Canning Methods That Are Not Safe

There are no safe short-cuts in home canning. That is Bernardin’s own sentence, and the rest of it draws the line this page is about:

Heat processing all high acid foods in a boiling water canner and low acid foods in a pressure canner — for the appropriate time for the food and jar size — are the only currently approved methods for safe home canning.

Two methods are approved. Everything below is one Bernardin names as faulty, unsafe or potentially dangerous. If you have already put food up by one of them, skip to what to do with those jars.

Corrected 19 September 2026: this page told you to clean up after a suspect jar with a bleach solution — in the disposal list and again in the FAQ. No source we hold publishes a disinfectant for that surface. HealthLink BC’s disposal steps name no disinfectant: hands with warm soapy water, then wash every utensil and surface. Their file is unreachable from our tooling today (HTTP 000), so this follows the wording this site already carries on forty other pages from an earlier read of it. Québec’s home-canning page, which we also cite, contains no mention of bleach or chlorine at all. Nothing about bagging, binning or keeping the jar sealed has changed.

The eight, at a glance

Method	What it looks like	Why it is on the list
Open-kettle	Cook in a pot, ladle hot into jars, lid on, wait for the ping	The filled jars are never heated to and held at the temperature that destroys spoilage organisms
Oven canning	Filled jars baked in the oven instead of a canner	Jars can break or explode; dry heat penetrates the jar slowly and the food is not heated sufficiently
Steam canning	Jars on a rack in a shallow covered pan of boiling water	The steam is not pressurized, the temperature is uneven, and no safe processing times exist for it
Microwave canning	Devices marketed for canning in a microwave	Not shown to destroy all bacteria present, and they do not heat uniformly
Vacuum sealing	A sealer attachment pulling a vacuum on a mason jar	No heat treatment at all, so organisms can grow under the vacuum seal
Paraffin wax	Melted wax poured over jam or jelly instead of a lid	The wax shrinks and pinholes; mould follows, and mould can mean mycotoxins
Inverting or blanketing	Turning jars upside down, or wrapping them to cool slowly	Rarely reaches a temperature that destroys microorganisms, and the warm spell can promote growth
Rubber rings and glass lids	Wire-bail jars, zinc caps, flat rubber rings	Failure rate too high to be recommended for home canning today

Why “it has always worked” is not the test

This is the objection worth answering first, because it is usually the true one. Somebody’s grandmother sealed jam under wax for forty years and nobody ever got sick.

Bernardin answers it directly, and their phrasing is better than any paraphrase:

Some people may experience “good luck” with outdated methods…and then one year everything spoils! Each growing season is different. Heat-resistant bacteria are not necessarily present in every piece of produce. IF these organisms are absent…and that’s a big IF…under-processed foods might keep. When they are present, the food spoils.

Their instruction from there is one line: always assume these microorganisms are present.

That is why every entry above is written as a rule about the method rather than a judgement about your jar. The method does not become safe because a particular batch survived; the batch survived because that year’s tomatoes happened not to carry the organism.

Open-kettle canning

The commonest of the eight, and the one most likely to be in a handwritten family recipe. You cook the food in an open saucepan, ladle it hot into jars, and put the lids straight on, hoping the seal forms as it cools.

Bernardin: “Open-kettle canning is not safe and is not recommended for food preservation.” Their reason is that regardless of how long or how hard you cooked it in the pot, the filled jars are never heated to and held at the temperatures that destroy spoilage microorganisms, and the step that drives air out of the jar to form a real vacuum is missing entirely.

What that produces is a lid that looks right. It may curve downward, sit there for months, and then unseal — or not unseal, and simply be a low-vacuum jar of under-processed food.

Simply getting lids to curve downward on jars will not, in itself, prevent food from spoiling.

Oven canning

Bernardin is blunter about this one than about anything else on the list: “Not only is it unsafe, oven-canning can be extremely hazardous.”

Two separate problems. Jars can break or explode from the temperature swing when the oven door opens — that is a burn and a face full of glass, not a food-safety question. And dry heat penetrates a jar of food very slowly, so the contents never get hot enough for long enough. Oven temperature also varies with the accuracy of the regulator and how heat circulates in that particular oven, which is the opposite of the controlled, known temperature a process time assumes.

Bernardin lists oven sterilisation of empty jars separately, as a cause of thermal shock and breakage. Jars are sterilised in water, not in the oven.

Steam canning

Worth separating carefully, because the name misleads. A steam canner is a shallow covered pan: jars sit on a rack, water boils underneath, steam circulates. It is not a pressure canner and shares nothing with one but the word.

Do not confuse steam canning with pressure canning… Steam in this device is not pressurized.

Unpressurised steam sits at the boiling point, so it cannot reach the 116 °C that Québec names as the safe temperature only a pressure canner can reach — the whole point of a pressure canner. Bernardin adds that these devices do not create a steady flow of steam nor hold an even temperature, so “it is impossible to know if the heat has penetrated the food properly.”

And the practical instruction, which is the one that matters: the processing times in tested recipes are for a boiling water canner or a pressure canner. Do not apply them to a steam canner — safe times have not been established for it.

Microwave canning

Devices have been sold for this. Bernardin’s answer is four words long at the end: “Do not use these devices to can food at home.”

Their objections are that the devices have not been shown to destroy all the bacteria present, that they do not heat uniformly, and that food siphoning out of the jars and lid failures both occur.

Vacuum sealing instead of processing

This one catches careful people, because the appliance genuinely does what it claims. A jar attachment on a vacuum sealer really will pull a vacuum on a mason jar.

The vacuum is not the preservation. Bernardin:

Since there is no heat treatment to destroy spoilage microorganisms and botulism bacteria, these organisms can grow under the vacuum seal.

