Atmospheric Steam Canning

A few years ago I picked up a non-pressurized steam canner.  It was a model that's no longer made; initially, I was primarily interested in it because it was stainless steel and held eight quarts.  My graniteware canner held seven quarts, and when filled with enough water to cover quart jars, tended to gurgle water a bit.  I had to deal with boiling water bubbling up under the lid and out.  It wasn't the most fun to contend with.

The canner I got had a temperature gauge on the lid and instructions for steam canning, so I gave it a try and it was much easier.  At the time the National Center for Home Food Preservation (NCHFP) site warned against non-pressurized (or atmospheric) steam canning, saying they had been inadequately researched.  Still, for acidic foods it seemed to have potential.  The important thing needed for acidic food processing is to get the contents of the jars to boiling (actually, somewhat less than that is okay) for a few minutes.

Atmospheric steam canning is more convenient and quicker.  Less water needs to be heated, making the process faster and the canner lighter in weight than it is when water-filled, though the weight factor isn't as important as it might seem--it does not take much water to fill the canner up when a full load of jars is being processed, since the jars displace so much water themselves.

I tried a few loads with the steam process and noticed the jars in fluid things (e.g., pickles), was boiling when I took them out of the canner.  That suggested the steam got the jars to an adequate temperature. I processed most things a few extra minutes when steaming.  The canner also can be used for boiling water processing--though some steam canners cannot, since the lid fits on the body below the level of the top of the jars.

I then ran across a peer-reviewed study:  Paola Whitmore, et al., "Home processing of acid foods in atmospheric steam and boiling water canners." Food Production Trends 2015; 35(3):150-160. I had seen another article previously.  Essentially, Whitmore and colleagues put thermocouples in canners and in jars to measure temps with boiling water and steam processes.  The temperatures reached during processing both ways were essentially the same.  They included temperature graphs that show how slowly things cool off--which you have probably noticed if you have canned--and indicated that steam with equivalent processing times to boiling-water canning was fine.  The NCHFP now agrees, citing the Whitmore article.

Canning is probably not an area where a  lot of grant-funded research is being done these days, so this might be the only article on the subject for a while.  This process can't be used for low-acid foods, since they need pressurized steam (temps of 240 F) to kill botulism spores.  However, steam for acidic foods at normal pressure lightens the load a little when processing.