agriculture, ‘all low-acid foods should be sterilized at temperatures of 240° to 250°F, attainable with pressure canners operated at 10 to 15 PSIG. PSIG means pounds per square inch of pressure as measured by gauge. […] At temperatures of 240° to 250°F, the time needed to destroy bacteria in low-acid canned food ranges from 20 to 100 minutes. The exact time depends on the kind of food being canned, the way it is packed into jars, and the size of jars. The time needed to safely process low-acid foods in a boiling-water canner ranges from 7 to 11 hours; the time needed to process acid foods in boiling water varies from 5 to 85 minutes.’(5) Furthermore, the length of the process time is unique to altitude. Why? Because of the difference in the boiling point of water. At sea level, water boils at 212°F but at 10,000ft in elevation the temperature of the boil drops to just 194°F. This means that the aforementioned 7 to 11 hours required at sea level to ensure safe processing rises as the temperature falls. Suddenly the old-time romance of preserving homegrown vegetables plucked from the garden begins to lose its appeal…

So perhaps it’s best to ditch the glamorous notion of home preserving and just pick up some canned goods from the grocery store like everyone else. Not so fast.

If you are looking to err on the side of caution it might be wise to note that even large manufacturers can be plagued by problem pathogens. In 2018, an issue involving the otherwise innocent-sounding Fiesta Corn, Del Monte recalled more than 64,000 cases of the product due to fears of ‘contamination by spoilage organisms or pathogens, which could lead to life-threatening illness if consumed.’(6) According to its website, Del Monte has in place a broad-ranging suite of measures to ensure food safety: ‘Significant efforts and resources go into maintaining food safety programs that include good agricultural and good manufacturing practices (GAPs and GMPs) in our farms, as well as Sanitation Standard Operating Procedures (SSOPs), environmental microbial testing and HACCP programs in our fresh cut plants. Implementation and verification of these programs is done through internal and external audits, continual training of our personnel and certification of our facilities against well recognized food safety standards accepted by customers.’(7) So perhaps home canners shouldn’t feel too guilty…

In the light of issues with corn, peppers, beets, and tomatoes maybe it’s better to stick to a less plant-forward diet? Well, not if the salmon recalls of 2019 are anything to go by. Information from Marler Clark, a law firm specializing in food safety, revealed a spike last year in Clostridium botulinum contamination, this time with seafood as the vector. Smoked Alaska Seafoods, Inc. of Wasilla, AK, withdrew its 6.5 oz. containers of Smoked Silver Salmon due to contamination fears, and AWERS Inc. of Bellevue, WA, recalled its Grained Salmon Caviar that had been distributed to states of the Pacific Northwest, and to New York and Canada. In the latter case, test results from the Canadian Food Inspection Agency (CFIA) showed ‘a lower than normal salt content, which can foster an anaerobic environment which is necessary to breed the Clostridium botulinum bacteria.’(8) Furthermore, on the east coast, Mill Stream Corp. (Sullivan Harbor Farm) of Hancock, ME, voluntarily recalled its Cold Smoked Salmon because ‘the product’s water phase salt (WPS) tested below 3.5%.’(9) According to the article published by Marler Clark, this was significant because the labeling instructions for the product recommend it be ‘kept refrigerated at or below 38ºF and that the product may be frozen.’(10) However, with a WPS  under 3.5%, the refrigerated – and therefore partially thawed – salmon would be an ideal vector of pathogenic contamination.

And there’s one more alimentary vector of the bacterium that deserves the spotlight if only because, perhaps mercifully, it is rarely seen on dinner plates below the 49th parallel: aged whale flipper.

