Open kettle, oven, inversion, dishwasher, sun: one mechanism each
Sources last opened 2026-08-26.
The short version
There is a standard list of canning methods that do not work: open kettle, ovens, microwaves, dishwashers, inversion, sun, slow cookers, and "dry canning" in both of its meanings. The USDA guide names several of them and gives one reason for the whole set — that these practices do not prevent all risks of spoilage [F-10.1]. That is true and it is almost useless, because it tells a reader nothing about what is actually going wrong.
SDSU Extension published a page in 2023 that does the job properly: a specific mechanism for each method, in one sentence each [F-10.2]. That page is the backbone of this one, and it is worth reading in full at the source.
The mechanisms fall into three groups. Some methods never reach a high enough temperature. Some reach it unevenly or unpredictably. And one — open kettle — reaches it in the pot and then reintroduces the problem while the jars are being filled. All of them produce a sealed jar, which is why all of them are believable.
Eight methods, eight mechanisms
Each of these gets one paragraph, because one specific reason is more useful than a page of general warning. The citations are separate for each because the evidence is separate for each.
Open kettle
What it is. Food is cooked in a pot, ladled hot into jars, and lids go on. The filled jar is never heat processed.
Why it fails. Two reasons, from two sources. SDSU: although jars may seal using this method, bacteria are not destroyed and can easily grow and multiply in the 'sealed' jar [F-10.2]. And the guide's own glossary adds the other half — foods canned this way often spoil because they become recontaminated while the jars are being filled [F-1.7]. Even where the pot was hot enough, the ladle, the funnel, the jar rim and the air are not sterile, and nothing after that point applies heat.
The consequence, with a date. In June 2024 an outbreak of eight cases of foodborne botulism in Fresno County was traced to home-prepared nopales — jars sterilised empty, raw salted cactus added, lids on, stored six weeks in an outdoor shed. All eight were hospitalised, six went to intensive care, and all recovered [F-12.4]. That is this method, applied to a low-acid vegetable.
The oven, with wet food in the jars
What it is. Filled jars are heated in a domestic oven instead of in water or steam.
Why it fails. Dry heat. SDSU: heat distribution may be uneven, especially in the oven, which uses dry heat, and this type of heat is slow to penetrate jars [F-10.2]. Air is a poor conductor compared with water or steam, so the oven's air temperature is not the jar's temperature and the two can be far apart for a long time.
And the glass. NCHFP: manufacturers of canning jars do not recommend baking or oven canning in the jars, and it is very risky with regard to causing jar breakage [F-10.3].
The oven, with dry goods — "dry canning", meaning two
What it is. Flour, rice, oats or beans are heated in closed jars in an oven, to make them shelf-stable for longer.
Why it fails. University of Illinois Extension, February 2023, gives the mechanism nearly everybody misses: all food has moisture, even dried food, and when dried food is put in a closed jar in the oven, the moisture moves up to evaporate but some pockets of moisture may remain [F-10.5]. Those pockets are the problem — a sealed jar with a low-oxygen environment and a damp spot in it. The same page notes that Mason jars are not designed to be heated in an oven, which increases the risk of shattering, and that there is no research supporting the claim that it extends shelf life [F-10.5]. NCHFP's own page agrees that this is not a recommended science-based method of storing dry foods [F-10.5].
"Dry canning", meaning one — leaving out the covering liquid
What it is. A tested recipe, packed without the liquid it calls for.
Why it fails. NCHFP: canning vegetables or any food pieces without the covering liquid that was used in process research can result in under-processing and, in the case of vegetables, a risk of botulism [F-9.12]. The liquid carries heat to the middle of the jar, and the schedule assumes it is there.
Inversion
What it is. The filled jar is closed and turned upside down so the hot contents heat the lid and pull a seal, instead of the jar being processed.
Why it fails. SDSU: the food in the jar does not reach a high enough temperature for the appropriate amount of time to ensure food safety, and it can compromise the seal [F-10.2]. Both halves are worth keeping. It under-processes, and the seal it produces is less reliable than the one a proper process produces — which is the opposite of what the method promises.
The dishwasher
What it is. Filled jars ride through a dishwasher cycle in place of a canner.
Why it fails. SDSU: there is little control over the temperature, pressure and processing time [F-10.2]. A dishwasher's cycle is designed around detergent chemistry and water use, not around holding a known temperature for a known number of minutes. You cannot verify any of the three variables the process depends on.
Note the separate, legitimate use: the guide allows a dishwasher for preheating empty jars on a complete regular cycle — while stating in the same breath that these washing and preheating methods do not sterilize jars [F-2.3]. Preheating empty jars and processing filled ones are different operations.
