Well you certainly imparted some info I didn't know Cathy. I have not canned in a few years and didn't know I couldn't use my glass cooktop to can. I use my pressure saucepan to cook on it quiet frequently.

Beverly

I wonder if it is only some glass cooktops. In our home from 1977 to 1980, I had one of the very early Corning glass cook tops. The home came with it along with a bunch of fruit trees and a large garden area. I canned on a lot - mainly hot water bath canning of jams and fruits but also some pressure canning of vegetables. I didn't have any problems with seal failure or spoilage.

On the other hand, our current home had a glass cooktop until we redid the kitchen a few years ago. Three of the burners were "smart" burners with a temperature sensor to control the temperature of the pan -- in theory. In practice they were lousy for anything without a perfectly flat bottom - just wouldn't heat well at all because they would get the air between the pan and the burner up to temp and think they had done there job. If your burners are like that, they would interfere with canning.

The fourth burner didn't have the temperature control and could get super hot. I think that I could have achieved a good boil for canning on that burner.

(I'm skeptical of the limitation being to keep the glass from cracking because the non-temperature burner on mine could get so hot and the top never cracked. Maybe only some brands use glass that has the problem.)

Cathy, You are such a good student. I learned that I can't bake really great bread like Teresa...

I have a glass cook top in the butlers pantry. Cooked on that thing for years but never canned on it. I guess I need to go to NC with you and learn how to can Murcotts. Ummm, BTW, WTH is a Murcott?

There is a big difference in glasstops. I intensely disliked the one I had before moving where I am now. I don't mind my new one at all. It doesn't cycle so much as the old one and heats up faster too.

My glass cooktop is 18 years old; it does cycle on and off but obviously it hasn't broken even with my uninformed treatment of it. I don't know if the Ceran glass is the newer glass; that's something I'm going to have to look into.

Karen, A Murcott is a "Honey Tangerine"; it's small to medium size, smooth easy to peel with a lovely fragrance. Some seeds, rich, red flesh is sweet as honey, and very juicy! packed with our deluxe items.

I've got one of those new fangled induction tops. I made Habanero jelly and peach jelly on it this summer. It doesn't cycle on and off like the old electric burners. Love my induction!

Cathy, you can come here to can anytime and like Karen I need to go to NC to can the Murcott jam for Dad. He loves the stuff and is hoarding it now that he is low.

There is a big difference in glass cooktops and some can be canned on, some can't. There's also a problem with pan size, if you pan is bigger than the size of the burner (or the sensor in the burner), that will also make it cycle on and off and not reach the necessary temperature within the specified amount of time. That's why so many people on Harvest buy free standing burners, either propane that they can set up outside, or a higher wattage electric burner, just for canning.

I have a cheap gas cooktop, but it works fine for canning. You know you can come here. Or maybe we should all just go to Sherry's again, LOL.

Annie

FYI

1. Headspace...

The amount of headspace is specific for each food. Improper headspace can lead to an improper seal, contents boiling out of the jar, or discoloration of the food.

- Low-acid foods, vegetables, meats: 1-inch headspace

- High-acid foods, fruits and tomatoes: 1/2-inch headspace

- Juices, Jams, Jellies, Pickles and relishes: 1/4-inch headspace

Jars should be filled with enough liquid to completely cover the food. Remove any air bubbles from inside the jar by running a plastic or rubber knife into the jar to remove trapped bubbles. More liquid may need to be added after this step.

2. Methods of packing jars...

Along with proper headspace, you'll get less air in your jars if you use the hot pack method when filling your jars. When you use a raw pack method, pack the jar as tightly as possible because air will be lost during processing causing the food to shrink. The exception to this is when canning corn, lima beans, potatoes and peas. They should be loosely packed because they absorb liquid and expand during heating.

Hot packed foods should be loosely packed because shrinkage or air loss has already happened.

3. Glass/smooth range top...

The range top used for canning should have a very large burner to provide enough heat to boil a large volume of water. It is recommended by researchers at the University of Missouri that ceramic or smooth-top ranges not be used for home canning because the bottoms of canners may not be entirely smooth, and even heating will not occur. If the pot is larger than the burner area, the heat may discolor the stove surface. High heat may actually cause the cook-top to fuse with the pot or pan.

-Grainlady

Thanks for the additional learning and offers. The instructor recommended several alternatives like the freestanding burner. She also suggested buying an old stove to put in the garage--that ain't gonna happen! I did some googling and found a 1300 Watt burner that had good reviews from canning people AND it was $50 off, so I ordered.

