Low acid vegetables and botulism
Canning vegetables from your garden can be fun to do, and what could be more delicious than a wintertime meal prepared with the vegetables you preserved during the summer.
Canning low-acid foods, which include red meats, fish, poultry, and all vegetables (except for most tomatoes) requires special care. Low-acid foods can support the production of the deadly botulism toxin if these foods are not processed properly in a pressure canner. A pressure canner heats food to high temperatures (240° to 250° F or higher) and destroys the spores that produce the botulism toxin. A boiling water bath canner, which can be used for canning pickles or fruit, heats food to boiling temperature (212° F) which is not high enough to ensure safety for canning vegetables and other low-acid foods.
Clostridium botulinum bacteria are the main reason why low-acid foods must be pressure-canned to be safe. Clostridium botulinum is a common soil microorganism that produces a very deadly toxin or poison. This food poisoning is called botulism, which is the most deadly food poisoning known. Home-canned foods are responsible for over 90% of all cases of foodborne botulism. Therefore, all vegetables to be canned must be washed thoroughly and peeled, trimmed, or chopped as directed.
Clostridium botulinum spores can be destroyed by pressure canning the food at a temperature of 240° F or above for a specific period.
Botulinum spores are on most fresh food surfaces, but because they grow only in the absence of air, they are harmless on fresh foods. The conditions which favor the germination of these spores are low acidity (such as in vegetables and meats) and the absence of air (such as in a sealed canning jar). These Clostridium botulinum spores can be destroyed by pressure canning the food at a temperature of 240° F or above for a specific period. If you find timetables on recipes for processing low-acid foods in a boiling water bath canner, do not use them. Research has shown that these timetables present a very real risk of botulism.
Pickling or canning salt can be added for flavor, but does not prevent spoilage. Spices and herbs may be added in small amounts, but butter, fat, flour, rice, barley or pasta should never be added unless the tested recipe directs you to do so. Adding ingredients that are not called for in the recipe may result in an unsafe product.
Successful processing in a pressure canner requires attention to several details:
- Vent pressure canners for 10 minutes at the start of processing. Venting drives air from the canner. If air remains trapped in the canner, the canner will not reach pressure, or pressurization will take a long time. A poor, unsafe product will be the result.
- Adjust for elevation. When pressure canning meats and vegetables, it is important to adjust processing pressure for elevation. The highest altitude in Minnesota is 2,000 feet.
- Dial gauge, up to 2,000 ft. –11 pounds pressure (11 psi)
- Weighted gauge, up to 1,000 ft. –10 pounds pressure (10 psi)
- Weighted gauge, above 1,000 ft. – 15 pounds pressure (15 psi)
- Keep an eye on pressure. Start counting processing time when the correct pressure is reached, and regulate heat to maintain a steady pressure.
- Bring the pressure back up and retime the entire process if at any time the pressure drops below the processing level. Fluctuating pressures can cause jars to lose liquid and damage seals or lead to under-processing and unsafe food.
- Allow the canner to depressurize when the timed process is completed. Do not force-cool the canner.
- Reprocess within 24 hours, if necessary. If jars fail to seal, remove the lid and check the jar-sealing surface for tiny nicks. If necessary, change the jar, add a new, properly prepared lid, and reprocess within 24 hours, using the same processing time. Otherwise, refrigerate the jars and use within 2-3 days, or freeze the jars for later use.
Reviewed by Kathy Brandt, University of Minnesota Extension educator, 2013.