Sun canning
What it is. Filled jars are left in direct sun.
Why it fails. SDSU: it does not produce high enough temperatures to kill bacteria in foods and may not distribute the heat evenly throughout the product [F-10.2]. There is not much more to say. The gap between the temperature a jar reaches on a hot day and the temperature the food needs is enormous, and this is the one method on the list where nobody has to be convinced by an argument about heat penetration.
Microwaves and slow cookers
Why they fail. SDSU on microwaves: there is no standard temperature among microwaves, and many have different heating properties [F-10.2]. On slow cookers: they do not achieve a high enough temperature to kill off harmful bacteria [F-10.2]. One is an uncontrolled variable; the other is a ceiling.
The three families, once you step back
Never hot enough. Sun, slow cooker, and — for low-acid food — anything atmospheric at all. The governing figure is that low-acid food is sterilised at 240° to 250°F, reached with a pressure canner operated at 10 to 15 PSIG [F-1.2], and a vessel open to the air tops out at 212°F at sea level [F-6.3]. Time does not substitute for temperature here.
Hot but uncontrolled or uneven. Oven, microwave, dishwasher. The heat exists but nobody can tell you what the food's temperature was or how long it stayed there, and process schedules are entirely statements about temperature and time.
Hot in the pot, then compromised. Open kettle, and inversion as a substitute for processing. The food was hot; the jar was filled in a non-sterile world; nothing heated the sealed jar afterwards [F-1.7].
Every one of the eight produces a sealed jar. That is the common feature and it is the reason all of them survive. The seal is the visible outcome, and it is the outcome that carries no information about the process.
What to do instead, by what you were trying to achieve
You wanted to preserve a low-acid food. A stovetop pressure canner holding four or more quart jars, using the tested schedule for that specific food from NCHFP. Or the freezer, which needs no schedule at all.
You wanted to preserve an acid food and did not want to buy a canner. A boiling water canner is a deep pot with a rack in it, and a steam canner is now research-tested for acid and properly acidified foods under stated conditions — see the steam canning page for the seven of them, one of which rules out a lot of recipes.
You wanted to make jars of dry goods last longer. Airtight containers, cool and dark, and a freezer for the things that go rancid. Nothing about heating them in an oven has been shown to extend shelf life [F-10.5].
You wanted a seal on a jar of something. Refrigerate it. A sealed jar in a fridge is a normal food storage container and raises none of these questions, because the temperature is doing the work instead of the process.
A note on the pages that teach these
They are not fringe. Several rank well, several are attractively photographed, and at least one presents open kettle, dry canning, water bath, fermentation and freezing as five equivalent alternatives with no tested citation anywhere on the page [F-13.14].
That is worth naming without contempt. These methods were, in some cases, taught by extension services and cookbooks generations ago, and were superseded as thermal process research improved. Somebody's grandmother did do it that way. A page that repeats it is repeating something it was taught, and a reader who follows it is doing what a reasonable person does with a confident, well-presented source.
The useful response is not to argue about who is a real canner. It is to ask the question that separates a tested method from a folk one: which document is this time and this temperature from, and can I open it? For every method on this page, there is no such document, and the pages above are where you can read the people who would have written it explaining why.
The primary source's list, and notice how little it explains
Open-kettle canning and the processing of freshly filled jars in conventional ovens, microwave ovens, and dishwashers are not recommended, because these practices do not prevent all risks of spoilage.
USDA Complete Guide to Home Canning, 2015 revision, p. 1-10
The link above opens the document itself, so a printed page number leads somewhere. What this document is, and who maintains it.
Opened and read by this site on 2026-08-26.
This is the whole of what the 2015 guide says on the subject, in the paragraph headed "Equipment and methods not recommended". It names four things and gives one reason for all of them.
"Do not prevent all risks of spoilage" is doing very little work. It does not say what the risk is, or which step fails, or why. A reader who wants to understand rather than comply gets nothing from it, and a reader who has just read a confident blog post explaining exactly how well oven canning works gets less than nothing.
That gap is not a criticism of the guide, which is a procedures manual rather than an explanation. It is the reason the next quote matters so much.
One mechanism per method — the most useful page in this whole subject
Processing jars in the dishwasher is unsafe, as there is little control over the temperature, pressure and processing time.
The food in the jar doesn't reach a high enough temperature for the appropriate amount of time to ensure food safety, and it can compromise the seal.
Although jars may seal using this method, bacteria are not destroyed and can easily grow and multiply in the 'sealed' jar.
