The University of Wisconsin-Madison conducted research showing that an atmospheric steam canner may be used to safely can naturally acid foods such as peaches, pears, and apples, or acidified-foods such as salsa or pickles. The steam canner uses only ~2 quarts of water (compared to 16 quarts, or more, in a boiling water canner) so you heat less water and processing can start more quickly. We tested two types of canners in the laboratory, a Back to Basics canner (pictured left) and a Victorio model (pictured right). These are the only two types of steam canners that the University of Wisconsin has tested as safe for home canning of acid foods, other canners have not been tested and are not recommended at this time.
What does a tested steam canner look like? The two types of steam canners that we tested in our laboratory have a shallow base and a tall domed lid. Jars of food sit on a rack in the base, above hot, boiling water, and are covered by the domed lid. There are vents holes in either side of the lid, just above where the lid joins the base, where steam vents during the canning process. The only difference in the two types of canners that we researched is that the Victorio model that we tested had a knob on the top of the lid with a temperature gauge that indicated approximate temperature within the canner; in our research this gauge appeared to be quite accurate.
Can you simply adapt a boiling water canner as a steam canner? I have gotten many calls and emails asking if a consumer can adapt a boiling water canner (BWC) to a steam canner by placing a rack in a boiling water canner, adding a limited amount of water, and waiting for steam to escape from the lid atop the canner. The University of Wisconsin does not recommend this approach. In order for any canning process to ensure safe food is produced, there must be enough heat for a long enough time to kill pathogens and spoilage organisms that would make the product unsafe when stored on the shelf. We tested both models of canners using thin thermometers, or thermocouples, tracking temperature at several points inside the canner and inside containers of several types of food to verify that the recommended canners and recipes would produce safe food. We compared our results to a boiling water canner that was used as recommended. For other steam canner styles or adaptations of a boiling water canner to be proven safe, a researcher would need to test the canner in a laboratory to ensure a safe product can be produced.
The University of Wisconsin recommends the following guidelines for using either of the steam canner models that we tested.
Guidelines for using a Steam Canner for Home Food Preservation*
- Foods must be high in acid, with a pH of 4.6 or below. Foods may naturally be high in acid (most fruits) or have added acid. Either a Boiling Water Canner or a Steam Canner may be used to safely preserve foods high in acid.
- An up-to-date, research-tested recipe is used. Approved recipes for boiling water canning may be safely adapted for use in a steam canner. Acceptable recipes are available from sources such as the National Center for Home Food Preservation or in the Wisconsin Safe Food Preservation series.
- Make the following adjustments to an approved recipe for a boiling water canner: at the processing step, place filled jars on the canner rack above hot/preheated water. Place the lid on the canner and heat, on high, until the canner vents. A full 6-8” column of steam will flow out of the vent holes in the canner. Once the canner continuously produces a full column of steam, start timing. Process time is based on the time for a boiling water canner. Adjust heat, as needed to ensure the canner vents during the entire process time.
- Jars are processed in pure steam at 210-212°F. Steam should flow freely from the canner vent(s) during the entire process, or the food is considered under-processed/unsafe. You may wish to insert a thermometer in the vent port during a test run to check processing temperature.
- Adjust processing time for elevation. Add 5 minutes to processing time for each 1,000 feet above sea level. Check your elevation at any location in the world using this handy tool.
- Jars must be heated prior to filling and filled with hot liquid (raw or hot pack). Jars of half-pint, pint, or quart size may be used, depending on the jar size acceptable in the recipe.
- Processing time should be limited to 45 minutes or less, including any modification for elevation. The processing time is limited by the amount of water in the canner base. When processing food, the canner should not be opened to add water. Regulate heat so that the canner maintains a temperature of 210-212°. A canner that is boiling too vigorously can boil dry within 20 minutes. If a canner boils dry, the food is considered under-processed and therefore potentially unsafe.
- Cool jars in still, ambient air. Jars should be cooled on a rack or towel away from drafts.
*These recommendations apply only to the Back-to-Basics or Victorio models of steam canner pictured on this web page. We did not test the safety of other models or styles of steam canners and can make no recommendation for use of other appliances. For more food safety and food preservation updates, subscribe to the blog. Stay well and preserve safe food! Barb