Grainlady, She did say I needed only 1/4 inch headspace for my jam just as you mentioned. I had about an inch in my quart jars.

Regarding headspace - Obviously there will be an entirely different product to air ratio with headspace of a given height in a 4oz jelly jar versus in a quart jar. There will even be a significant difference between narrow and wide mouth pints, etc.

I've often read, as here, that proper head-space is important but I've rarely if ever seen a recipe state what size or shape jar should be used. Is there something magical about height over volume ratios, or is there a standard jar that one should use unless otherwise stated in the recipe?

Might want to add: low acid foods require pressure canning.

Food: The headspace requirement is the same no matter the shape of the canning jar. If you are using approved recipes they state what size volume canning jar to be used. You may reduce, but not increase volume, such as jelly in a 4oz jar as opposed to using a standard jelly jar of 8ozs. A quart of jelly is not recommended for home cannning and alot of jelly!
When I am canning, say, spaghetti sauce, I normally use pint (16oz) size jars, but I also have ready 1/2 pint (8oz) jars if there is not enough product for proper headspace in the pint jars.
I always have 4oz jars ready when making jams and jellies in addition to the standard 8 oz jars.
Hope that helps.

Thanks for the discussion.
I mentioned the pint and the quart only because I filled the pints to higher level because it seemed like such a little amount and I wanted to be generous as these were gifts. That was my only silly rational.

Re FOAS's comment about headspace, there is an alternative point-of-view. I think agencies like the NCHFP don't bring this up because learning about food preservation is complicated enough for a lot of people, without adding to it. A simple rule-of-thumb re headspace (as cited by Grainlady) is sufficient.

But if you're interested in something with a bit more precision, here it is:

I don't have time to type out the entire passage, but Beverly Alfeld, who has devoted a lot of time to researching canning issues and safe canning practices, has a different take on the issue of headspace. At the very least, she provides food for thought:

In order to safely seal a canning jar, an adequate vacuum needs to exist in the air-space between contents and the lid. That air-space needs to be about 6 percent and never less than 5 percent of the entire jar's volume. The contents or food product in the jar will then take up about 94% of the jar's space. Some books speak in terms of inches . . . but this measure is not always an accurate measure, because the shape of the jar, the circumference of the rim, and the height of the jar will all impact the headspace. A tall, slender, 8-ounce canning jar filled to 94-percent capacity would not have the same fill requirements in inches (from product top to jar top) as a short, squatty, 8-ounce canning jar filled to 94-percent capacity. So, the percentage of fill in a jar is a better indicator of how far to fill a canning jar than the number of inches, or fractional inches, from the top of the canning jar to the prepared product fill line.

If consistently using one type of canning jar, determine the correct level of the fill line, and fill all identical jars in the same way each time you use them. . . .A regular-mouthed, upright quart might have a headspace of 3/4 inch to 1 inch, depending on the style of jar you have and if the contents are chunky and more likely to float up. A jardiniere that is likely to expand might be filled with a slightly larger headspace than, say, applesauce. Be careful not to overpack a jar of jardiniere. It is better to under pack than to overpack.

Excerpted from a canning and recipe development manual, Pickles to Relish by Beverly Ellen Schoonmaker Alfeld.

Having said that, consider the practical implications of too-little or too-much headspace. If the headspace is too great then with a product like jam there is a greater likelihood of oxidation. With a full-sugar jam, the likelihood of spoilage is slim, even if the jam weren't processed, so the main "risk" is an aesthetic one. (Perhaps mold, depending upon conditions.)

With a product which expands, like pie filling, too little headspace results in siphoning and a greater likelihood of seal failures.

Carol

Ok, I'm going to ask what is probably a stupid question -- not because I intend to can (the idea scares the bejebus out of me due to some of my sister's spectacular failures...botullism anyone?), but because I'm just darn curious: What is/was the purpose of paraffin wax in canning? My great aunt often used it, but my grandmother never did.

Renee, I'm not an expert on the subject but I know my DGM used the wax on her jams and jellies. I think they did that instead of HWB to seal the jam/jelly. Pour hot wax on top and once it cooled it sealed the stuff in the jar.

Cathy, glad you learned some new things in your class. I've been lucky with my smooth top so far. It's the one I bought up at the cabin and is now my back up range out in the garage. The right front burner expands to twelve or thirteen inches and is perfect fit for my big stock pot that I HWB with. I'm sure I would have challenges with a normal canner as they don't have a flat bottom.

David