why can't oven canning ever be safe?

jeminijadAugust 31, 2010

First, I am aware that BWB and pressure canning are the only recommended methods of preserving food today. This isn't going to be a boiling green beans thread.

What I am wondering is why oven canning high acid foods is condemned as always unsafe. I understand that air is not as efficient a conductor of heat as water, so 200 degree air won't transfer heat into the food as well as 200 degree water. But why wouldn't sterilized jars, set into a water bath (say 3/4 up the jar) in a 275 oven for 4 hours, for high acid/high sugar foods only, reach a safe temperature? Is the main concern here jar breakage?

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readinglady(z8 OR)

It might be. AFAIK, though, nobody knows as testing hasn't been done with that particular set of conditions.

There's a huge problem with oven variability and unreliability, as well as various "hot spots" and "less-hot spots" depending upon the oven. So one of the main issues is that it would be almost impossible to come up with a consistently safe time to accomodate every possibility out there.

On the other hand, boiling water is boiling water, so regardless of how long it takes a particular stove to come up to heat (even a wood stove), once at temperature processing results will be reliably safe for every appliance.

I might also add that 4 hours in an oven is can be incredibly energy-wasteful.


    Bookmark   August 31, 2010 at 4:22PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

Agree. It is very wasteful of energy not to mention much less efficient. Manufacturers of canning jars do not recommend baking or oven canning in the jars. Does it result in safe food? Who knows. Do jars break? Yes, so disadvantages? Yes. Does it have any advantages over a BWB? Can't see how it could.


    Bookmark   August 31, 2010 at 5:18PM
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Linda_Lou(SW Wa.)

Oven Canning Oven-canning is extremely hazardous. The oven canning method involves placing jars in an oven and heating. In oven canning, product temperatures never exceed the boiling point, and uniform heat penetration cannot be assured. It is, therefore, not considered safe to use for home canning. Because this process fails to destroy the many bacteria, including the spores of Clostridium botulinum, it can cause the food to become toxic during storage. Also, canning jars are not designed for intense dry heat and may explode resulting in serious cuts or burns.

You have to have the water over the tops of the jars by 1 to 2 inches. Just part way up will not suffice.
How would you keep water boiling the entire time to process jars ? It take boiling water for a determined time for each food to kill the types of bacteria in the jars that would cause spoilage as well as get a true vacuum seal. Not having the water boiling the entire processing time would not allow for the food to be safe.
Processing on the stove top is much more effecient and faster.

    Bookmark   August 31, 2010 at 5:27PM
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val_s(z5 central IL)

a huge problem with oven variability and unreliability

I would have to say this would be my biggest concern. When we bought our home 23 years ago, it came with a stove and fridge. The first few times that I cooked in the oven, I couldn't figure out why things weren't getting done when they were supposed to. Some things cooked too fast, some too slow. Needless to say, the oven temperature "thingy" was out of whack. Good excuse to buy a new stove :-)


    Bookmark   August 31, 2010 at 6:42PM
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I raised my own tomatoes, so I knew they were high acid. They could make chapped lips squeal. I open kettle/ oven canned them for 20 years without losing a jar, but I was extremely careful and clean. Also I knew my oven.
Oven canning of dry ingredients might or might not work, but it won't hurt you. Bacteria won't grow in noodles that won't grow bacteria in a box on the shelf. Flower might get bugs, but it probably won't. Oven sugar? Can't hurt you. If the jar seals, ants can't get in. I seal-a-meal sealed a 5 pound bag of flour, and forgot it in a drawer for ten years. I thought it would be bad, but no bugs, and I couldn't tell any difference in bread, pie crust, or gravy. I didn't tell, and my family didn't say a thing.

    Bookmark   August 27, 2011 at 11:46PM
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readinglady(z8 OR)

They could make chapped lips squeal. Doesn't necessarily mean they were high-acid. They could just have been low sugar.

Cleanliness is no guarantee either. Botulism can grow with clean product in a sterile jar.

But, if someone wants to can jams in the oven, cognisant of the possibility of jar breakage, that's their prerogative. So is sealing of dry goods, though I would think you could achieve the same results with lids prepped in hot water and hot jars. I doubt you'd even need to use an oven.

