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Home bakers, especially those in the growing cottage foods industry often want to know how to can bread or cake in jars. Their goal is to produce a shelf-stable bread in a jar to sell. The expectation is that the baker can bake in the jar and then close it with a canning lid. .The bad news is, breads, cookies, pastries, etc. canned this way are dangerous and a like environment for botulism to grow.
Almost all recipes for quick breads and cakes are low-acid and have the potential for supporting the growth of a bacteria like Clostridium botulinum when sealed in a jar. Put simply, when you seal a low acid food (like breads) in a jar that prevents the free passage of air, you create a perfect environment for botulism to grow. Many cakes and quick bread recipes often have little or no acid resulting in a pH range above 4.6. A pH of this level will support the growth of pathogenic organisms that cause foodborne illnesses.
And don't fool yourself into think that it is sterile because you quickly sealed hot bread in the jar. Botulism spores are airborne and tough. That amount of heating and method would neither kill them nor exclude them. Clemson University's research showed a high potential for problems:
"Clostridium botulinum spores are abundant in nature but will only grow and produce toxin in unrefrigerated high moisture foods that are low in acid and exposed to little or no oxygen. These conditions occur in low acid canned foods; low acid canned foods must be processed under pressure at temperatures of 240°F or higher to make sure that the heat resistant spores are killed. Research at Penn State has shown that low acid canned bread or cake products may support the growth of Clostridium spores."
Sometimes. Not often. The risks are too great even with commercial equipment. And when it is done, you will see these products made commercially with additives, preservatives and processing controls that are not available for home recipes. As a further caution, the manufacturers of canning jars like Ball, Kerr, Mason don't endorse baking in their canning jars. Home canning jars are intended for use in hot water baths or pressure canners. They are not designed to withstand the thermal stresses that occur with dry oven heat. This inevitably leaks to breakage.
It should also be noted that most (if not all) states cottage food laws prohibit canning breads.
Don't' can bread in jars! Penn State says:
"Breads and cakes canned in glass mason jars are not safe to eat. The risk for contracting a food-borne illness is high due to improper heating methods."