Home canning: tested recipes only

Clostridium botulinum occurs naturally in soil and so gets onto vegetables and other foods. Its spores are very heat-resistant. In a closed jar they find exactly what they like: little oxygen, moisture and – in low-acid foods – hardly any acid. There they can germinate and produce a nerve toxin.

Acid inhibits this bacterium. For sufficiently acidic foods such as most fruits, tested instructions therefore allow a water bath. Low-acid foods such as vegetables, meat, beans and soups, on the other hand, need a pressure canner. Under pressure, water only boils at a temperature higher than 100 °C, and only this higher temperature reliably kills the spores.

That is why tested recipes are so precise: they specify acidity, jar size, headspace, method and time. These values were determined in the laboratory so that the heat reaches the centre of the jar for long enough. Even a thicker sauce or a larger jar can change that.

Use only current instructions from official or scientifically testing bodies, for example the USDA or the National Center for Home Food Preservation, or equivalent bodies in your own country. Follow them exactly: ingredients, quantities, jar sizes and times are not changed. Under these instructions tomatoes usually need added acid, even though they taste sour.

At higher altitude water boils at a lower temperature. Tested instructions therefore include altitude adjustments – anyone canning in the mountains must follow them.

After cooling, check that all lids have sealed. Jars that have not sealed go into the fridge and are eaten soon, or are reprocessed according to the instructions. Store the finished jars cool, dark and dry, and label them with contents and date.

Jars with bulging or loose lids, gas, cloudiness, leaked liquid or an unusual smell are not opened or tasted but disposed of safely. Important to know: the toxin can be present without any visible sign at all. So the correct method is the real protection, not the check at the end.

If botulism is suspected – for example vision problems, difficulty swallowing or speaking, or paralysis after eating preserved food – call the local emergency number (112 in the EU) immediately. This is an emergency.

Inherited family recipes, canning in the oven or dishwasher, or simply turning hot jars upside down are not considered safe by recognised bodies. In an oven, dry air heats the jars unevenly, and turning jars over does not heat the contents sufficiently.

Canning is a fine craft for calm times: if you have a garden, you can preserve harvest surpluses. But it takes time to learn, suitable jars, new lids or intact seals, and for low-acid foods a tested pressure canner.

In the middle of an emergency, without a reliable heat source, without experience and without instructions, canning is not a good idea. For an emergency stock, commercially produced tins are the safer choice. This entry deliberately contains no recipes, times or pressures; regional recommendations may differ.

Many people have canned for years using grandmother's recipes without getting ill. But that does not mean these methods are safe – botulism is rare, and a single mistake can be enough. Modern laboratory testing has shown which methods are reliable and which are not.

The USDA publishes the “Complete Guide to Home Canning” for this, and the National Center for Home Food Preservation provides tested methods online. Both stress: low-acid foods must be pressure canned.