Why Oven Canning is a Bad Idea

Why Oven Canning is Not Recommended - LivingHomegrown.com

I figure for every question I get from my readers – via comments or emails – there may be a few others with a similar question.

But when I get one question over and over, I know it’s time to write up that discussion as a blog post so that everyone can benefit from the information.

Such is the case with today’s post.

Why Oven Canning is Not Safe - LivingHomegrown.com

What is Oven Canning?

I have been asked several times lately about “oven canning”

  • Is it safe?
  • Have I ever done it?

In case you are not familiar with the term, Oven Canning is when you place jars of preserves in the oven to “process” them rather than processing them in a water bath.

A typical reader question goes like this:

“I’m wondering if it is okay to process in the oven instead of the water bath. I’ve heard of people placing the filled jars into the oven at 220 degrees for 15 or 20 minutes. It seems like this would work because you are placing the jars in the same temperature for the same amount of time. And it sounds easier. What do you think?”

I’m so happy to answer questions like this because it gives me an opportunity to not only squelch some bad information going around the internet, but to also explain the reasoning and the science behind the USDA recommendations on this matter.

I believe that by understanding the “why”, we care better equipped as canners to make these safety decisions on our own.

First – The Big Picture:

I know it sounds logical that this technique would work and yes, some people do it even though they know it is considered dangerous. Back in the day, I even tried it myself.

But canning recommendations have changed and I know better now. I would not do this today.

Let’s first look at the big picture here. When I can, I want to be sure that what I feed my family is as safe as I can make it.

If I’m going to the trouble to can in the first place, I don’t want to use any technique that is questionable. Using questionable methods pretty much defeats the purpose of working toward healthier food for my family.

I’m figuring you probably feel the same way.

That said, if I can look at the science behind a method or find testing that demonstrates the safety of something against USDA policy, I might consider it.

For example:

I do use Weck canning jars occasionally even though they are not recommended by the USDA.

But there is a big difference here:

  • Weck jars are tested and recommended via European countries.
  • The USDA is not recommending against them.
  • The USDA simply has not taken the time to test and recommend themselves.

Oven Canning is different.

The USDA strongly recommends against the practice of oven canning.

And Master Food Preservers are taught to preach against it. Here’s why:

Why Oven Canning is Not Safe - LivingHomegrown.com

Why Oven Canning is NOT Recommended:

1) Dry Heat: Although the oven is set to 220 degrees, there is a difference between dry heat (oven) and wet heat (water bath). The heat penetrates the food at a different (slower) rate. There are no tested standards for how long something must be baked in a home oven to reach the proper temperature internally to kill bacteria and create the proper vacuum seal vs. when we just cook the food in the jar. If the entire contents did not reach the proper temperature, the mold and bacteria are not completely killed. And we know for sure that a straight swap out of time is not long enough. The big problem is that no one knows for sure how long is long enough without baking the contents.

2) Thermal Shock: There is a risk that the jars can break – especially with the lids on and especially when removed from the oven. The jars are not tested or manufactured for this type of dry heat canning. And thermal shock is more likely with dry vs. wet heat. This could be dangerous if they crack or explode while you are removing them from the oven.

3) Vacuum Seal: Some people bake the jars and then put on the lids as they come out of the oven. The same problems mentioned above apply. But when doing it this way, you are more likely to get a weak vacuum seal (not as strong of a vacuum as water bath). Also this two-step process allows more entry points for surviving bacteria – especially mold.

NOTE: Although mold is not poisonous, it is a carcinogen and should be avoided.

Here’s the Thing:

I realize that despite the warnings, a small percentage of people still do this. And no, there are not 100′s of people rushing to the emergency room with shards of glass in their eye from jar explosions. (But I have heard stories of broken jars).

My question is this: Why choose oven canning over water bath processing when the results and  safety are questionable (beyond the possibility of broken jars)?

Oven canning is not easier or faster than water bath processing. In fact, it should take longer because the dry oven heat should be given more time than wet heat.

Perhaps it boils down to people not believing the safety concerns or they just feel lucky. They are free to make the choices that they feel most comfortable with.

For me, the whole point is to feel confident in what I am feeding my family. So, I do not oven can. As a trained Master Food Preserver, I do not recommend it.

