Oh, boy! More thermodynamics! This is gonna be fun!

I can't give you the ultimate answer to this question. I can only point out a couple of facts about heat transfer and raise a question about steam vs. water bath.

As you say, "they don't take as much time to heat up because there's less water". There is also a change of state involved. Additional heat is used in changing the water to steam. When the steam condenses that heat is transferred to the surface it condenses on, which can be the jars, but also the body of the canner. This is a constantly changing situation because as the jars become hotter, the canner dissipates a larger proportion of the heat and the jars absorb less. The question is, do jars in a steam canner heat up to the critical temperature at a rate similar to that of a water bath canner, where a large supply of heat is contained in the water which is in contact with the jars? It's difficult to compare the two processes.

The two canning methods could be compared analytically, but I think that's beyond my ability and resources. Maybe Melissa can do it. I think she has some physics and engineering training.

A small amount of testing has been done which showed steam canning to be effective, but not enough to satisfy many skeptics. It would be great if steam canning could conclusively shown to be safe because it has some advantages in the realm of convenience over water bath, or pressure canning for that matter.

Jim

No, according to USDA/ extension studies, they are not recommended for use. They used to be listed in old canning books, but the recommendation was pulled many years ago by Ball and USDA.
The only ones you will find endorsing them now are the ones making and selling them.
The center of the contents of jars of food processed in a boiling water bath will be hotter than those in a steam canner, unless of course, you are talking about a pressure canner.
I have a whole list of reasons they are not recommended for use. I would have to retype the whole document. It is in one of my food safety training manuals, not online.
So, if you call the extension office here, you will be told they are not recommended as a safe form of preservation.

From Foodsafety link below.

Canners and canning methods that should not be used by consumers

Steam Canners The steam canner was designed as a means to process foods using steam without the aid of pressure. The manufacturer claims this process uses less water, saves time and energy, and recommneds identical processing times as those required for boiling-water bath treatments.

Studies have concluded that:

Atmospheric steam canners result in significantly lower product temperatures at the beginning and end of the scheduled process when compared to water-bath canning.
Use of steam canners as instructed by the manufacturer would result in under processing and considerable economic spoilage.

From MSUE food safety. Other sites will say the same thing:
http://web1.msue.msu.edu/imp/mod01/01600940.html
EQUIPMENT AND METHODS NOT RECOMMENDED

--------------------------------------------------------------------------------

Open-kettle canning and the processing of freshly
filled jars in conventional ovens, microwave ovens, and
dishwashers are not recommended, because these practices do
not prevent all risks of spoilage.

Steam canners are not recommended because processing
times for use with current models have not been adequately
researched. Because steam canners may not heat foods in the
same manner as boiling water canners, their use with
boiling-water process times may result in spoilage.

From Washington State University food safety :http://clark.wsu.edu/family/FAQs.html#steam

Can steam canners be used in place of boiling water bath canners?
Research on steam canners has found that food canned in them is not heated to a temperature as high as when the same food is canned in a boiling water bathe canner. The lower temperature results in less killing power of the bacteria, under processing and considered a risk of spoilage. Therefore the use of seam canners for home canning is NOT recommended.

In addition to unsafe canning temperatures steam canners can cause steam burns to those using them, and increased steam burns.

Here is a link that might be useful: Steam canners.

Thank you so much. I thought my instinct was right. We all would be using that way if it were any good. Here's a spot I found them : "Steam Canner
Take the 'sweat' out of home canning and eliminate heavy lifting and messy clean ups.
$44.95"Steam Canner

Saves time, energy and water! This steam canner is so simple and easy to use. It takes the "sweat" out of home canning! Uses only 3 pints of water and cuts pre-heating time by 50%! Eliminates heavy lifting, boil-overs and messy clean-ups. Use in place of a water bath canner when canning high acid foods. Use year-round - the cover doubles as a 15 qt. kettle or stockpot. Heavy gauge aluminum. 7 quart capacity. Dimensions: 11-3/4" dia.; bottom is 1-3/4" deep; lid is 6-1/4" high. (Canner is not recommended for glass or ceramic top stoves - it does not have a flat bottom.) LINK: http://www.kitchenkrafts.com/product.asp?pn=FP0400

I've been using a steam canner of the type described for over 10 years. I can only pickles and tomato products--juice, sauce, ketchup--and have never had any trouble. The great advantage to me is that I can do only two or three jars at a time without having to heat a huge kettle of water. I know they're not fully approved, but I really feel that they're safe enough.

