I’ve been doing a fair bit of preserving lately. Jams, relishes, pickles, bottled fruit, and now I’m venturing into the wonderful world of pressure canning. Big girl pants are officially on!
But there’s one word that keeps popping up whenever I research safe preserving methods. Botulism.
It’s a rather frightening-sounding word, isn’t it? And while botulism is incredibly rare, it’s something I think every home preserver should understand. Not because we need to be terrified of our lovingly filled jars sitting on the pantry shelf, but because a little knowledge goes a long way towards keeping our food safe.
So, let’s unpack it.
What Exactly Is Botulism?
Botulism is a rare but potentially life-threatening illness caused by a toxin produced by the bacterium Clostridium botulinum. The bacteria themselves are found naturally in our environment, particularly in soil and dust. That means they can find their way onto the vegetables and fruit we harvest from our gardens. Nothing particularly unusual about that.
The problem arises when the bacteria form spores, which are remarkably hardy little things. Under the right conditions, these spores can germinate, allowing the bacteria to grow and produce a powerful toxin.
And what are those conditions?
- Very little or no oxygen
- Low acidity (a pH above 4.6)
- Sufficient moisture
- A temperature that allows bacterial growth
Sound familiar? A sealed preserving jar can provide just such an environment if its contents haven’t been prepared and processed correctly.
You Can’t See It, Smell It or Taste It
Now, here’s the really important bit (and scary, if you ask me). You cannot reliably see, smell or taste botulinum toxin. A jar might look absolutely perfect. The lid might be beautifully sealed. The contents might look just as appetising as the day you preserved them. But none of that guarantees the food is safe.
Yes, bulging lids, leaking jars, unusual smells and bubbling contents can indicate spoilage. However, botulinum toxin can be present without any of those warning signs. Needless to say, if you suspect a jar has been improperly processed or may be contaminated, don’t taste it. Follow appropriate disposal guidance.
It’s All About Acidity
One of the most important things I’ve learned about preserving is the difference between high-acid and low-acid foods. High-acid foods, such as most fruits, have a natural acidity that prevents Clostridium botulinum from growing and producing toxin. This is why properly prepared high-acid preserves can be safely processed using a boiling-water bath.
Low-acid foods, however, are another story. We’re talking about vegetables such as carrots, green beans and potatoes, along with meat, poultry and seafood. These foods don’t have sufficient natural acidity to prevent the bacteria from growing. And that’s where pressure canning comes in.
Why Doesn’t a Water Bath Do the Job?
A boiling-water bath reaches approximately 100°C at sea level. That’s perfectly suitable for processing many high-acid foods when following tested recipes. Clostridium botulinum spores can survive boiling-water temperatures however, therefore the water bath method is not suitable for low-acid foods.
A pressure canner uses pressure to raise the temperature of steam inside the canner to approximately 116–121°C, depending on the pressure and altitude. These higher temperatures, maintained for the correct processing time, are necessary to safely preserve low-acid foods.
It’s not simply a matter of boiling something for longer. And no, an ordinary pressure cooker isn’t automatically a suitable substitute for a pressure canner.
But Nana Never Used a Pressure Canner!
This is something I’ve found particularly interesting while looking through my old New Zealand cookbooks. Aunt Daisy’s cookbooks and Tui’s Commonsense Cookery books are wonderful reminders of how previous generations preserved their harvests.
Hot filling, the overflow method, turning jars upside down, and sealing preserves while piping hot were all familiar practices. And plenty of people will tell you that their mothers and grandmothers preserved food this way for decades without any problems.
I’m certainly not suggesting otherwise. However, surviving a particular method doesn’t necessarily mean that method is consistently safe. Food science has come a long way, and we now understand considerably more about the conditions that allow harmful bacteria to grow.
For example, turning a jar upside down might help create a seal as the contents cool, but it doesn’t replace the heat processing required for a safely preserved product. Likewise, a lid that has popped down tells us a vacuum has formed. It doesn’t tell us whether the food inside has received sufficient heat treatment.
And there’s another important distinction: some traditional jams, pickles and relishes may be sufficiently acidic to prevent botulism, but that doesn’t automatically make every old recipe or hot-fill method suitable for shelf storage. The ingredients, proportions and processing method all matter.
What About Vinegar and Lemon Juice?
Both can play an important role in safe preserving.
Vinegar is commonly used to acidify pickles and relishes, while lemon juice or citric acid may be required in certain fruit and tomato recipes. However, adding a splash of vinegar or a squeeze of lemon doesn’t automatically make a low-acid food safe for water-bath processing. The final acidity of the entire product matters, not simply whether something tastes sharp or tangy.
That’s why tested recipes specify particular ingredients, quantities and processing instructions. Changing the ratio of vegetables to vinegar, adding extra ingredients or thickening a recipe can alter how safely it preserves.
A little creativity in the kitchen is wonderful. When it comes to shelf-stable preserving, though, some things really are best left unchanged.
Is Botulism Common?
Thankfully, no. Foodborne botulism is extremely rare, particularly when compared with many other foodborne illnesses. But rare doesn’t mean harmless.
Botulinum toxin attacks the nervous system and can cause blurred or double vision, drooping eyelids, difficulty speaking or swallowing, muscle weakness and, in severe cases, paralysis affecting breathing. It is a medical emergency. If botulism is suspected, seek urgent medical attention. Don’t wait for symptoms to improve.
So, Should We Be Worried About Home Preserving?
I don’t think we should be frightened of preserving. Quite the opposite. It’s a fabulous way to make the most of seasonal abundance, reduce food waste and fill our pantry shelves with delicious homemade goodies.
But I do think we should respect the science behind it. For me, that means following tested recipes from reliable sources, using the correct equipment, paying attention to processing times and altitude adjustments, and resisting the temptation to improvise when safety is involved.
It also means understanding that different foods require different preserving methods. A jar of strawberry jam isn’t the same as a jar of green beans. And a beautifully sealed jar isn’t necessarily a safely preserved jar.
One of the things I love about modern homesteading is bringing old-fashioned skills into today’s world. I’m a great believer in doing what you can with what you have, and learning from the generations who came before us. But that doesn’t mean we can’t improve on traditional methods when new knowledge becomes available. Nana had her preserving wisdom, and we have the benefit of modern food science. I reckon that’s a pretty good combination.
So, whether you’re bottling peaches, making your favourite relish or venturing into pressure canning for the first time, take the time to understand the process.
After all, the whole point of preserving our harvest is to enjoy it later (not to preserve bacteria and make ourselves ill).
Happy preserving!