What are your thoughts on this?

Please let me know in the comments.

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About the Author

Theresa Loe is the Co-Executive TV Producer and the On-Air Canning/Homesteading Expert for the national PBS gardening TV series, Growing A Greener World. She is a lifetime canner and a graduate of the Master Food Preserver Program. She studied both sustainable horticulture and professional culinary arts and she is a wrangler of chickens and two teenage children. (Not necessarily in that order.)

{ 25 comments… add one }

  • Laura June 27, 2014 at 7:00 pm

    I have been waterbath canning for a few years, but am nervous about pressure canning. I did buy a canner. But it is still on the shelf. I haven’t used it yet.

    Reply edit
    • theresa July 4, 2014 at 5:21 am

      Yes – Pressure canning is a big step. It definitely takes time to get comfortable with it. I will have to cover some pressure canning posts here soon.

      Reply edit
    • Sue Prewitt August 22, 2014 at 10:41 pm

      Once you do, you will be amazed it is so easy and wonder what you were nervous about.

      Reply edit
      • theresa August 23, 2014 at 5:19 am

        Hi Sue,

        I have oven canned. I did it before going through training as a Master Food Preserver many years ago. I totally agree it is easy. But I won’t do it anymore for all the reasons above.

        Reply edit
  • Suzanne Lucero June 28, 2014 at 10:53 am

    Thanks for writing about this. I’ve never heard of oven canning before; now that I have, I’ll be able to recommend against it and use your common-sense arguments against the practice when asked.

    Reply edit
  • Anna June 28, 2014 at 6:44 pm

    I can see why some people would try it because they already HAVE an oven but they’d have to buy something to can the normal way. I’m not saying it’s a good rational but I can can see some people thinking that.

    Reply edit
    • tash August 15, 2014 at 10:57 am

      I agree. I think people do oven canning (or want to do it) because they don’t have a pot big enough to can in. My biggest pot (prior to buying a special canning one) was barely big enough to can 4oz jam jars. And then you have to find a place to store such a large contraption. However your oven is already there and it already has a place to live when not in use.

      Reply edit
  • Brenda Wagner June 29, 2014 at 2:31 pm

    I have a gas oven and oven canning to me would seem like the same thing as putting them on the stove top above the flame. I see broken jars in the future as you have pointed out. Love your website!

    Reply edit
    • tash August 15, 2014 at 10:59 am

      I don’t understand. As pointed out above it’s completely not the same, no matter if you use gas or electric putting something in a dry oven is totally different then putting something in boiling water. Next time you make chicken boil one and bake one, are they different?

      Reply edit
  • Jen August 10, 2014 at 1:16 pm

    When I began canning I read about oven canning and that’s the way I can :) My jams, chocolate sauce, tomato sauce have all turned out just fine, plus I gotta say when I read/hear about a government agency “recommending” something, I tend to do my own research as I can’t help but believe there’s an ulterior motive with them. Happy canning!!

    Reply edit
    • theresa August 15, 2014 at 11:52 am

      Jen – I understand your reluctance to believe a government agency. But even the manufacturers of the jars tell people not to oven can. I’m glad it works for you. But I feel more comfortable following the recommendations of the people who make the jars. Besides, I don’t find water bath canning difficult.

      Reply edit
  • Amber August 22, 2014 at 11:08 pm

    We do oven canning of dry goods, such as dry corn, beans, and rice. In this case, you’re not trying to cook anything, you simply want to kill any bug eggs that might be in the dry product, as happens with most bulk dry products, and provide a vacuum seal that will exclude air and moisture from getting to the dry product during storage. The oven canning process usually will drive out any extra moisture that has accumulated in the product since it’s been dried. I know a lot of people store these products in plastic buckets with mylar bags and oxygen absorbers, but half gallon jars work rather well for smaller quantities of the bulk product.

    Reply edit
  • Jennifer @ Recipe for Random August 23, 2014 at 8:53 am

    I had wondered about oven canning. Thanks for the education.

    Reply edit
  • Becky August 25, 2014 at 8:30 am

    I’ve never heard of oven canning. Wonder if it’d be any safer to put the jars in a water bath and then can in the oven?