An anaerobic sealed jar at room temperature is not a neutral container — it is the exact environment Clostridium botulinum prefers. Public Health Ontario’s review lists the growth envelope as a pH above 4.6, less than 2% oxygen, 4–50 °C, and water activity above 0.85. A vacuum-sealed jar of low-acid food on a shelf satisfies every one of them.

Bernardin’s own recommendation for the attachment: dry ingredients and non-perishable staples only. Flour, rice, dried beans, coffee, dehydrated food. Nothing wet, nothing destined for the pantry shelf as a preserve.

Paraffin wax

Still sold, still in old jam recipes, and off the list for two reasons that compound.

First, the seal fails in ways you cannot see. Melted paraffin covers the surface and reaches the jar rim, then shrinks away from the glass as it hardens, letting air at the food. Even where it looks solid it can carry microscopic pinholes.

Second — and this is the part that surprises people — you cannot skim the mould off. Bernardin:

Research has shown that mold growth may produce mycotoxins, that can be very harmful to human health. Mycotoxins can migrate like invisible threads through the food to the bottom of the jar. Thus, skimming the surface mold will not remove mycotoxins.

That sentence is why mould on home-canned food is a discard and never a rescue, wax or no wax.

Inverting jars, and wrapping them in blankets

Two versions of the same idea: use the food’s own heat to do the processing. Turn the filled jars upside down for a few minutes so the hot contents sterilise the lid, or wrap the hot jars in towels so they cool slowly.

Bernardin’s finding is that food preserved this way “rarely, if ever, reaches temperatures sufficient to destroy microorganisms.” And the blanket version is worse than merely ineffective:

Keeping the jars warm in blankets can, in fact, be dangerous. The prolonged warm environment may promote bacterial growth inside the jars.

Slow cooling is not gentle processing. It is time spent in the temperature range where bacteria grow.

Rubber rings, glass lids, wire bails and zinc caps

The antique jars — wire-bail with a glass lid and a flat rubber ring, or one-piece zinc porcelain-lined caps. Bernardin does not recommend them for home canning today, on the grounds of failure rate.

They are lovely, and they are excellent for dry storage, refrigerator pickles and cut flowers. They are not for heat processing. The two-piece SNAP lid exists because it vents during processing and then holds a measurable vacuum — a rubber ring does neither predictably.

Recipes published before 1988

Not a method, but the reason most of these methods are still circulating.

General home canning guidelines were updated in the late 1980s to meet today’s standards of quality and safety. Home canning recipes or books published prior to 1988 may contain outdated information which could affect the safety of the end result.

Bernardin’s own line draws it at 1988, so that is where we draw it too. A pre-1988 book is not automatically wrong about everything, but it is exactly where paraffin, open-kettle and oven canning are still written down as normal — and it may carry processing times set for equipment and lids that no longer exist.

Corrected 19 September 2026: this page gave the boundary as 1989 in four places, while the Bernardin sentence quoted directly above it says books published prior to 1988. The year is theirs and ours was a year wider. If you set aside a 1988 book on our figure, Bernardin’s own line does not put it in the warned set — though a book from either side of the boundary is still worth checking against a current edition, because that is when the guidelines moved.

This applies hardest to a handwritten card. A recipe copied from a relative has no publication date at all, so the method it names is the only clue you have.

What we are not going to tell you

We cannot tell you that a particular jar made this way has gone bad. That is not the standard, and it is not knowable by looking. The botulinum toxin is odourless and tasteless; it does not cloud the food, and a jar can be dangerous and look perfect.

What we can tell you is what the authority publishes: these methods were not tested, no processing time exists for them, and the barrier a shelf-stable jar depends on was never established. That settles it without anyone needing to inspect anything.

What to do with jars you already made this way

Treat them as food that was never processed and has been sitting at room temperature.

1. Do not taste it, and do not taste it to check. There is no amount small enough to be a test.
2. Do not open the jar if you can avoid it. Keep it sealed.
3. Bag it. Put the unopened jar inside a heavy garbage bag, seal it, then put that inside a second waterproof container with a lid.
4. Bin it in a covered rubbish bin that your household, children and pets cannot get into.
5. Clean up. Wash your hands well with warm soapy water, and wash every utensil and surface the jar or your hands may have touched — HealthLink BC’s steps, which name no disinfectant.

How to tell if home-canned food has gone bad carries the full disposal routine, including what to do if a jar has already leaked.

If food from a jar like this has already been eaten, do not wait to see whether it passes. Double or blurred vision, drooping eyelids, slurred speech, difficulty swallowing, a dry mouth or muscle weakness — call 911 or go to an emergency department immediately. Botulism antitoxin works best given early.

The two methods that are approved

For completeness, since the whole page is defined against them:

• Water-bath canning — high-acid foods only: jams, jellies, fruit, pickles, and tomatoes with added acid. A boiling water canner is a large deep pot with a lid and a rack. Bernardin’s Complete Book asks for a pot 7.5 cm (3 in) taller than the jars (p.10); the water itself goes 2.5 cm over the lids (p.7), which is the floor of the 1–2 inch range their website gives.
• Pressure canning — the only safe method for low-acid food: vegetables, meat, poultry, seafood, soup. It reaches 116 °C; a boiling water canner cannot.

You do not need to buy a purpose-made canner to do the first one properly — see water-bath canning without a canner, which is Bernardin’s own tip rather than a workaround.

Related

• How to tell if home-canned food has gone bad — the inspection routine and full disposal steps
• Mould on home-canned food — why skimming is never the answer
• Botulism: what every Canadian canner needs to know — the organism these rules exist for
• Water-bath canning without a canner — the approved method, without buying anything
• How we source — why Bernardin and not the first blog result