However, it's all personal choice. We try to point out benefits and risks. It's up to the individual to decide what to accept and what to reject.


    Bookmark   August 28, 2011 at 1:49AM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

I raised my own tomatoes, so I knew they were high acid.

Sorry but there is no such thing as a "high acid tomato". All varieties have been pH tested numerous times and charts are available for the results.

As Carol said, oven canning and open-kettle canning are your choice, risks and all. But neither proven to be unsafe approach is something to be advocated for others.


    Bookmark   August 28, 2011 at 9:50AM
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I met a woman at a farmers market this weekend who has a new small business and cans in an industrial kitchen. I asked her about her methods of canning because she creates all of her own recipes. I wanted to know how she knows her recipes are safe and whether she has had them tested. She told me not to worry and explained the oven canning process that she uses. Her products are in 1/4 pint ball jars. She makes tomato and fruit jams.

Do you know if oven canning is ok for food companies (maybe they have some special equipment)?
Is there a reason she can make up her own recipes and sell to the public? For example, is testing required?
Do you think it is safe to eat her jams? I brought a group of students to the market and some I them wanted to buy the jam.
Is there a way I can make up a recipe (fruit/tomato/pickle or low acid vegetable) and test it for safety?

Thanks so much for your thoughts on this!

    Bookmark   September 26, 2012 at 8:40AM
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readinglady(z8 OR)

I worked in a cannery as a college student and don't know of any commercial cannery that cans in ovens. Generally they use pressure canners and their methods are determined by the standards of the FDA.

The fact that someone uses an "industrial" kitchen is irrelevant. Using an oven is not recommended for canning for the reasons posted above, and a commercial oven would be no different.

Her jams are probably safe (not sure about the tomato jam depending upon the recipe) as most fruits are high acid and jams are also high sugar. So the product itself is for the most part low-risk regardless of the process.

It sounds as if your first step is to do a lot of reading and studying to bring you up to speed on the principles of canning and the safety issues before worrying about "making up a recipe." I'd recommend the NCHFP self-paced online course. It's free and a great way to begin.

As far as your seller is concerned, I wouldn't touch anything she sells aside from a high-acid fruit jam. Many farmer's markets are not adequately regulated and there is risk with some of the products sold.


Here is a link that might be useful: NCHFP Home Page

    Bookmark   September 26, 2012 at 11:52AM
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Do you suppose that she meant that she cooked the food in the oven to reduce the liquid, then canned it the traditional way and sealed it in a boiling water bath. Otherwise, that's just scary.

    Bookmark   September 26, 2012 at 12:35PM
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You don't say what state the farmer's market is in. Laws vary from state to state, but AFAIK, if you are a commercial food processor (and not a cottage industry, as some states allow farmers to process their own fruit into jams and jellies without testing), you must have documented procedures on file. I can't imagine that oven canning would be allowed - but then again, apparently the Blue Chair Jam company does it, since that is the method described in their/her cookbook (an alternate method is described as "according to the manufacturer's instructions" - we can only assume that means BWB as described on Ball's website).

    Bookmark   September 26, 2012 at 1:10PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

As far as your seller is concerned, I wouldn't touch anything she sells aside from a high-acid fruit jam. Many farmer's markets are not adequately regulated and there is risk with some of the products sold.

Ditto what Carol said.

And the seller's "don't worry" and "making up her own recipes" comments would have inclined me to contact the local health department about her too. Most counties and states have at least some rules the seller has to abide by when selling to the public.

Do not make up your own recipes is the #1 rule of safe home canning.


    Bookmark   September 26, 2012 at 2:49PM
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Tuggy3(9b NorCal)

I would like to make some of the jams in the Blue Chair Jam cookbook but I am wary of the oven thing also. I wonder how she sells those jams in big name stores in California. We have a lot of regulations. My question is how do you convert the oven times that a book like that recommends to water bath processing times without overprocessing and losing the fresh flavors. I've never processed anything high acid under ten minutes.

    Bookmark   December 4, 2012 at 2:50AM
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readinglady(z8 OR)

The standard time for any sweet preserve in 8-oz. jars is 10 minutes BWB. Sometimes I see a recipe which specifies a longer time but 10 minutes is perfectly adequate.


    Bookmark   December 5, 2012 at 5:14AM
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