Home Canning Q&A - NFReads.com

This article was reviewed by the following canning experts gratis:
– Tracy Falbe, author of Home Canning Guide for Everyone Who Eats
– Jennifer Gomes, author of Steam Canning for Beginners: Get Started Using the Best, FASTEST Way to Can!

How to know if canning jars are sealed
Should all jars pop when opened?
How to remove air bubbles when canning
What to do if canning jars don’t pop/seal
How long to process jam
How to measure jar gaskets
Cold pack vs hot pack canning
Best tomatoes for canning juice
How long do canned beans last once opened?
How long does canned jam last?
How long does canned salsa last?
How long does canned chili, corn or green beans last in the fridge?
How long do homemade pickles last unopened?
How long do opened pickles last?
How long does canned venison last?
How to store mason jar lids
How to wash mason jar lids
Can canning lids be reused?
How to make mason jar lids one piece
Why are mason jar lids two pieces?
Can you put mason jars in the oven?
Can you microwave mason jars?
How much liquid does a mason jar hold?
Wide mouth vs regular mouth mason jars
How to label mason jars
How to open a canning jar
How to store empty canning jars
What is the difference between canning salt and table salt
Pickling lime vs pickling salt
What is ball pickle crisp?
Can you use sea salt for pickling?
How to thicken salsa before canning
How to thicken bean soup naturally
How much water do you put in a pressure canner?
Pressure cooker vs pressure canner
How to use a pressure canner with a weighted gauge
How to test a pressure gauge
Can you double stack jars in a pressure canner?
Pressure canning vs water bath
How to make calcium water
Pickling vinegar vs white vinegar
Can you can Pico de Gallo?
Can you pickle straight eight cucumbers?
Can you pickle burpless cucumbers?
Can you eat pickling cucumbers raw?
Can you peel cucumbers before pickling?
Do bay leaves have tannins?
What does alum do for pickles?
Best cucumbers for making pickles
What is a jelly bag?
How to can green beans without a pressure cooker
Can you can yellow tomatoes?
Clear Jel vs Sure Jell
Can you can on a glass top stove?

How to know if canning jars are sealed

Make sure it’s been at room temperature for a day, then take the screw bands off and check that the middle of the lid is slightly curved downwards by looking at it from the side at eye level and/or by running your finger over it. It should feel firm with no give if you press down on it, and tapping on it with a spoon should give a high-pitched sound. Finally you can also lift the jar by the lid (without the band of course) and check that it doesn’t fall off, which it shouldn’t if it’s sealed properly.

Should all jars pop when opened?

It’s possible that the popping sound could be dull or barely audible depending on a number of things (like whether your finger was on the middle of the lid when opening it). That’s why it’s best to check before opening one using the method in the previous answer above, but if you’ve already opened a jar without doing so and don’t think you heard a pop it’s best to play it safe and throw it out.

What to do if canning jars don’t pop/seal

If it’s simply the lack of a popping sound you’re concerned about, as mentioned in the previous answer above (though that was about opening, but it still applies to cooling) it’s not as reliable an indicator of a proper seal as manually checking it with your eyes and fingers. So what if your canning jars don’t seal? Check for the following causes:

Used a jar not designed for canning
Lid wasn’t screwed on tightly or it was screwed on slanted
There was food debris on or around the rim
Insufficient/inconsistent canning pressure, temperature and/or duration
You secured the ring too tightly – you only need to use the strength of your fingers, not wrestle it with your entire arm or body – or the ring itself is damaged
The jar was under or overfilled – the minimum/maximum depends on the recipe, and invisible pockets of air inside ingredients can end up leaving too much empty space inside in total
If a pressure canner was used, cooling it rapidly can break the seal
The jar was cracked/broken, particularly around the rim
Damaged or faulty lid (or its sealing gasket)

How to remove air bubbles when canning

Use any non-metallic utensil (plastic, wood, silicone etc. otherwise you could damage the jar – you might not be able to see the damage but it could worsen once it’s heated) to thoroughly push and prod the food downwards. Keep in mind there could be air bubbles in the center that you can’t see. And remember that this isn’t just for appearances – too much air can prevent a seal from forming properly.

