As Nebraskans begin to pick vegetables from their home gardens, many of them will decide to can their extra produce. While this is an excellent way to preserve food for future use, following instructions and taking certain precautions are important to guarantee proper produce preservation and to prevent illness.
Clostridium botulinum is a bacterium that has a spore that exists in the soil. In the absence of oxygen, like in jars containing canned foods, these spores will change to live cells and multiply rapidly. These bacteria release a toxin which causes botulism, a deadly form of food poisoning. Signs of botulism usually appear 12 to 36 hours after eating contaminated food. Botulism causes impaired vision, slurred speech, muscle weakness and paralysis that can result in death.
Clostridium botulinum cannot grow in acidic environments, so acidic foods, such as fruits, are safe from this form of contamination. However, meat, vegetables and other low-acidity foods must be canned properly to kill the bacteria.
If someone has never canned before and has the freezer space, freezing food is recommended over canning because it prevents Clostridium botulinum from growing. For long-term preservation, store frozen foods in a stand-alone freezer with a temperature below 0 degrees Fahrenheit. Do not freeze food in regular refrigerator-freezers. These units are meant for short-term storage and may not prevent frost buildup on food.
For those who still want to can, the first step to good canning is to find a good canning jar. Mason-type jars with self-sealing lids are best and can be used many times as long as they are cleaned between each use. Lids should be used only once. Do not use commercial jars, such as ones for mayonnaise, because they are more likely to break or not seal properly. Check all jars for cracks and make sure the seal is tight.
Next, the jars can be packed with raw produce, which is more suitable for vegetables being cooked in a pressure canner. Food also can be hot packed, meaning it has been brought to a boil, simmered for 2 to 5 minutes and then promptly canned. Hot packing removes excess air from the food tissues and increases shelf life, and it is best used with boiling water canning. Regardless of which method is used, all juices, water or other liquids added to the cans should be heated to a boil.
The amount of headspace, the space between the liquid in the jar and the jar lid, is very important because the food will expand. Be sure to check specific headspace requirements for each food. Do not just estimate because the vacuum could be destroyed or the can could burst.
When canning vegetables, meats and other low-acid foods, use a pressure canner because Clostridium botulinum spores can still survive in a boiling water canner. A boiling water bath can be used to process acidic foods such as jams and jellies, fruits and pickled products. Check currently published food preservation cookbooks or websites for correct pressure, time and altitude adjustment for canning each specific food.
After processing, check to make sure the lid has been properly sealed by one of three methods. Press down on the middle of the lid. If it springs up, the jar is not properly sealed. Tap the lid with the bottom of a teaspoon, and if it's properly sealed it will make a ringing, high-pitched sound. Finally, when the jar is held at eye level, the center of the lid should be concave, not flat or bulging.
Store canned foods at a temperature between 50 and 70 Fahrenheit. Do not place cans near hot pipes, a range, a furnace, in uninsulated attics or in direct sunlight.
For more information on canning, visit UNL's Food Preservation.