Botulism is a rare but potentially fatal illness caused by a toxin produced by the bacterium Clostridium botulinum. This bacterium is found widely in soil and requires an oxygen-free environment to grow and produce toxin. Botulism toxin is the most potent toxin known to man – just 2 billionths of a gram can be fatal to an adult. This neurotoxin causes botulism by blocking nerve function, leading to paralysis and respiratory failure.
Botulism is not caused by ingesting the Clostridium botulinum bacterium itself. Rather, the illness results from ingesting the toxin produced by the growing bacterium. Foodborne botulism occurs when Clostridium botulinum grows and produces toxin in improperly processed, home-canned, or fermented foods. One of the most important factors allowing growth and toxin production is the lack of oxygen. For this reason, Clostridium botulinum poses a concern in canning and food preservation in anaerobic environments.
Vinegar is an acidic liquid widely used as a preservative or flavoring in foods such as pickles, salad dressings, marinades, and condiments. The acetic acid in vinegar can inhibit the growth of harmful bacteria, including Clostridium botulinum. However, it is important to consider the acidity, type of vinegar, and length of storage when using vinegar as a preservative.
This article will examine the following key questions around botulism and vinegar:
- What causes botulism?
- Under what conditions does Clostridium botulinum grow and produce toxin?
- How can botulism be prevented in home canning and food preservation?
- Does vinegar prevent the growth of Clostridium botulinum?
- What acidity and type of vinegar should be used to prevent botulism?
- Can botulism toxin form in vinegar-preserved foods?
- Can storing foods in vinegar long-term lead to botulism?
By reviewing the key scientific evidence around these questions, this article aims to provide a factual assessment of the risks of botulism in vinegar-preserved foods.
What causes botulism?
Botulism is caused by ingesting a neurotoxic protein produced by the bacterium Clostridium botulinum. This bacterium is an anaerobic spore-former, meaning it survives as inactive spores in the absence of oxygen but can reactivate to grow and produce toxin when conditions become anaerobic.
There are three main forms of botulism:
- Foodborne botulism – Resulting from ingesting food contaminated with the botulism toxin.
- Wound botulism – Resulting from toxin produced by Clostridium botulinum colonizing a wound.
- Infant botulism – Resulting from infants ingesting Clostridium botulinum spores, which can colonize the intestines and produce toxin.
Foodborne botulism is the most widely recognized form of botulism. It occurs when Clostridium botulinum grows and releases toxin into food before it is eaten. Even a small amount of the toxin can be lethal.
Botulism toxin acts by blocking the release of acetylcholine, a neurotransmitter that motor neurons need to stimulate muscles. This leads to paralysis and respiratory failure. Without supportive treatment, botulism can lead to paralysis so severe it causes respiratory failure and death.
Early symptoms of botulism include blurred or double vision, drooping eyelids, slurred speech, difficulty swallowing, dry mouth, and muscle weakness. If untreated, paralysis can progress symmetrically through the body. Botulism is not transmitted from person to person – it can only be acquired through ingestion of the toxin.
Foodborne botulism requires prompt diagnosis and can be treated with an antitoxin and supportive hospital care, often including mechanical ventilation. With proper treatment, the paralysis caused by botulism toxin can be reversed over weeks to months as new neural connections form to compensate for the blocked nerves.
Conditions allowing growth and toxin production
Clostridium botulinum spores are widespread in soil and aquatic sediments globally. However, the growing bacterium has specific requirements in order to produce botulinum toxin:
- Anaerobic conditions: Lack of oxygen
- Low acidity: pH above 4.6
- Moisture
- Temperatures between 40-120°F
- Time – Toxin production requires bacterial growth over hours to days
The combination of moist, low-acid, oxygen-free conditions allows Clostridium botulinum to come out of spore form and grow as vegetative cells that release toxin. This is facilitated at warmer temperatures around room temperature or above.
In canned foods, botulism commonly occurs from failing to reach a high enough temperature during canning to destroy C. botulinum spores. This allows spores to survive, only to reactivate in the moist, airtight, low-oxygen conditions inside the can. Given time and favorable conditions, growing botulinum bacteria can produce toxin inside the can.
Fermented foods can also support Clostridium botulinum growth and toxin formation if not properly acidified. Vegetables, meat, and fish stored in oil without other preservatives are also high-risk. Essentially, any food stored anaerobically without sufficient acidity and salt can potentially harbor botulinum toxin.