How long to process jam

After initially reaching a high boil then lowering to a light boil:

15 minutes if the jars aren’t sterilized
5 minutes if sterilized jars are used
1 additional minute for every 1000 feet (300 meters) above sea level

How to measure jar gaskets

This assumes you’re referring to a “Weck jar” (note that this canning system isn’t USDA approved and could be unsafe) or any non-canning jar with a gasket: the diameter of the inner circle of the gasket (the width of the round empty space) should be equal to or a bit smaller than the outer diameter of jar rim (including the thickness of the glass) above the ledge. The diameter of the entire gasket (its widest measurement) should be equal to or a bit larger than the diameter of the jar ledge.

Cold pack vs hot pack canning

Cold pack canning uses raw ingredients while hot pack canning involves cooking the food first then adding it to the jar, which helps reduce air pockets in and between ingredients. Any liquid added to either still needs to be boiled.

Best tomatoes for canning juice

The best tomatoes for juice seem to be darker-fleshed ones (though not exclusively) – people have had positive experiences with Amish Paste, Pantano Romanesco (less viscous), Rutgers (highly regarded as a juice tomato), Brandywine, Kelloggs Breakfast (sweet), Cherokee Purple (also popular for juicing) and Marglobe Supreme (acidic).

How long do canned beans last once opened?

Some sources recommend up to 3-4 days in the fridge, but this can vary depending on (but not limited to):

Whether you have contaminated it with anything (such as a used spoon)
The internal temperature of your fridge (which itself can vary depending on things like the weather, size of your fridge, how old it is,how much is inside it, etc.)
Whether the beans also contain preservatives and how much of them (including natural ones like salt and vinegar)
Whether you added any preservatives to it after opening (like salt and vinegar)
Whether you leave the can open or put the contents in a sealed container
To be extra safe it’s best to just freeze any leftover beans – any food left in the fridge for more than a day starts losing a lot of flavor anyway (at the very least).

How long does canned jam last?

The National Center for Home Food Preservation suggests up to a year for properly stored unopened jam (and probably also for similar foods like jelly and apple butter), and after which taste, texture and nutrition will begin to deteriorate (this will vary depending on ingredients and storage conditions) though it may be continue to be safe to consume long past that point – as long as the seal isn’t broken (assuming it was canned properly in the first place of course) unopened preserves generally don’t go bad.
Once opened they should be safe for about a month in the fridge. Whether opened or not however, the aforementioned numbers can vary depending on things like what ingredients and how much sugar is used, how often and hygienically you consume it, etc.

How long does canned salsa last?

Same as the previous answer above for general info, but unopened storage recommendations for best flavor increases to 1.5 years while post-opened numbers actually go down as it keeps for less than a week at best even when refrigerated.

How long does canned chili, corn or green beans last in the fridge?

3-4 days once opened, though it can vary depending on the factors listed here.

How long do homemade pickles last unopened?

Technically it’s possible for them to not go bad for years or more in terms of microbial spoilage if the seal stays intact (applies to pickle relish as well) – commercial pickles can last a year and possibly more past their printed expiration date and there is as least one anecdote about opening and eating one over a decade old, though that’s not necessarily recommended! After a year or so though they will begin to lose their crunchiness and may eventually turn to mush (some ingredients faster than others).

How long do opened pickles last?

As pickling is a method of preservation in itself, it has a much longer edible time span after opening (and kept in the fridge) – up to the better part of a year depending on the strength of the brine/vinegar solution. Note that all the pickles must be completely below the top of the liquid and anything above that will spoil much quicker as it’s no longer protected from microbes in the air.

How long does canned venison last?

For properly canned meat, the UN says they are safe for up to 4 years (which goes down to 1 year for hotter climates). And there is at least one blogger who has claimed to have tried canned elk that was 4 years old (and that “It was excellent” – https://thecamogame.wordpress.com/2007/10/29/how-long-is-canned-venison-good/) – though the author prefaces it with a warning suggesting that the same results are not guaranteed for you, followed by the usual warnings about making sure it was canned properly in the first place and to watch out for rust as that can be a cause of contamination (especially as more time passes).

How to store mason jar lids

Below are storage ideas/tips for mason jar lids (make sure everything’s dry first!):

Plastic bags sealed with clamps or ziplocks
Boxes or containers – airtight ones may help prevent rust, and compartmentalized ones help with organization and sorting
Flipping them over and placing them on the top of jars with the ring placed around it loosely so that it’s all together
Rings can be strung on clothes hangers, strings/ribbons/cables, paper towel holders, broom handles (on separate ones sorted by size if you like)

How to wash mason jar lids

This depends on the brand, but generally manufacturers recommend you wash them gently with soap and water using a soft sponge – anything harsh can damage seals and coatings (and dishwashers are too harsh as well). That may be enough to get possible dust and debris out of new lids, but for getting smells out of used lids many people swear by the following (used for anywhere from a few hours to days):

Ground coffee
Mustard powder
Vinegar, baking soda, salt or a combination of these
Hydrogen peroxide
Sunlight

Can canning lids be reused?

