This is NOT canning! - 5 preserving methods you should not rely on
Spend enough time reading about home preserving and you'll eventually come across something like this:
"My grandmother always filled the jars while they were boiling hot, turned them upside down and kept them in the pantry. Nobody ever got sick."
Or perhaps:
"I've canned tomatoes in the oven for 30 years."
And one of my favourites:
"The lid sealed, so obviously it worked."
These methods aren't unusual. Some have been used for generations, and depending on where you live, you may even have grown up watching your parents or grandparents preserve food this way.
But there's an important distinction between getting a jar to seal and safely processing food for shelf-stable storage.
When I talk about safe home canning on this blog, I mean processing food using an appropriate, tested method: generally boiling-water canning for suitable high-acid foods and pressure canning for low-acid foods.
So let's look at some methods that might produce something resembling a jar of canned food — but aren't substitutes for proper canning.
1. Turning jars upside down
Let's start with one that is incredibly common, particularly here in Europe.
You make jam, tomato sauce or another hot product. You pour it boiling hot into a clean jar, screw on the lid and turn the jar upside down.
After a while you turn it back over.
The lid has pulled down.
Success?
Not quite.
The cooling contents can create a vacuum inside the jar, which may cause the lid to seal.
But here's the important part:
A sealed jar and a safely processed jar are not the same thing.
Turning the jar upside down doesn't provide the controlled processing time and temperature required by a tested canning recipe.
There is another problem too. Food can come into contact with the sealing surface of the lid while the jar is inverted, potentially interfering with the quality of the seal.
If you're following a tested water-bath recipe, there's simply no reason to turn the jars upside down.
Fill them correctly, apply the lids and process them upright for the specified time.
2. Open-kettle canning
Open-kettle canning sounds more sophisticated than it really is.
The basic idea is simple.
Food is cooked thoroughly in a pot and, while still extremely hot, poured into sterilized jars. The lids are applied immediately and the heat from the food causes the jars to seal as they cool.
Again, you'll often get a beautiful vacuum seal.
But the filled jars haven't actually been processed.
Cooking the food beforehand doesn't replace processing the food inside the sealed jar.
During proper canning, the entire jar and its contents are subjected to a specified process for a specified amount of time.
With open-kettle canning, microorganisms can potentially be introduced while transferring the food into the jars. The jars then receive no subsequent processing step.
For something like jam, the difference between the two methods isn't particularly difficult anyway.
Instead of filling the jars and stopping there, put those filled jars into your water-bath canner and process them according to the tested recipe you're following.
It's one extra step.
3. Oven canning
Oven canning appears in old preserving books and still circulates online.
There are a few variations.
Sometimes filled jars are placed in the oven and heated.
Sometimes people put jars containing dry foods such as flour, rice or beans into the oven and call this "dry canning."
Neither should be confused with tested home-canning methods.
An oven uses dry heat, while established home-canning processes rely on heat transfer from boiling water, steam under pressure or other specifically tested conditions.
The temperature displayed on your oven isn't the same thing as the temperature being achieved throughout the food inside a jar.
And there's another issue.
Standard home-canning jars are designed for home-canning processes, not necessarily prolonged exposure to dry oven heat. Heating them this way can increase the risk of thermal stress and breakage.
If you're preserving a high-acid food, use an appropriate boiling-water or atmospheric steam-canning process where recommended.
If you're preserving low-acid food, use a pressure canner.
Your oven is excellent for bread.
Leave the Mason jars out of it.
Think about heat transfer for a moment
Think about the difference between opening a hot oven and putting your hand near the heat inside it, compared with accidentally touching boiling water.
Your oven might be sitting at 180°C, yet you can open the door and briefly reach into that hot air to remove a baking tray.
You certainly couldn't put your hand into a pot of boiling water at just 100°C.
Why?
Because air and water transfer heat very differently.
Hot air is a relatively poor conductor of heat. Water transfers heat to an object much more efficiently, and steam can transfer enormous amounts of energy when it condenses.
That's one reason you can't look at an oven set to 120°C or 150°C and think, "That's hotter than boiling water, so surely it will work for canning."
The temperature displayed on the oven isn't the whole story.
What matters in canning is how heat moves through the canner, through the glass jar and ultimately through the food all the way to the coldest part of that food.