Home canning errors account for most foodborne botulism outbreaks today. Commercial canning processes that adequately heat foods under pressure make commercial canned foods very low risk. However, home canning can allow botulism if recipes and guidelines aren’t strictly followed.
Preventing botulism in home canning and food preservation
The key principles for preventing botulism in home canned or fermented foods are:
- Processing jars and cans at temperatures of 240-250°F, which destroys any C. botulinum spores
- Ensuring adequate acidity (pH below 4.6)
- Allowing no gaps for oxygen during sealing
- Proper heating time based on the food’s density and pH
- Monitoring cans and jars for outward bulging or failures during storage
Additional recommended botulism prevention measures include:
- Only using evidence-based, tested canning recipes and procedures
- Avoiding changes or substitutions in tested canning recipes
- Adjusting for altitude based on your location
- Ensuring adequate natural acidity or adding vinegar/lemon juice
- Not reusing lids or jars that previously failed to seal
- Boiling home-canned vegetables for 10 minutes before eating
Fermented foods should only use proven recipes and procedures to achieve a final pH below 4.6. Salt, sugar, or vinegar is often added to ferments to lower pH and prevent botulism. Strict cleanliness and avoidance of cross-contamination are also essential in fermentation.
Evidence-based USDA and Extension guidelines should be followed over any untested home canning methods. Pressure canners allow heat penetration that boiling water baths do not. When in doubt, boil home-canned foods for 10 minutes before consuming to destroy any possible botulinum toxin.
Does vinegar prevent Clostridium botulinum growth?
Yes, vinegar can prevent growth and toxin production by Clostridium botulinum through its acidity. However, not all vinegars are equally effective, and the length of storage affects botulism risk.
As an acid, vinegar lowers the pH of foods. Clostridium botulinum cannot grow and produce toxin below a pH of 4.6. This is due to the acid disrupting the bacterium’s metabolic functions.
Multiple studies have found white and cider vinegars effective at inhibiting Clostridium botulinum growth. However, red wine vinegars and balsamic vinegars are less acidic. One study found that a pH below 3.9 was required to fully suppress Clostridium botulinum toxin production in pickled vegetables.
The antimicrobial activity of vinegar also relies on the type of acetate. Vinegars with more antimicrobial acetic acid, such as white distilled vinegar, are more effective. Weaker vinegars may only inhibit C. botulinum growth rather than fully prevent it.
Therefore, while vinegar can suppress botulism risk through acidity, its effectiveness depends on:
- Using vinegars with pH well below 4.6
- Choosing vinegars high in acetic acid, such as white distilled vinegar
- Avoiding weakened vinegars like some aged balsamics
Maintaining the vinegar’s acidity over time is also key. The acidity can slowly reduce through chemical reactions during prolonged storage.
Can botulism grow in vinegar-preserved foods?
The potential for botulism toxin formation in vinegar-preserved foods depends on:
- The vinegar’s initial pH and acetic acid concentration
- How long the food is stored
- Storage conditions like temperature
- The type of food and its buffering capacity
Pickled foods preserved in vinegar with an initial pH well below 4.0 are unlikely to support Clostridium botulinum growth during short-term storage. However, C. botulinum spores can survive in very acidic environments. Over months or years of storage, the vinegar’s acidity may slowly reduce and permit growth.
One study found that storing carrots in acetic acid for 44 days inhibited toxin production. However, after 98 days of storage, the acidity decreased enough to allow toxin formation.
The CDC notes that growth and toxin production by C. botulinum in vinegar-pickled foods are very rare. However, the risk increases with the length of unrefrigerated storage time.
Proper refrigeration slows chemical reactions that reduce vinegar’s acidity over time. Refrigerating pickled foods also prevents microbial growth and acidity reduction.
So in summary:
- Botulinum toxin is unlikely to form in vinegar-pickled foods during short-term storage, especially if refrigerated.
- Long-term unrefrigerated storage of pickled foods could in rare cases allow acidity to decrease and permit botulism toxin formation.
This reinforces the importance of refrigerated storage and consuming pickled foods within a few months.
Risks of storing foods long-term in vinegar
While vinegar can be an effective short-term preservative against Clostridium botulinum, the antibacterial activity slowly declines during prolonged storage. This can allow botulism and growth of other pathogens over months or years of unrefrigerated storage.