The USDA recommends against reusing mason jar lids as it makes it more likely for them to rust or for the seal to fail, unless it’s one that’s specifically designed to be reused. Lots of people claim they reuse single-use lids without any problem but some do admit that used lids do have a higher failure rate. Even for the ones that appear to be fine at first, the seals may not hold up over time.

How to make mason jar lids one piece

The intended purpose is important here: if it’s for canning, then it’s a very bad idea, even to buy one-piece jar lids in the first place as they require precise commercial-level canning practices. If it’s because you just want to reuse them for non-canning purposes (e.g. to store grains, salt, marbles etc.) then it’s just a simple matter of gluing them where they overlap, ideally using food-safe adhesives if you’re going to store food and giving it plenty of time to dry.

Why are mason jar lids two pieces?

Because air needs to vent out during the canning process so that a vacuum forms as it cools and the contents shrink, and unless you use commercial-level canning systems (with precise control of temperature, timing, pressure, etc.) you need the gap between the two pieces to ensure this happens properly. As a bonus it also reduces costs as you can reuse the outer ring and only need to repurchase the middle of the lid rather than an entire lid.

How to store empty canning jars

If you don’t have that many then you can just place them on any flat area like a shelf or cupboard, but I suspect you wouldn’t be reading this if that were the case! It can be tempting to just throw them all in a giant bag, but that’s a quick way to end up with chips, cracks or even worse dangerous broken shards of glass. The best solution is to stack them in a sturdy box (plastic crate, wooden or thick cardboard box, etc.) with:

Each layer of jars covered by a piece of cardboard (or several sheets of paper/newspaper)
Each jar separated from the ones next to it with either cardboard dividers or by individually wrapping them in layers of newspaper/paper
If the box you’re using has holes in it (e.g. a crate) and you want to reduce any dust that gets in to make it easier to clean before using, you can line the inside of the box with a plastic bag first then tie or clamp the bag after it’s filled.
See also: lid storage tips

Can you put mason jars in the oven?

This is unsafe both mechanically and biologically:

Whether empty or filled, they’re not made to withstand oven temperatures and can crack into sharp pieces (if you’re lucky) or burst apart and send those sharp pieces flying everywhere.
Assuming you’re really lucky and the glass stays intact, if you were using it for canning then your luck will run out when you open it up to eat it. While the oven itself can reach temperatures even higher than botulism-spore-killing pressure canner temperatures, without the steam the heat is conducted into the jar at a slower rate so the middle of the jar won’t reach a sufficient temperature unless you increase the canning time (which isn’t something you just estimate) – BOOM, botulism. If you do bake it for long enough that the middle of the jar gets as hot as it can get for a sufficient amount of time, you’re still not out of the woods yet. Without the pressure of a pressure canner the inside of the jar will only ever reach the boiling point of water (212F/100C degrees) which isn’t enough to kill botulism spores, so unless the food is sufficiently acidic – BOOM, botulism.

Can you microwave mason jars?

Generally yes, as long as:

the lid is off
it has a microwave safe symbol
it isn’t damaged/cracked or too old
it doesn’t contain anything frozen

How much liquid does a mason jar hold?

Canning jars generally top off at half a gallon, and US versions come in the following volumes (equivalent amounts in different units of measurement on each line):

8, 16, 32 and 64 ounces, or
1, 2, 4 and 8 cups, or
0.5, 1, 2 and 4 pints, or
0.25, 0.5, 1 and 2 quarts, or
0.0625, 0.125, 0.25 and 0.5 gallons, or
236.58, 473.176, 946.35 and 1892.71 milliliters (rounded to 2 decimals)
For international/metric jars (again, equivalent amounts in different units of measurement on each line):
0.125, 0.25, 0.5 and 1 liters (multiply by 1000 for milliliters), or
4.23, 8.45, 16.91 and 33.814 ounces (rounded to 2 decimals), or
0.53, 1.06, 2.11 and 4.23 cups (rounded to 2 decimals), or
0.264, 0.528, 1.057 and 2.113 pints (rounded to 3 decimals), or
0.132, 0.264, 0.528 and 1.057 quarts (rounded to 3 decimals), or
0.033, 0.066, 0.132 and 0.264 gallons (rounded to 3 decimals)

