Can you can items using a deep frier instead of a pressure canner?

Question

In this question about canning safety, the answer states that the only safe way to can low-acid foods is by using a pressure canner, since only they will be able to reach the temperatures required to sterilise the food.

However, I can think of another, fairly common piece of kitchenware that can also reach the temperatures involved pretty easily: the deep frier. While boiling water is limited to the boiling point of water, oil can reach much higher temperatures.

As such, would it be possible to safely can low-acid foods by immersing the sealed cans into a deep frier?

Answer 1

In theory, it may be possible to kill all of the contaminants inside your jars using a deep fryer. However, I would not call it "safe," since there is possibly more risk of getting hurt from hot splattering oil than there would be from improper canning methods.

Water at sea-level air pressure boils at 100C. This means that a pot of water will never reach more than 100C, unless the pressure is increased. A pressure canner increases the boiling point of the water inside, so that the contents of the canner can reach the higher temperatures necessary to kill all bacteria and such. Since heat moves from areas of high concentration to areas of low concentration, eventually the temperature in the canner equalizes, and all of the jars reach the same internal temperature as the water inside the canner. This means that the jars themselves must also be at high pressure, since otherwise the (mostly) water inside of them would never have been able to reach such high temperatures.

Which brings us to the other thing that pressure canners do: they provide a "counter-pressure" to the pressure inside the jars, which prevents the jars from exploding. The "absolute" pressure on the jars is mostly irrelevant, but if the pressure inside the jars was much higher than outside, then the force pushing the glass of the jars "outward" might be more than they were built to handle, and the jars can explode. If the pressure outside and inside the jars is the same, the jars are safe and stay intact.

A deep-fryer does the first step (allowing the cans to reach high temperatures), since oil boils at a significantly higher temperature than water. The cans will get heated by the oil, which is at atmospheric pressure, and therefore the pressure inside the cans will increase. This is because the water inside "wants" to boil, since its temperature is greater than 100C, but it has nowhere to go, so the pressure just builds up. If the jar is not able to handle the stress of the high pressure inside of it, it can explode. And to make things worse, it's submerged in a bunch of really hot oil when it does so.

I'm pretty sure that's not something you want in your kitchen.

(As @JohnEye pointed out in a comment above, it is possible that the "weak spot" in a jar would actually be in between the lid and the glass, so that instead of exploding to relieve the pressure, the contents of the jar would just leak out and the jar would never get much above boiling point. This is prevented in a pressure canner because of the back-pressure from the contents of the pressure canner that keep the jar's contents in place. If this is the case, then even it theory the jars would never become safely canned).

Answer 2

That’s a nice thought, but physics make it impossible. It’s not about the external temperature, but about internal temperature, in the jars.

The contents of the jar are (hugely simplified) water, which means that the temperature inside the jars can’t exceed the boiling temperature of water, only after all water has turned into steam will the temperature rise again. (See video here, for example.)

Boiling point depends on environmental pressure (as those cooking high in the mountains observe daily) and the pressure canner creates an environment where all objects inside are exposed to a given pressure level. The non-pressurized fryer (or other heat source from your list) means that while the surrounding medium (fat, air in an oven…) is significantly hotter, the core of your product to be canned remains at 100°C tops (give or take a bit, depending on elevation and how tightly you screwed on the rings).

Practically speaking, with a fryer you are bound to create one or another kind of kitchen disaster, with possibilities ranging from cracking jars from thermal shock and over violent sputtering, to content being pushed outside and explosive evaporation of water in hot fat. For “more harmless” heat sources like ovens, the physics described above still make the suggestion not feasible.