Canning Day

What the day is

Canning Day is observed on October 23, which sits at the tail end of the northern preserving season, after the last of the tomatoes and apples are in.

Origin and background

The origin of the observance is not documented. The technique has a precise and slightly military origin: in the 1790s the French government offered a prize for a method of preserving food for armies on campaign, and Nicolas Appert, a confectioner, won it by sealing food in glass jars and heating them. He published his method in 1810.

He had no idea why it worked. Germ theory was decades away, and Appert assumed air was the problem. Pasteur's work in the 1860s supplied the explanation: heating kills the microorganisms present and the seal prevents new ones entering.

That gap between working practice and understanding matters, because the modern safety rules come from the understanding rather than the practice. The serious hazard is Clostridium botulinum, which forms heat-resistant spores, thrives without oxygen, and produces one of the most potent toxins known. It cannot grow in high-acid conditions, which is why the rules split sharply in two.

High-acid foods, including most fruit, pickles and properly acidified tomatoes, can be safely processed in a boiling water bath. Low-acid foods, including vegetables, meats and beans, require a pressure canner, because only pressure raises the temperature high enough to destroy the spores. This is not a matter of preference or tradition; water-bath canning of low-acid food is genuinely dangerous.

How to take part

• Use a tested, current recipe from a recognized extension service or canning authority rather than a family method of unknown provenance.
• Pressure can anything low-acid. There is no safe shortcut.
• Check seals after cooling and refrigerate anything that did not seal.
• Label everything with contents and date. Unlabeled jars get eaten last or not at all.
• Discard without tasting any jar that is bulging, leaking, spurting or smells wrong.
• Start with something high-acid such as apple butter if you are new to it.