Wide mouth vs regular mouth mason jars

Some people think that “wide mouth” simply means the opening is as wide as the jar (so that the jar ends up looking like a cylinder) while “regular mouth” means the opening is smaller than the width of the jar. This is a flawed definition and only applies to certain jar sizes – the fact is there are two standard set-in-stone jar mouth sizes so even the “wide mouth” ones won’t be as wide as the jar on larger jars (such as half-gallon ones which don’t even have regular-mouth versions):

Regular mouth: outer width/diameter of 2.75 inches and rim width of ~3/16 inches
Wide mouth: outer width/diameter of 3 inches and rim width of ~3/16 inches

There are no other mouth sizes (despite there being, including metric versions, at least 8 different jar capacities) because there are only two lid sizes – imagine having to buy 8 different sizes of lids! Combined with the previous answer above about jar volumes you should have a pretty picture of how big standard mason jars are overall.
The main benefits of wide mouth include being easier to add and remove larger ingredients and being easier to clean. However they can be more expensive and more difficult to find (both jars and lids) and harder to open (and yet also easier to accidentally open cylindrical ones as it’s easier for the lids to bump into things). Also with some pressure canner models you may not be able to fit as many in them.

How to label mason jars

Location: Labeling the lids rather than the body means you don’t need to worry about washing them off since the lids will be disposed (as they should be), with the disadvantage that you can’t read them if they’re high up on a shelf or stacked. What to include: At a minimum a label should have the contents and the canning date, but there’s nothing stopping you from including more details such as ingredients, recipe, how you were feeling that day, etc. What to use: Writing on them directly with a permanent marker without a label is quick and easy to clean (especially with some alcohol or other cleaner/solvent) but the writing can end up being invisible if the contents are dark too. Adhesive labels or masking tape fixes this problem but sticking many of them can become a chore, not to mention taking them all off again (and dissolvable ones don’t always seem to come off any better). Though at least with computer printed ones you can include a ton of writing/info (or even pictures) without having to write it all by hand.

How to open a canning jar

If twisting it doesn’t work then try prying it open by sticking a spoon, fork, butterknife, can opener hook, etc. between the lid and the jar (take care not to crack the tip of the jar). If all else fails, then puncturing the lid will break the seal/vacuum and make it much easier to simply lift the lid off.

What is the difference between canning salt and table salt

Regular table salt contains anticaking agents to prevent clumping and many are also fortified with iodine, while canning (or “pickling”) salt is virtually pure sodium chloride and is also much finer (almost dust-like) so that it dissolves more quickly (it needs to be stored completely sealed or else it will draw water from the air and stick together, eventually becoming one solid lump). While in terms of safety it’s fine to use ordinary table salt for canning, the added substances in it can affect the canned food’s color and make it cloudy.

Can you use sea salt for pickling?

Sea salt and some brands of Kosher salt (that don’t contain any additives) don’t have the problems that table salt has outlined in the previous answer above, but they’re not finely ground like canning salt (the grains are even larger than in table salt) so they may not dissolve evenly unless you crush them into a powder-like consistency yourself.

Pickling lime vs pickling salt

Pickling lime is food-grade calcium hydroxide and is not the same thing as pickling/canning salt. It was popularly used in the past to make pickles more crunchy, but because it is alkaline/basic it can raise the PH (weaken the acidity) to levels that make it dangerous for the water bath method (which is what is typically used for pickles) by allowing botulism spores to survive. To prevent this, veges soaked in pickling lime needs to be rinsed then soaked in fresh water 3 times to eliminate as much of the lime as possible before being processed.

What is ball pickle crisp?

Ball pickle crisp is made of food-grade calcium chloride and is yet another additive used to help make pickles more crunchy. Unlike the pickling lime mentioned above, you don’t need to soak then wash it off – it can simply be added to the jar ingredients.

