"Dry Canning" is Not Recommended

If you have heard the term "dry canning," it's most likely in the context of canning dry foods such as grains, rice, beans, or nuts for long-term storage. The other popular reference to "dry canning" involves pressure canning vegetables without added liquid. Neither of these are recommended methods of home food preservation!

Dry Canning of Dried Foods

This is putting dry foods such as flour, dry beans, nuts, and other similar products into mason-style jars, placing lids on the jars, and then heating the filled jars in an oven set around 200°F for a period of time. Or jars of food are placed in the oven without lids, with lids placed on the jars when they come out of the oven. In either case, a seal is formed; however, it is not a true vacuum seal as achieved during processing in a water bath or pressure canner. While this may seem like an excellent way to store these foods, this storage method is not research-tested and can result in spoiled food or potential illness if consumed.

Dry Canning Dry Food Issues

The National Center for Home Food Preservation Preserving Food at Home blog article, "Dry Canning Isn't Canning to Me," talks about the safety issues of this process.

• Without testing, we do not know if this is a time and temperature process that will kill bacterial spores or mold spores.
• All foods contain a certain amount of moisture (water). While this process will cause moisture to move to the surface of the food where it evaporates, dry food may retain pockets of moisture within the food. These moisture pockets in a sealed jar could support the growth of mold or bacterial spores such as Clostridium botulinum and Salmonella, especially if home-dried foods are canned.
• There is currently no research to support this process as a method to extend the shelf life of dry foods.
• Nuts and grains may go rancid more quickly as the heating process increases oxidation.

Long-Term Storage of Dry Foods

If you want to store dry foods for the long term, Utah State University Extension's publication "A Guide to Food Storage for Emergencies" is an excellent resource. The publication describes appropriate methods for heating and cooling dry foods before packaging for insect control as well as freezing as an alternative method. Their website has information specifically on storing dried milk, dry beans, rice, sugar, and wheat as well as other foods.

Dry Canning of Vegetables

In dry canning vegetables, the vegetables are placed in the jar (hot or raw) with no added liquid and then processed in a pressure canner according to the processing times and pressure from a recipe that calls for added liquid.

When canning low-acid foods, such as vegetables, it is critical to precisely follow a research-based, tested recipe to ensure a safe final product. This includes preparation of the vegetable, filling the jars, the amount of liquid to add, headspace, and amount of pressure to process at based on altitude.

Dry Canning Vegetables Issues

Both the National Center for Home Food Preservation "Dry canning raw vegetable is an unsafe practice," and Dr. Barbara Ingham's "Unsafe canning practice: 'dry canning' vegetables" articles outline the following reasons why this is extremely dangerous because of the potential for botulism poisoning.

• Liquid added to the jars is critical for proper heat transfer during the processing of the food. The liquid allows for convection heating. As the liquid in the jar heats in the canner, it begins to circulate, resulting in movement within the jar. This movement of the hot liquid allows for a quicker, more efficient transfer of heat throughout the jar, including the coldest spot in the jar. The processing time of tested recipes is based on this method of heating.
• Without added liquid, heat is transferred via conduction, a slower method of heat transfer. In this case, the food is heated in layers. For example, the outer layer of the vegetable (near the outside of the jar) gets hot, the heat from this layer transfers to the next layer, and so on until the innermost layer is heated. Without testing, we cannot say that this process will occur in the same amount of time that it does when liquid is added since the process is much slower.
• Bacteria and bacterial spores (Clostridium botulinum) are more sensitive to wet heat and will be killed more rapidly. If vegetables are canned without the added liquid, the possibility exists that the C. botulinum spores may not be destroyed. In the jar's low-acid, anaerobic environment, the spores may become vegetative and produce the botulinum toxin. The toxin does not change the appearance of the food, the smell, or the color, and it only takes a very, very small amount to cause illness.

What Does All This Mean to Me?

Canning is truly a science; therefore, it is essential to follow and use tested and research-based recipes from reputable sources of information. These include the USDA Complete Guide to Home Canning, National Center for Home Food Preservation website, So Easy to Preserve from the University of Georgia, and Extension publications such as Penn State Extension's "Let's Preserve." When using other sources and social media, review the information critically and verify with reputable resources.

Remember, when canning, safety must come first!

References

Ingham, B. (2020, June 18). "Unsafe canning practice: 'dry canning' vegetables." University of Wisconsin-Madison Division of Extension. Retrieved August 4, 2021.

National Center for Home Food Preservation. (2020, April 16). "Preserving Food at Home: Dry canning isn't canning to me." Retrieved August 4, 2021.

National Center for Home Food Preservation. (2020, June 25). "Preserving Food at Home: Dry canning raw vegetables is an unsafe practice." Retrieved August 4, 2021.

Brennand, C. P., & Hendricks, D. J. (n.d.). "Food Storage in the Home." Utah State University.