    Reply edit
    • theresa August 25, 2014 at 3:05 pm

      Hi Becky,

      No, the issue is that the internal temperature of the contents does not reach the proper temperature as fast in the oven. So you really don’t know how long is long enough. In other words, you can just switch out a 10 min water bath processing time for 10 min in the oven. Also, the temperature fluctuations can happen as you remove the jars. People still try to get around all those issues though. I find it easier to just water bath can.

      Reply edit
  • Melissa August 26, 2014 at 11:32 am

    I was taught that it was OK to sterilize & heat my jars in the oven (then finish off in the water bath), which works well for me because my pot is slightly rusty and sterilizing in the boiling water leaves a residue on and in my jars- defeating the point of have them sterile prior to filling. I’ve been doing this for years and never had an issue. Is this method unsafe?

    Reply edit
    • theresa August 26, 2014 at 7:29 pm

      Hi Melissa,

      You ask a great question.

      When it comes to sterilizing jars, you can skip the step completely if you are water bath processing the food for 10 minutes or longer OR if you are pressure canning the product. Both of those situations will sterilize the jars as they process. The only time you need to sterilize is if you are processing for something for a short time (like 5 min in a water bath).

      You can heat the jars in the dishwasher to warm them but this does not sterilize them. Just run them through a cycle and keep the door closed as you start to prep your food. They stay hot for a good hour.

      If you have a recipe that is less than 10 minutes in the water bath, you have two choices.

      1) You can increase the water bath canning to 10 minutes so that you don’t have to sterilize at all. You can do this with no problem in most cases. The only time it is an issue is with pickles which can lose crispness with longer processing.

      2)You must sterilize for boiling the jars for 10 minutes. There really isn’t any other approved method. Now, do I think you will fall over dead by not boiling the jars. No. You shouldn’t get sick from skipping the boiling. What you are trying to prevent is spoilage and molds.

      Using the oven is not recommended by the makers of the jars. Plus, you really are only heating them – not sterilizing them. To truly sterilize them you have to bake them at a high heat and that can cause breakage.

      So – the bottom line is that I would recommend that you just increase your processing time to at least 10 minutes. Then you only need clean, warmed jars. It is the easiest solution and saves you a step. :-)

      Reply edit
  • ann cronin September 6, 2014 at 10:30 pm

    I was wondering if it’s ok to sanitize your jars in the oven. Then fill and process using the water bath method.

    Reply edit
    • theresa September 8, 2014 at 5:58 am

      Hi Ann,
      The makers of the lids do not recommend heating in the oven because it dries out the rubber sealant (the plastisol) on the lid. It can give you more failures if dried out or if it gets micro-cracks.

      Reply edit
  • Angela Carberry September 21, 2014 at 9:59 am

    Just came across this site and I’m surprised no one has mentioned the method by which I’ve been making jams and jellies for years, which is. I sterilize my jars, put in the contents and allow them to seal at room temperature. I tried putting them in a water bath once and ended up with several leaking and a real mess. Now I’m wondering if I’ve potentially been putting everyone at risk with my gifts?

    Reply edit
    • theresa October 23, 2014 at 5:32 am

      Hi Angela,
      The method you describe is not an approved method because it does not create a strong vacuum seal. The jars must be water bath processed to expel the air and pull a proper vacuum on the jar. Because you have been doing this with jams/jellies, you are not at risk for botulism because of the high acid content of your contents. So you are okay there. However, you are at risk for molds and other bacteria (some molds are considered carcinogens)because you have a weak vacuum seal. It is a good idea to do the water bath method in the future. The stronger vacuum seal also greatly reduces the risk of losing your seal over time.

      Reply edit
  • Lynda October 4, 2014 at 6:24 pm

    This year I oven canned zucchini bread in quart jars while the lids were tightly screwed on. Do you think this unsafe to eat , also?

    Reply edit
  • Pat October 27, 2014 at 7:24 pm

    I saw a recent post on another blog (can’t remember which) that talked about oven canning. But that was for dry goods, dehydrated, not jelly or anything ‘wet’. It was only for dry canning, nothing else. It didn’t, tho, tell of the problems with oven canning like you did. Thanks so much, I had wondered.

    Reply edit

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