How to thicken salsa before canning

The only safe way to do this (according to the University of Minnesota) is by using small Italian tomatoes or adding tomato paste. There are (possibly unsafe) recipes and anecdotes about using a 1:500 ratio of guar/xantham gum by weight (as well as being used in commercially canned products), but reputable sources warn against using cornstarch, flour and other starch-based thickeners as these can prevent proper and even heat conduction in the jar (by clumping up and/or thickening too much or too quickly) leaving germs unkilled – you can die from it – so it could apply to any other thickener as well. While there are specially modified cornstarch products designed to be used for canning like Clear Jel (be sure not to use any other non-canning versions of it like the “instant” version) they should only be used in a trustworthy recipe that specifically uses it.

How to thicken bean soup naturally

The USDA only approves okra, potatoes and beans for safe thickening of soups – anything else (no matter how natural) is off limits including dairy and any starchy products like grains/flours for reasons listed in the previous answer above.

How much water do you put in a pressure canner?

This can be confusing if the device instruction manual tells you to refer to the recipe while the recipe tells you to read the manual, but it seems to be 3 inches for most models – manuals that use volume units of measurement (e.g. quarts) may specify different amounts of water for different loads and usually still end up being about 3 inches anyway.

Pressure cooker vs pressure canner

For USDA standards a pressure canner has to be large enough so that 4 quart jars with their lids can fit inside it. This ensures that its volume is sufficient for heating and cooling times that are safe enough to prevent botulism. Many pressure cookers are too small to be safe, and even ones that are large enough don’t have precise and consistent temperature and pressure controls that are crucial for safe canning.

How to use a pressure canner with a weighted gauge

This varies by manufacturer so it’s best to refer to their instructions and if the gauge was acquired separately it will usually only work for specific models. In general though it’s a simple concept – a gauge that is adjustable by adding or removing metal weights to increase or lower the desired pressure inside the canner, and once the gauge begins to rock/vent is when that has been reached. You will need to use more weights to reach the same amount of pressure at higher altitudes – again refer to manufacturer for specific instructions.

How to test a pressure gauge

First of all this is for dial gauges – weighted gauges can’t and don’t need to be tested. With dial gauges the manufacturer usually offers testing for free (usually only if you bring it in yourself, otherwise you will have to ship it at your own expense – they only need to take the lid though, not the whole thing!) and some may even test models from other manufacturers too (check with them first). You might also want to ask an auto repair or HVAC shop.
Gauges should be tested at least once a year as they are a major cause of canning failures.

Can you double stack jars in a pressure canner?

Yes as long as there’s enough vertical space, and it’s also best to:

Ensure the first layer is completely full
Spread the second layer out evenly if there are less jars than in the first layer
Use a rack in between the layers.
No change in pressure, duration or water is needed.

Pressure canning vs water bath

With water baths you basically immerse filled canning jars in boiling water (anything large enough to fit the jars and water works) and logically the contents of the jars will never get hotter than 212F/100C degrees – this isn’t enough to neutralize botulism spores (even if the “adult” bacteria die) so it’s only safe for canning foods with an acidity level sufficient to do so (below a pH of 4.6 – don’t estimate this by taste or “feel”!). So what about everything else? That’s where pressure canning comes in, where jars are steamed in a pressure canner (not just any old pressure cooker) where the steam heats up the food to temperatures high enough to neutralize botulism spores.

How to make calcium water

“Calcium water” is food-grade monocalcium phosphate powder mixed with water, and it’s used to help pectin gel in jam making. Most likely you don’t need to purchase it separately yourself as it often comes included with home jam-making pectin products, like the ones from Pomona.

Pickling vinegar vs white vinegar

There is no difference in the basic makeup of “pickling vinegar” or white vinegar – they both are a mixture of water and acetic acid. Whether it’s “pickling”, white, brown, apple cider, as long as its strength is at least 5% (the percent of acetic acid in it) it’s safe to use for pickling (though taste is another matter) as that level will neutralize botulism spores. “Pickling vinegar” is more a marketing gimmick than anything else, and they tend to be sold in larger containers and some with a higher acidity (which ironically could actually end up being dangerous as you might want to dilute it to match a recipe that asks for a lower acidity, and in the process you could accidentally dilute it too much to an unsafe level).

Can you can Pico de Gallo?

Technically yes and there are recipes for it, but once you do so it’s hard to call it pico de gallo anymore as everything that differentiates it from salsa (relatively large chunks of uncooked ingredients with minimal liquid) would be gone – it would be a bit like mincing a steak into a burger patty and still calling it a steak.

Can you pickle straight eight cucumbers?