Yes, you did read that correctly – whale fins. According to the State of Alaska Epidemiology Bulletin ‘the Alaska Section of Epidemiology (SOE) received report of three adults presenting to [a hospital] emergency department complaining of nausea, vomiting, dizziness, and double vision. All three patients had eaten at a potluck meal on January 1, 2019.’ The ensuing investigation involved monitoring all of the potluck diners of whom ‘9 (64%) reported at least one symptom compatible with botulism during January 1–11. Four (44%) persons experienced mild symptoms and were managed as outpatients […] Five (56%) experienced symptoms severe enough to warrant closer observation or hospitalization; one of whom subsequently received a diagnosis other than botulism. […] One of these patients was intubated, sustained multiple cardiac arrests, and subsequently died 13 days after symptom onset.’(11) Interviews with the attendees established a list of 17 possible contamination suspects and laboratory test along with patient serum samples ultimately identified the culprit as type E botulism contracted through the ingestion of whale meat.

Did you hear the sound of that door slamming? That was our appetite leaving the building…

So where does this leave hungry diners? Well, if mastering home canning is still on your bucket list, the go-to might actually be fermented foods – think sauerkraut or kimchi.

Fermented foods are safe from contamination by Clostridium botulinum for one simple reason: competitive exclusion. In lacto-fermentation – the kind used to create pickles, kimchi, kefir, kombucha, and sauerkraut – an environment is created that is not conducive to the generation of botulism. As Tim Hall writes in Debunking the Botulism Fear, ‘Fermenting foods creates an environment that is antagonistic to botulism. It’s what scientists call “competitive exclusion.” Beneficial bacteria begin to acidify the food, a condition C. botulinum doesn’t like. Adding salt to a ferment also reduces C. botulinum’s ability to grow, and encourages beneficial bacteria to take over. There are also other “competitive factors” that beneficial bacteria and fungi create in smaller quantities to exclude pathogenic varieties.’(12) The beneficial lactic acid bacteria can number in the hundreds and tend to include strains such as Leuconostoc mesenteroides and Lactobacillus plantarum which are probiotics. In contrast to Clostridium botulinum, these bacteria actually make the GI tract happy and healthy so crafting fermented foods might be an option if home canning is on your radar. As an article available through ScienceDirect notes: ‘Kimchi has antioxidative and antiaging, antimutagenic, anticancer, antiobesity, and other health benefits. Moreover, [it exhibits] antioxidative, anticancer, immune-stimulatory effects, and antiobesity and other probiotic activities. Kimchi is safe from the contamination of NO3, NO2, nitrosamines and biogenic amines, and NaCl content. The health-promoting, probiotic, and functional food properties of kimchi can be optimized or increased by manipulating the types and amounts of ingredients, as well as by using appropriate probiotic starters and kimchi preparation methods.’(13)

‘Optimized or increased’? From a food science perspective that sounds like a call to action. Let the fermentation challenge commence!

Pasta sauce or probiotics – which would you rather see on your (non-pewter) plate? We’d love to know your thoughts!


  1. http://www.epi.alaska.gov/bulletins/docs/b2019_05.pdf
  2. ibid
  3. https://www.cdc.gov/botulism/people-at-risk.html
  4. https://www.cdc.gov/botulism/consumer.html
  5. https://nchfp.uga.edu/publications/usda/GUIDE01_HomeCan_rev0715.pdf
  6. https://www.cnn.com/2018/12/13/health/del-monte-corn-recall-bn/index.html
  7. https://www.freshdelmonte.com/our-company/science/food-safety/
  8. https://www.botulismblog.com/botulism-recall/salmon-with-botulism-risk/
  9. https://www.botulismblog.com/botulism-recall/botulism-risk-in-cold-smoked-salmonella/
  10. ibid
  11. http://www.epi.alaska.gov/bulletins/docs/b2019_05.pdf
  12. https://www.wildfermentationforum.com/viewtopic.php?f=7&t=3654
  13. https://www.sciencedirect.com/topics/food-science/kimchi

1 thoughts on “What Every Home Canner Needs to Know…

  1. Pingback: Perilous Pickles, Renegade Relishes, and the Case of Contamination - Food Contact Surfaces

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