Why Canning Works — and Why Only Tested Methods.
Killing botulinum spores at 100 °C would take more than five hours. At 116 °C in a pressure canner it takes minutes. That is why low-acid foods are never canned in boiling water.
A sealed, heated jar is a perfect home for Clostridium botulinum if any of its spores survive: no oxygen, no competition, and, in low-acid food, nothing to stop it growing and making its toxin, which can kill in tiny amounts. Commercial canning of low-acid food is therefore designed for a 12D process — enough heat at the coldest point of the can to reduce the spores by twelve logs, a trillion-fold. The D-value of the spores is about 0.21 minutes at 121 °C, and with a z of 10 °C it is far longer at the temperature of boiling water.
Two consequences follow for home canning. Acid foods (pH 4.6 or below, XIII.7) — fruit, jams, pickles, tomatoes with added acid — can be processed in a boiling-water bath, which kills the moulds and yeasts that can grow in them. Low-acid foods — vegetables, beans, meat, fish, soups — must be processed in a pressure canner at 115–121 °C. And the processing time depends on the jar size, the food's density and packing, and altitude, so it must come from a tested procedure (USDA, the National Center for Home Food Preservation), never from guesswork. The workshop shows the arithmetic; it gives no processing times.
Below: the minutes needed at each temperature for a 12-log reduction of proteolytic C. botulinum spores (D(121.1 °C) = 0.21 min, z = 10 °C), with boiling water and pressure-canner temperatures at the altitude you choose. This is the logic only; jars also take time to heat through.
- Explain why botulism drives canning rules.
- Explain the 12D concept.
- Choose between a water bath and a pressure canner.
- Find and follow tested procedures.
The arithmetic of a trillion
At 100 °C a 12-log reduction of botulinum spores would take about 5.4 hours, and that is at the coldest point of the jar, not counting the time to heat it through; no boiling-water bath can do it reliably. In a pressure canner at 69 kPa (about 115 °C at sea level) it takes about 10 minutes, and at 121 °C, 2.5. Altitude lowers both: at 2 000 m water boils at 93.3 °C and the 69 kPa canner reaches only 111.0 °C, which is why tested procedures raise the pressure at altitude.
This plate deliberately gives no processing times. Those depend on the food, the jar, the packing and the canner, and they were set by measuring heat penetration in real jars. Use only current tested procedures — the USDA Complete Guide to Home Canning, the National Center for Home Food Preservation, or your national equivalent — with a dial or weighted-gauge pressure canner for low-acid foods. Never can in an oven, a slow cooker, an electric multi-cooker not tested for canning, or by inverting jars. And discard, without tasting, any can or jar that is bulging, leaking, spurting or smells off.
Find the tested procedure for something you would like to preserve (for example, tomatoes) on the National Center for Home Food Preservation's website. Note how the time changes with jar size and altitude, and that acid is added to tomatoes.
Terms
- 12D process
- Heating enough to reduce botulinum spores by twelve logs at the coldest point.
- Pressure canner
- A sealed pot that reaches 115–121 °C under steam pressure.
- Boiling-water bath
- Processing jars submerged in boiling water; for acid foods only.
- Heat penetration
- The time for the centre of a jar to reach the process temperature.
Sources
- The 12D botulinum cook: D(121.1 °C) = 0.21 min, z = 10 °C for proteolytic Clostridium botulinum spores (standard values in thermal-processing texts, e.g. Fennema's Food Chemistry and the FDA low-acid canned food regulations, 21 CFR 113).
- USDA National Institute of Food and Agriculture, Complete Guide to Home Canning, 2015 revision; National Center for Home Food Preservation.