Food preservation is essential because it extends the length of time during which the food is nutritionally viable and safe to eat. Most fresh fruits, vegetables and animal products spoil quickly without refrigeration, dehydration or preservation. The Clemson Cooperative Extension states that the most popular and effective preserving techniques employ salt, sugar, vinegar or brine.
According to the Clemson Cooperative Extension, food preservation works because the preserving agents prevent bacteria, mold and other potentially harmful organisms from growing on the food. Dry preserving techniques, such as packing food in salt, draws water out of the food, making it an inhospitable environment for microorganisms. Wet-preserved foods undergo heat processing that kills microorganisms, and the vacuum seal created by proper canning prevents life-sustaining oxygen from entering the container.
Most bacteria and fungal spores cannot live without oxygen and do not survive prolonged immersion in boiling water. A notable exception is the bacterium Clostridium botulinum, which causes a dangerous and potentially fatal disease called botulism. Clostridium botulinum is unusually hardy and thrives in anoxic environments; oxygen is toxic to it. It also survives boiling temperatures that kill most other bacteria. However, it cannot survive acidic environments such as pickle brine.
About.com expert Leda Meredith explains that non-acidic preserves such as unpickled vegetables, soup stock and all wet meat preserves do not contain sufficient acid to kill Clostridium botulinum. The only safe technique for making these preserves is to process the jars in a pressure canner, which subjects them to temperatures much higher than boiling water.