Tested water-bath and pressure-canning processes account for that heat transfer.
Putting a jar in a hot oven does not reproduce the same process.
4. Using a pressure cooker instead of a pressure canner
This one causes a lot of confusion because the names sound almost interchangeable.
Pressure cooker.
Pressure canner.
Both operate under pressure. Both use steam. Both can reach temperatures above the normal boiling point of water.
But they aren't automatically interchangeable.
A tested pressure-canning process doesn't simply say:
"Get this food really hot."
The process depends on a combination of factors including pressure, temperature, processing time, jar size, food density and the heating and cooling characteristics of the canner itself.
Traditional pressure canners are designed to hold multiple canning jars and maintain the conditions required for established pressure-canning processes.
Small pressure cookers heat and cool differently.
That matters because the heating and cooling periods form part of the overall thermal process the food experiences.
So if your recipe says pressure canner, don't assume your ordinary pressure cooker can take its place.
5. Assuming a multicooker can pressure can because it has a pressure setting
Then we arrive at the modern version of the same problem.
Electric multicookers are fantastic appliances.
They can pressure cook, slow cook, steam, sauté and sometimes do another seventeen things that apparently required separate appliances when I was growing up.
But a pressure-cooking function does not automatically make an appliance suitable for pressure canning.
This includes assuming that because an appliance reaches a particular pressure or temperature, you can simply use pressure-canning times intended for a traditional pressure canner.
You can't safely make that assumption.
Some appliances even have buttons labelled "canning", which understandably makes the situation even more confusing.
For low-acid foods, use equipment for which reliable pressure-canning guidance exists and follow the manufacturer's instructions alongside an appropriate tested recipe.
A button on the front of an appliance isn't a substitute for a validated process.
"But the lid sealed!"
This deserves its own section because it's probably the biggest misconception in home canning.
A seal is not proof that food was safely processed.
You can demonstrate this yourself without canning anything.
Put very hot water into a jar, apply a lid and allow it to cool.
There's a good chance you'll create a vacuum.
The lid may pull down.
It may even make that wonderfully satisfying PING!
That doesn't mean you've canned the water.
It means the cooling contents created lower pressure inside the jar and pulled the lid against the rim.
When we check the seals on properly canned jars, we're checking whether the final part of the process worked.
We're not using the seal to determine whether the entire process was safe.
That's an important difference.
"But we've always done it this way"
I understand this argument.
Food preservation existed long before laboratories started publishing home-canning recommendations.
Families have passed preserving methods from one generation to another, and many of those traditions are wonderful.
But "nobody I know became sick" isn't the same thing as demonstrating that a preservation method reliably controls the relevant hazards.
Food safety is largely about managing risks that you cannot necessarily see.
Clostridium botulinum is the classic example in home canning because under the right conditions its spores can germinate and produce botulinum toxin.
You can't reliably determine that a jar is safe simply by looking at it, smelling it or checking whether the lid is sealed.
That's why modern canning recommendations are based on controlled processes rather than whether a jar looks normal when we open it six months later.
Safe canning isn't about making everything sterile
It's also worth clearing up another misconception.
Home canning isn't about somehow making every jar absolutely sterile.
It's about using the right combination of acidity, temperature and processing time to control the microorganisms that could make the food unsafe or cause it to spoil.
That's why acidity matters so much.
High-acid foods can generally be processed safely at boiling-water temperatures when an appropriate tested recipe is followed.
Low-acid foods require the higher temperatures achieved in a pressure canner.
You can't compensate for using the wrong method simply by processing the jars "a bit longer."
Canning is a process, not a sealed jar
This is probably the easiest way to remember all of this.
Canning isn't the act of putting food into a jar and getting the lid to seal.
Canning is the entire process.
Preparing the food correctly.
Using the correct jar size.
Leaving the specified headspace.
Using the appropriate canning method.
Making altitude adjustments when necessary.
Reaching the required temperature or pressure.
Processing for the specified amount of time.
Cooling the jars correctly.
And finally, checking that the jars have sealed.
That last little ping is satisfying.
But it's only one part of the process.
So if someone hands you a jar of tomato sauce and proudly tells you:
"Don't worry — I turned it upside down and it sealed perfectly."
You now know the question that actually matters.
How was it processed?
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