Canning vegetables from the garden can be fun to do, and nothing tastes more delicious in the dead of winter than home-grown vegetables. Canning low-acid foods, including red meats, fish, poultry, and all vegetables (except most tomatoes) requires special care. Low-acid foods can support production of the deadly botulism toxin if they are not processed properly in a pressure canner. Pressure canners heat food to high temperatures (240 to 250 degrees F or higher) and destroy 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 degrees F) which is not high enough to ensure safety for canning vegetables and other low-acid foods. Clostridium botulinum bacteria Clostridium botulinum bacteria are the main reason low-acid foods must be pressure-canned for safety. Clostridium botulinum is a common soil microorganism that produces a toxin, or poison. This food poisoning is called botulism, which is the most deadly food poison known. Home-canned foods are responsible for more than 90 percent of all cases of foodborne botulism. All vegetables to be canned must be washed thoroughly and peeled, trimmed, or chopped as directed. Botulinum spores are on most fresh food surfaces, but because they grow only in the absence of air, they are harmless on fresh foods. Conditions that 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). Clostridium botulinum spores can be destroyed by pressure canning foods at a temperature of 240 degrees F or above for a specific period of time. 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 high risk of botulism. Adding ingredients 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 in the recipe may result in an unsafe products. Safe and successful canning Successful processing in a pressure canner requires attention to several details. o 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. This results in unsafe, poor quality products. o Adjust for elevation. When pressure canning meats and vegetables, it is important to adjust the processing pressure for elevation. The highest altitude in Minnesota is 2,000 feet. A dial gauge should be set to 11 pounds of pressure per square inch (psi) for elevations up to 2,000 feet. A weighted gauge should be set to 10 psi up to 1,000 feet. Weighted gauges should be set to 15 psi for elevations above 1,000 feet. o Keep an eye on pressure. Start counting processing time when the correct pressure is reached, and regulate heat to maintain a steady pressure. o 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. o 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 rim 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 for later use.