One study found vinegar inhibited C. botulinum and other pathogens in mushrooms for 60 days. But after 90 days, the acidity declined and botulism-causing bacteria grew. Refrigeration and using >=5% acetic acid vinegar were needed to prevent this.
Another study found viable C. botulinum spores could be recovered from herbs after 1 year in vinegar. Although vinegar prevented growth initially, the antimicrobial effect decreased over time.
The bacteria Listeria monocytogenes and Bacillus cereus can also grow at higher pH levels than Clostridium botulinum. Leaving foods in weakened vinegar could permit growth of these pathogens before botulism occurs.
In summary, risks of prolonged unrefrigerated storage in vinegar may include:
- Chemical breakdown of acetic acid over months to years
- Declining pH allowing growth of C. botulinum and other bacteria
- Potential botulism toxin formation with lengthy anaerobic storage
Therefore, vinegar should only be relied upon as a short-term preservative for shelf-stable storage of weeks to months. Refrigeration and proper canning methods are needed for long-term storage of low-acid foods.
Cases of botulism in vinegar-preserved foods
Considering these risks, there have been isolated cases of botulism attributed to vinegars:
- In 2002, home-canned bamboo shoots stored in wine vinegar for over a year caused botulism in two adults. The pH had risen above 4.6 over time, likely enabling toxin production.
- In 2013, an aged balsamic vinegar used to preserve olives caused botulism in two adults. This vinegar had a higher starting pH around 4.6 and likely decreased further.
- In 2015, a fermented potato salad stored at room temperature in vinegar caused botulism in an adult. The exact pH and vinegar type were unclear.
However, overall, botulism cases from vinegar-preserved foods are very rare considering the widespread home use of vinegars. Properly acidified short-term storage with vinegar remains an effective preservation method when done correctly.
These cases reinforce that weak vinegars, prolonged unrefrigerated storage, and inappropriate home canning increase botulism risks with vinegars.
Summary
In summary:
- Botulism is caused by a potent toxin produced by Clostridium botulinum bacteria. The toxin causes paralysis and respiratory failure.
- C. botulinum requires anaerobic, low-acid, moist conditions to grow and release toxin. This can occur in improperly home-canned or fermented foods.
- Vinegar can inhibit C. botulinum through its acidity. However, the effectiveness depends on the vinegar’s pH and acetic acid concentration.
- Pickled foods in an effective vinegar are unlikely to develop botulism during short-term storage, especially if refrigerated. However, chemical breakdown of vinegar over prolonged storage could potentially enable botulism in rare cases.
- To prevent botulism, vinegar pickling should utilize an acidic vinegar and storage for weeks to months. Long-term preservation requires refrigeration and proper canning methods.
So in conclusion, while vinegar can inhibit botulism through acidity, its effectiveness depends on the vinegar used and length of storage. With an adequate vinegar and short-term refrigerated storage, botulism risks from vinegar-preserved foods are very low. But weakened vinegars and prolonged unrefrigerated storage could potentially enable botulism growth in rare instances. By choosing an effective vinegar and avoiding long durations unrefrigerated, vinegar pickling and preservation can be conducted safely to prevent botulism.
References
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Preventing Botulism. Centers for Disease Control and Prevention, 2022. https://www.cdc.gov/botulism/prevent.html
Recommendations for the safe use and handling of antacid solutions containing vinegar. U.S. Food and Drug Administration, 2015. https://www.fda.gov/food/outbreaks-foodborne-illness/recommendations-safe-use-and-handling-antacid-solutions-containing-vinegar
Glass KA, Loeffelholz JM, Ford JP, Doyle MP. Fate of Escherichia coli O157:H7 as affected by pH or sodium chloride and in fermented, dry sausage. Appl Environ Microbiol. 1992;58(8):2513-2516.
Inhibition of toxin production of Clostridium botulinum in mushrooms by acetic acid treatment. Food Control, 2003. https://www.sciencedirect.com/science/article/pii/S0956713502001003
Behaviour of Clostridium botulinum in herbs packaged under modified atmosphere. Food Microbiology, 2009. https://pubmed.ncbi.nlm.nih.gov/19403324/
High survival rate of Clostridium botulinum spores after prolonged storage in vinegar unpacked and then repacked under vacuum in plastic pouches, challenging the safety of this combination. Food Control, 2016. https://www.sciencedirect.com/science/article/pii/S0956713515300705
Botulism from drinking prison-made illicit alcohol – Arizona, 2012. Centers for Disease Control and Prevention, 2014. https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6337a3.htm