Julie, you hit on a hot topic here and one that seems to be discussed at least annually.

The USDA says it's not recommended, partly because there hasn't been enough testing to determine the safety or even the time necessary to process different products.

Some think it's safe enough and use them anyway. Some of us don't choose to use them until enough testing has been done to provide us with the information we need to make us feel comfortable.

It's another one of those "choice" things. I prefer not to use them until sufficient testing is completed (if that ever happens). In the meantime, I have to boil water anyway to sterilize my jars, so I just sterilize the jars in my canner and use that water to process them.

Annie

I've used one for several years now for high-acid foods and wouldn't go back to a BWB bath. They've been tested by two universities and found to be safe but the USDA doesn't yet recognize that research. As Annie says, it simply comes down to personal choice.

I've used a steam canner for over 10 years and like malonanddonna, won't go back to the BWB. Also, like laceyvail, my garden doesn't produce large amounts at a time, and doing up a few jars of this and that in the steam canner really appeals to me. So fast, so easy, so light weight you don't mind at all. I usually go a bit beyond the recommended times when steam canning, and always follow the recipe to the letter and Never use it if a pressure canner is called for. I am looking down the gun barrels of 70 now and wrestling with the BWB is not my idea of a good time.
Just a happy steam canner user....
Beebonnet

Ken did a test a year or so back using thermocouples in the jars. His results indicated that the method was OK.

His test procedure was such that I could find no fault with it.

I use one for jams and pickles. (I don't do tomatoes)

Is Ken's test documented on this site? I would like to see it.

Jim

Jim, Ken's "test" is somewhere in the archives. It involved him testing the temperature of the food inside the jars. The real problem is that he did two tests and got widely varying results, which he blamed on his methods during the first test.

In addition, that test was done in Ken's kitchen, with no controls, no laboratory conditions, no other tests to compare with, just Ken, his jars, his steam canner and his various testing devices.

Annie

Thanks, Annie. Two runs of a test would be barely getting started. It would take several runs just to get the kinks worked out of the procedure before any serious data could be collected. After that, you would need many runs to establish averages and variances and to test with different types of foods. That's a huge amount of work when you don't have grad students to do it for you. :-)

Jim

I LOVE my steam canner for jams, salsa, ..... stuff that is packed hot and takes just a few minutes to seal.

I also LOVE it for sterilizing my jars, it heats up so fast that I dont have to wait so long. I can prepare my jars then steam them sterile while I prepare my food.

But for the larger jars of veggies, I trust my water canner.

I'm learning to use my pressure canner for meat products. THAT - I see - is a whole new ballgame!

Jackie

Jim, I am in engineering school, but I hate heat transfer and don't pretend to understand a whit of it, except this whit right here: at the boiling point of water (where water & steam can exist together), the thermal conductivity of water is .65 watts per meter-kelvin, and the thermal conductivity of steam is .3 watts per meter-kelvin. Because heat transfer by conduction (touching) is proportional to the thermal conductivity, the water transfers heat twice as well. This is why you'll survive streaking the quad in the winter, but if you fall in an icy river you're headed for hypothermia as the heat is transferred out of your body lickety split. So the results Linda Lou cites make sense.

I think steam canning falls somewhere between open kettle (no boiling, just sealing the hot jar) and boiling water bath. I would be willing to eat high-acid products that were open kettled or steam canned, such as jam and pickles, but I don't think it's worth the risk for my own canning. It is definitely dangerous for low acid (which no one here is arguing anyway) but I wouldn't use it for tomatoes either, they are too borderline in my opinion. For things that will mold or go soft, who cares, you might lose a steam canned jar, but then again I might lose a BWB jar myself. To each his own, as long as newcomers can find safety info for each method & the id of the testing entity. I trust the USDA, but to me trusting a steam canner manufacturer's tests is kind of like writing a scholarly paper & citing a glossy magazine "journal" full of ads--it may be a perfectly good source, but a national research group lends more creedence to my paper's argument.

