Most foods require a pressure canner for safe preservation.
Most home cooks, with at least a casual interest in food preservation, have canned a few things over the years. It takes only a small investment of time and equipment to put a few shelves of pickles, jelly or preserves in the pantry, and they're easily "put up" using a large pot of boiling water. Those who have extra-large gardens -- or families -- often invest in a pressure canner, which makes it possible to preserve a wider range of foods.
How Canning Works
Canning works by accomplishing two specific goals. First, it heats foods to a food-safe temperature, to kill any potentially dangerous microorganisms. Second, it creates a sealed, airtight environment to prevent molds or bacteria from re-contaminating the food. The process works by heating food in loosely-sealed jars. As the air in the jar expands in the heat, it forces itself out between the jar and the lid. As the finished jars cool, the air shrinks again and creates a vacuum, tightly sealing the lid to the jar.
Potentially Dangerous Microorganisms
Microorganisms of all kinds surround us, abounding in everything from the air we breathe to our own skin. Most of them have no effect on us or our food, and many others have positive effects. However; a small handful cause spoilage in food, and an even smaller number can cause serious illness or death in humans. Most microorganisms can only flourish and reproduce at temperatures between 40 and 140 degrees Fahrenheit, the so-called "danger zone" for food safety. Heating foods to temperatures of 165 F and higher kills most dangerous microorganisms, or pathogens, leaving the food safe.
Spores and C. Botulinum
Although canning protects against most pathogens, some protect themselves against extreme heat or cold by forming spores, tiny armored pellets that protect them until conditions improve. One such pathogen is Clostridium botulinum. The spores protect the bacteria so well they can survive to temperatures of 240 degrees F, and begin to reproduce in the oxygen-free environment of low-acid canned goods. The bacteria produces a toxin, which makes humans deathly ill. Pickles, tomatoes and most fruit have enough acidity to kill C. botulinum, but most other foods must be pressure canned.
How Pressure Canning Works
Most people understand that the boiling temperature of water is 212 degrees F at sea level, and goes down by about 2 degrees for every 1000 feet of altitude. Pressure cookers and pressure canners reverse that process by hermetically sealing the pot. As steam builds up in the pot, it artificially raises the atmospheric pressure in the pot, and raises the boiling temperature of the water as well. This makes it possible for a pressure canner to heat food past the boiling point of water, to a temperature of 250 degrees F, which kills the C. botulinum spores.
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