Their thicker skin and watery flesh make them less ideal than pickling cucumbers, but that hasn’t stopped many people from having them turn out great – one benefit they have is their seeds are even smaller than many pickling cucumbers. Funnily enough, using them long before they reach their full 8 inches (when they are virtually seedless) seem to lead to the best results.

Can you pickle burpless cucumbers?

You can if you use baby ones, though more for non-fermented pickles. Large fully-grown ones contain more of a substance called elaterase (this is what makes them “burpless” or “bitter-free”), and this will cause them to become mushy when fermented.

Can you eat pickling cucumbers raw?

Yes many people do, though the definition of “pickling cucumber” doesn’t exactly seem to be set in stone. They do tend to have more prickles/spines on them, and as noted in the previous answer above burpless/bitter-free cucumbers aren’t really used for pickling so on average pickling ones will likely have more bitter compounds especially near the neck (which could lead to burps if you’re sensitive to them, but many people also find them more flavorful).

Can you peel cucumbers before pickling?

Yes, and some people do this with larger bitter ones. It does reduce flavor and crunchiness and it could get mushier quicker but otherwise it’s safe.

Best cucumbers for making pickles

“Best” can be subjective, but generally people lean towards medium-sized prickly/spiny ones (especially black spines) with small seeds (many report bad experiences with larger ones) and burpless cucumbers are also out. The overwhelmingly popular ones are “National Pickling” and “Kirby”, and it’s also hard to go wrong with most varieties with the word “pickling” in its name.

Do bay leaves have tannins?

Yes, and you only need one for 10 ounces of water for crunchy pickles.

What does alum do for pickles?

Aluminum potassium sulfate is used for making fermented pickles firmer, but unfortunately it doesn’t work for non-fermented ones. As for how much alum to use, any more than half a teaspoon per quart will actually make pickles mushier in addition to imparting a bitter taste. Some also avoid it because greater aluminum concentrations have been found in brains of dementia patients.

What is a jelly bag?

It’s used to make jelly, which unlike jams don’t have any solid bits from the fruit in it. It’s essentially a meshed pouch hanging from a tripod-like stand – pour mashed fruit in it and the juice will drip through to be collected for jelly making. The holes are small enough to not let fruit pulp/skin/seeds through while large enough to not get blocked up – either of these could happen with DIY alternatives like using cheesecloth or a coffee filter.

How to can green beans without a pressure cooker

First of all, this also applies to peppers, beans, chili, corn, carrots or any other non-sour/acidic foods.
Secondly, many people ask this question while referring to a “pressure cooker” but it’s not the same thing as a “pressure canner” which is what you what you should be using.
So without pressure it’s basically going to be a hot water bath, which is only safe for sufficiently acidic foods (and the same goes for methods using fryers, ovens or anything else without pressure, which aren’t recommended anyway). So the only safe way to can them without pressure is to pickle them. Note that this is only for vegetables – beef or any other meat needs pressure full stop.

Can you can yellow tomatoes?

Yes, and no changes to red-tomato recipes are needed. Same with purple, green or any other color tomatoes – you can even mix different ones together. If you’re using a water bath just remember to follow the USDA safety guidelines for canning tomatoes.

Clear Jel vs Sure Jell

These two have completely different uses:

Clear Jel (and similar non-GMO gluten-free alternatives like Ultra Gel and Cornabys E-Z Gel) is a modified corn starch used for safe thickening of canned foods (since other thickeners are unsafe, including ordinary corn starch). It is not the same thing as pectin.
Sure Jell is a mixture of pectin, citric acid and dextrose and is used in jam or jelly to help reduce fruit cooking time and make them “not runny”.

Can you can on a glass top stove?

Maybe if you get a number of things just right, but any number of those “things” (not limited to the ones below) could go wrong:

If the stove is one of those that pulses on and off to control the temperature, there won’t be a consistent boil which is fine for plain cooking but potentially dangerous for canning
If the bottom of pot/canner is wider than the heating element the sides/edges may not get enough heat (and you could also damage the surrounding areas)
If the bottom of the pot/canner isn’t completely flat (common with non-metallic ones) the heating element won’t be able to transfer heat properly (instead the heat could stay in the stove and damage it), and if the bottom of the pot is curved upwards (common with older ones) the empty space can actually forum a sealed vacuum and get the pot stuck so hard that you can’t remove it without breaking something

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