Q. What is open kettle canning and why is it considered unsafe?
Open kettle canning consists of filling (hopefully) clean jars with hot product and placing lids and screw-top rings on the filled jars. After the rings are tightened, the jars are inverted and left to cool. The heat softens the sealing compound on the lids and, as the jars cool, a vacuum is created and forms an apparent seal.
There are several reasons this method is considered unsafe.
Open kettle method omits the important step of processing, which ensures that the contents of the jars are sterile and the jars are properly sealed. Remember – just because a jar is sealed does not mean that its contents are sterile! The jars need to be kept at a sufficiently high temperature for a certain length of time to achieve this.
Inverting the jars can prevent the sealing compound from adhering to the glass. Proper processing involves keeping the jars upright throughout the heating and cooling phases, so the contents don’t interfere with the sealing compound.
Don’t assume that if the jar is sealed to the point that you cannot pry the lid off without a can opener, the contents within are completely germ-free. Even a few bacteria can multiply rapidly and cause spoilage inside a sealed jar. In a low-acid product, botulism contamination is a real (and potentially deadly) risk. Mold often develops on jams and jellies that have been subjected to open kettle canning. If mold is visible, even just a small amount on the top layer of product, the entire jar should be discarded. Fungi produce microscopic threadlike structures called hyphae that can produce toxins, rendering the jar’s contents unsafe to eat.
Any product that is going to be stored in jars at room temperature must be processed in either a boiling water canner, steam canner, or a pressure canner. Only food considered high acid can be processed in a boiling water or steam canner. Always follow a tested and approved recipe when doing any type of canning. Sometimes it can be confusing to determine whether a recipe is safe and approved, especially when searching online. Using AI to search for recipes can be especially perilous since it draws from both safe and unsafe online sources. As you can imagine, the unsafe sources vastly outnumber the safe sources. Safe recipe sources include the Ball or Kerr websites, USDA, any university or cooperative extension website (which should end in .edu), the National Center for Home Food Preservation, or Healthy Canning.
Many influencers, prepper and homesteading sites, and even many published authors are not necessarily safe sources.
Why should you only use tested and approved recipes from reliable sources? The approved recipes have undergone extensive testing under a variety of conditions, allowing a margin of error for safety. The food testing labs have instruments that can measure heat penetration as well as changes in acidity that can occur with ingredient substitutions. I recommend checking the online sources periodically since safety recommendations change as new testing results become available.
Los Angeles County
mglosangeleshelpline@ucdavis.edu; 626-586-1988; celosangeles.ucanr.edu/UC_Master_Gardener_Program/Orange County
ucceocmghotline@ucanr.edu; mgorange.ucanr.edu/
Riverside County
anrmgriverside@ucanr.edu; 951-955-0170; ucanr.edu/sites/RiversideMG/
San Bernardino County
mgsanbern@ucanr.edu; 909-387-2182; mgsb.ucanr.edu
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