Heat distribution may be uneven in both pieces of equipment, especially in the oven, which uses dry heat. This type of heat is slow to penetrate jars.
It doesn't produce high enough temperatures to kill bacteria in foods and may not distribute the heat evenly throughout the product.
South Dakota State University Extension, the why behind unsafe canning practices, live
Opened and read by this site on 2026-08-26.
SDSU Extension, updated 27 October 2023, written by Abigail Lambert under the direction and review of Curtis Braun, SDSU Extension Food Safety Field Specialist. The five passages above are, in order, the dishwasher, inversion, open kettle, the oven, and sun canning. The same page also covers microwaves, pressure cookers and slow cookers [F-10.2].
What makes this page unusual is that it does not lecture. Each entry names one specific failure, and each failure is a different kind of thing.
The dishwasher fails on control: you cannot set the temperature, you cannot hold it, you cannot time it.
Inversion fails on time at temperature, and adds a second problem — it can compromise the seal it is supposed to create.
Open kettle fails on what a seal proves. Note the scare quotes around 'sealed' in that sentence; they are the source's, and they are the argument.
The oven fails on heat transfer: dry air is slow to get through glass and into food, and it is uneven while it does it.
Sun canning fails on temperature outright. Nothing in it approaches the temperatures that matter.
The oven, from the people who maintain the guide — and a hazard to your hands
the jars are preheated in an oven (dry-heat), which is not recommended for canning jars. Manufacturers of canning jars do not recommend baking or oven canning in the jars. It is very risky with regard to causing jar breakage. There is no guarantee that the jars heated in this dry manner are sufficiently heated to sterilize them, as we do not have data on sterilizing jar surfaces by this dry-heating method.
National Center for Home Food Preservation, live site, University of Georgia
Opened and read by this site on 2026-08-26.
Two separate objections in one paragraph, and they are worth pulling apart because only one of them is about microbiology.
The glass. Canning jars are made for wet heat. Manufacturers do not recommend baking in them, and NCHFP calls the practice very risky with regard to causing jar breakage. That risk is immediate and physical: hot glass, in an oven, at head height in some kitchens.
The data. Read the last clause slowly — "we do not have data on sterilizing jar surfaces by this dry-heating method." That is not a finding that dry heat fails. It is a statement that nobody has measured it, so no guarantee can be offered.
This is the distinction that runs through the whole site. The oven's heat-transfer problem is a characterised mechanism [F-10.2]. The oven's jar-sterilising claim is an absence of data. Both lead to the same answer here, and they are not the same kind of statement.
'Dry canning', meaning one: leaving out the covering liquid
'Dry Canning' Not Recommended. There are also some other methods that are not recommended being circulated among home canners. Canning vegetables or any food pieces without the covering liquid that was used in process research can result in under-processing and in the case of vegetables, a risk of botulism.
The USDA and University of Georgia researched and science-based home canning processes are meant to be used with the food preparation and jar filling directions in the recommendation.
National Center for Home Food Preservation, live site, University of Georgia
Opened and read by this site on 2026-08-26.
The phrase "dry canning" means two different things and this is the first of them: packing food into jars without the liquid the tested recipe calls for.
The liquid is not a garnish and it is not there to stop the food drying out on the shelf. It is part of how heat gets from the outside of the jar to the cold spot in the middle, and the process time was calculated with it present. Take it out and the same clock delivers less heat to the centre. NCHFP names the consequence directly: under-processing, and for vegetables, a risk of botulism.
The second sentence is the general principle and it is worth more than the specific case. A tested process is the whole package — the preparation, the pack, the jar size, the liquid, the headspace, the time. It was validated as a set. Changing one part of it and keeping the time does not give you a slightly different tested process; it gives you an untested one that looks familiar.
The second meaning of "dry canning" — heating dry goods like flour or rice in jars in an oven — is on the same NCHFP page, described as also not a recommended science-based method of storing dry foods [F-10.5].
The named consequence: this is not a hypothetical
In June 2024, an outbreak of eight cases of foodborne botulism was caused by contaminated home-prepared prickly pear cactus (nopales); two patients required invasive mechanical ventilation. The outbreak led to a coordinated investigation and intervention by local, state, and federal agencies. All patients received timely treatment with heptavalent botulinum antitoxin, and all recovered.
CDC MMWR, botulism outbreak from home-canned nopales, Fresno County, 3 July 2025
Opened and read by this site on 2026-08-26.
MMWR, published 3 July 2025, on an outbreak in Fresno County, California in June 2024. Eight cases; all eight hospitalised; six admitted to intensive care; all recovered [F-12.4].