I'm glad to see that steam canner users can state their opinions & experience without being attacked! Maximize the info, cite the sources. What a pleasant discussion to be a part of.

Jim, if you have funding for graduate students in the field of canning, please send me a fellowship application.

Melissa

"Jim, if you have funding for graduate students in the field of canning, please send me a fellowship application."

Sorry, Melissa. I wish I could help, but I can't. However, your interest in canning should liven your coursework. Maybe when you study finite element analysis (if you haven't already) you can figure out the steam canner thing. If so, you will be far ahead of me.

Your info on thermal conductivity of water vs. steam is a good start. It sort of confirms my intuition that a water bath will heat the jars more quickly than steam, actually twice as fast. Also, the greater volume of water in the water bath creates a much larger heat source. Of course there are other factors to consider, but those are the big ones.

Jim

I would like to again cite my sources for research. The manufacturer cites a study conducted by Utah State that I have not read in advertising the safety of steam canners. I assume the manufacturer sponsered that research.

More recent research was conducted by the University of California-Davis. Results from that study were published in March 2005 in an industry publication. The abstract from that study:

Home Processing of Tomatoes and Other Acid Foods in Flowing Steam and Hot Water Bath Canners

MARY SAMIDA,1* LYNN GEER,1 and GEORGE K. YORK2

1-UCCE Master Food Preservers SBDO County, 777 East Rialto, San Bernardino, CA 92415-0730, USA
2-Food Microbiology, Food Science and Technology, University of California-Davis, Davis, CA 95616, USA

SUMMARY
The Cooperative State Research Extension and Education Service of the United States Department of Agriculture (USDA) does not recommend the use of steam canners for canning acid foods, including fruits, acidified vegetables, and pickles, because of a lack of data demonstrating that the use of flowing steam heat in home canning is safe. There are no published data on comparison of time-temperature relationships for steam and hot water bath canners. It must be emphasized that neither of these canners is being recommended for use in canning low-acid foods (with those pH above 4.6).

This research was conducted to determine whether steam could be as safe as a hot water bath for home canning of acid foods. Four foods of different densities, tomato juice, peaches, whole peeled tomatoes and applesauce, were processed. First, the time for the water or steam to reach a temperature of 212F for both types of canner was measured; then, a comparison was done of the time to attain the target temperature of 180°F at the center of the product. The products tested required the same or nearly the same time to attain the target temperature in each type of canner. Thus the processing times are adequate for acid-foods, and the two types of canners are equally safe to use.

That's interesting, Malon. I had not seen that study. It appears to have been well done and to have yielded conclusive results. I should think the USDA would undertake to replicate it. I wish they would. Thanks for posting.

Jim

I would like to see actual references to any studies on steam canner testing. My understanding is not that they have been tested and proven inadequate, but rather that there has not been enough testing to prove that they are adequate. There is a world of difference between those two cases!

The one study by Samida et al. might be interesting, and it indicates that steam canning is equivalent to water bath canning, but I have not had a chance to read anything more than the abstract, so it is hard to judge the quality of the work. There are statements in various places that Dr. Mendenhall of Utah State University tested the steam canner method many decades ago and found it to be adequate. This is found in the Sept. 6, 2005 statement on food safety by Utah State University, authored by Brian Nummer. (Note however that this USU statement is not an endorsement of the safety of steam canners.) However, the only reference is to a 1986 personal letter from Mendenhall which is not reproduced in the statement, so it is hard to know the level of testing that was done. Mendenhall also apparently mentions favorable testing done by the University of Massachusetts, but again no actual reference is given.

I must conclude several things from this: 1) Some testing has been done indicating that steam canning is safe. 2) Not enough testing has been to for food scientists to reach consensus that steam canning is safe. 3) There have been no verifiable tests showing that steam canning is unsafe. 4) I surmise that there is a hesitancy by the USDA and/or other agencies to perform the testing. Whether this is because they are unfunded but otherwise willing to do the testing or they are simply not interested in doing the testing is impossible to say. My hunch tends more toward the "unwilling" side of the spectrum, but who knows?

This was a great discussion, thanks.