Canning/Heat-preserving food

[Justin Loew]

Preserving food at home involves putting produce in glass jars and heating them to kill pathogens like bacteria, fungus, yeast, etc...

The USDA recommends AGAINST using an oven to preserve food. They only support the boiling water method, which involves submerging the jars in a pot of boiling water or in a pressure cooker.

I am unsure if the issue has ever been studied intensively. Theoretically, by heat diffusion, it should not matter whether the heat source is boiling water or hot air. Eventually the heat should flow into the "colder" jar full of food until it is equalized.

Some of the reasons people give against using an oven is that the jars might break, the temperature reading might not be reliable, etc... but I don't buy it. Water boils at 212. If you wanted to make sure you were heating your jars enough in the oven, one could just turn the heat up to 240 or 250, or whatever, to more than compensate for small deviations between oven models.

The only difference between the two methods is that I can divine is that the water boiling method might go faster since the number of molecules transferring heat would be greater, but is it that much of a difference?

 

[Evo]

There are a lot of reasons oven canning isn't safe.

Oven Temperature- the oven cannot be a pressure chamber,

1. which means that the food inside the jars will never get hotter than the boiling point of water (212F) regardless of how high the air temperature is inside the oven, Basic Law of Physics Folks!
2. Explosion Risk- Canning jars were not designed to be in a dry heat environment for a prolonged period of time and have been known to crack, shatter and in some instances even Explode during processing.
3. Manufacturers of canning jars do not recommend baking or oven canning in the jars.
4. Lack of Proof- There isn't a Single, research-based documentation/study to support that 'canning' any food in a dry oven as described on this web page or any page that proposes oven canning provides sufficient heating to destroy bacteria of concern.

In Short, Oven Canning is a potentially hazardous practice that can lead to food poisoning, botulism and death. There are a number of considerably safer methods of food storage and preservation

Read the article
https://www.budget101.com/content.php/3975-how-to-can-using-Dry-Oven-Canning

 

[sophiecentaur]

Hi and welcome to PF
As you say, water is a very effective and reliable medium for transferring heat (you suggest a valid reason why, in your post). I would imagine that boiling water under pressure would be a more reliable method for cooking food thoroughly and that would be why it is the chosen method for USDA. Google will produce a long list of links about this topic. This one is good for general reading. There are obviously a number of factors that apply to different foods and the length of time that preservation needs to operate for.
Alternative dry methods of heating are likely to be effective but they would be slower if you need to avoid frazzling / charring the outside of the food before the inside is 'cooked through'.
I think you would need to have a particular foodstuff in mind and its form before we could come to any useful conclusion about which method would be best.

 

[Justin Loew]

There are a lot of reasons oven canning isn't safe.

Read the article
https://www.budget101.com/content.php/3975-how-to-can-using-Dry-Oven-Canning

I understand points #2, #3, and #4 as practical matters, but I am trying to get a physics explanation.

In #1, are you saying that if you put a sealed jar of food in a 1000 degree oven, it will never get above 212, no matter how long it is in there? That seems to defy everything that is known about heat transfer/diffusion.

As far as having the jars under pressure, yes that will raise the temperature of the food in a pressure cooker (water method). Since the jars are sealed, the pressure should also rise somewhat (inside the jars) in the oven cooking method, just by ideal gas law consideration.

I guess there would be a way to test it out. Put a thermometer/sensor in the middle of a jar, put it in an oven, and see how warm it gets.

 

[russ_watters]

In #1, are you saying that if you put a sealed jar of food in a 1000 degree oven, it will never get above 212, no matter how long it is in there? That seems to defy everything that is known about heat transfer/diffusion.

It won't go above 212F until it boils away all the water in it, and/or pressurizes it, which obviously you don't want to do.

The problem with using more heat is that it doesn't promote even heating. Lower temperatures for longer durations provide more even heating.

 

[Justin Loew]

It won't go above 212F until it boils away all the water in it, and/or pressurizes it, which obviously you don't want to do.

The problem with using more heat is that it doesn't promote even heating. Lower temperatures for longer durations provide more even heating.

Water inside the jars boils no matter if it is in a water canner or in an open oven. With the jars being sealed, the pressure should rise in both of them, I would suspect.

 

[Evo]

I understand points #2, #3, and #4 as practical matters, but I am trying to get a physics explanation.

In #1, are you saying that if you put a sealed jar of food in a 1000 degree oven, it will never get above 212, no matter how long it is in there? That seems to defy everything that is known about heat transfer/diffusion.

As far as having the jars under pressure, yes that will raise the temperature of the food in a pressure cooker (water method). Since the jars are sealed, the pressure should also rise somewhat (inside the jars) in the oven cooking method, just by ideal gas law consideration.

I guess there would be a way to test it out. Put a thermometer/sensor in the middle of a jar, put it in an oven, and see how warm it gets.

Well, we aren't experts at canning, so google it for yourself if you don't like the answer. Be careful. thread closed.