The method matters here and it is described in the report. The empty jars were prepared by immersing them in boiling water. Chopped, uncooked nopales mixed with a small amount of salt went in. Metal lids went on. The jars were stored for six weeks in an outdoor shed [F-12.4].
That is open-kettle canning of a low-acid vegetable, which is exactly the method several currently-ranking pages present as a legitimate alternative. Everything else on this page is a mechanism. This is an outcome, with a date, a county, a case count and a DOI.
Two things this is not. It is not evidence about home canners as a group — MMWR describes a family preparing a traditional food and says nothing about trends or communities, and neither does this site. And it is not a reason to be frightened of canning. Eight people became very ill from one specific method applied to one specific food, and all of them recovered.
What people ask next
My jars sealed. Doesn't that mean it worked?
No, and this is the single most important sentence on the page. A lid that pulls down and holds is evidence that a vacuum formed as the contents cooled and contracted. That is all it is evidence of.
The guide lists what proper canning does: remove oxygen, destroy enzymes, prevent the growth of undesirable bacteria, yeasts and molds, and help form a high vacuum in jars [F-1.9]. The vacuum is last, and it is a consequence of the process rather than the mechanism.
SDSU puts it more bluntly for open kettle: although jars may seal using this method, bacteria are not destroyed and can easily grow and multiply in the 'sealed' jar [F-10.2]. Every method on this page produces seals. That is precisely why they are persuasive.
Is inversion the same as open kettle?
They overlap and they are not identical. Open kettle is about the process: hot food goes into jars and the filled jar is never heat processed. Inversion is about what happens next — the closed jar is turned over so that the hot contents heat the lid and drive the seal.
SDSU's objection to inversion has two parts: the food in the jar does not reach a high enough temperature for the appropriate amount of time, and the practice can compromise the seal [F-10.2]. The second half surprises people, because the whole appeal of inversion is that it seems to guarantee a seal.
Inversion is also used as a finishing step on top of a real boiling water process by some recipes and some jam manufacturers. That is a different situation from using it instead of a process, and the tested instruction after a real process is to let jars sit undisturbed while they cool [F-11.2].
What about the microwave, the slow cooker and the pressure cooker?
All three are on SDSU's list with their own reasons. Microwaves: there is no standard temperature among microwaves and many have different heating properties. Slow cookers: they do not achieve a high enough temperature to kill harmful bacteria. Pressure cookers: the heat during the processing time may not be enough to destroy bacteria and make the food safe to consume [F-10.2].
The pressure cooker one is the one people argue about, because a pressure cooker does reach canning temperatures. The problem is not the peak reading. It is that the tested processes were developed for stovetop pressure canners holding four or more quart jars, and that cool-down contributes lethality that a small vessel does not deliver. There is a whole page on that in the pressure hub: Can I use an Instant Pot or an electric canner.
Why do these methods have such staying power?
Because they are old, they produce a jar that looks correct, and until something goes wrong there is no feedback at all. A method that fails occasionally and silently is much harder to unlearn than one that fails visibly every time.
It is also genuinely easy to get wrong. A national food magazine with editors and a test kitchen published a video showing seafood being water-bath canned, and pulled it after food preservation professionals objected, saying "We made a mistake, and we promise to learn from it" [F-10.7]. The program director quoted in that coverage put the rule in one line: you cannot achieve a temperature of 240 degrees in anything other than a pressure canner [F-10.7].
If a staffed test kitchen can miss that, a home cook following a well-written blog post has not failed at anything.
I used one of these methods. What do I do with the jars?
Do not taste anything to test it. Tasting is the one check that cannot be undone, and the organism at issue does not reliably announce itself by smell, appearance or a bulging lid.
There is no reprocessing window to reach for here. USDA's reprocessing passage is written for a jar that failed to seal after a correct process, and it is not a repair procedure for a batch put through a method that had no tested process behind it [F-9.9]. Running such a batch again does not convert it into a tested one.
So treat the jars as spoiled rather than as questionable, and handle the disposal carefully rather than pouring the contents down a sink and rinsing the jar. What that looks like in practice, jar by jar, is set out on I did something the internet told me to. Now what.
Read these yourself
SDSU Extension — The 'Why' Behind Unsafe Canning Practices
extension.sdstate.edu
NCHFP — Equipment and Methods Not Recommended
nchfp.uga.edu
NCHFP — canning FAQs, including oven heating of jars
nchfp.uga.edu
University of Illinois Extension — Is canning flour safe?
extension.illinois.edu
USDA Complete Guide to Home Canning, 2015 revision
nchfp.uga.edu