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Pasteurization is one of the more mild forms of thermal processing that kills pathogens in milk and increases its safety and shelf life. Modern pasteurization uses high temperature and short time to kill pathogens in the milk. It kills off the most heat-sensitive pathogens while retaining the qualities of milk that consumers expect. This amounts to heating the milk to 161° F (72° C) for 15 seconds by running the milk through a tube of which the size and diameter takes the milk exactly 15 seconds to pass. Since that may be a bit more sophisticated than possible with primitive technology, you can also batch pasteurize the milk by heating it to 154.4°F (63° C) in a large container and holding it at that temperature for 30 minutes while it is stirred continuously. Americans would find it strange to walk into a store and purchase a bottle of milk that has been sitting on the shelf unrefrigerated for months but in Spain this is a common occurrence. The milk has been ultra-high temperature pasteurized by heating it to 280°F (138°C) for a minimum of two seconds. Milk pasteurized in this manner may be stored unrefrigerated for 6-9 months (using sterile packaging of course).
Pasteurization is one of the more mild forms of thermal processing and it works by denaturing the enzymes in the bacteria. Canning of food works similarly. It wasn’t until the early 1800s that we figured out how to preserve food by canning. If you are reinventing canning, you will need sturdy, air tight containers in which the food could be heated. How long and to what temperature the food needs to be heated depends upon the acidity of the food. Low acid foods must be heated more than high acid foods because the acid helps to denature the enzymes of the bacteria. You will need a pressure canner to sufficiently heat some foods. An open pot of water won’t get hotter than 212°F (100 °C) no matter how long you boil it. A closed pot in which the pressure can build allows you to heat the food to temperatures of 250°F (121°C ) for the 20 to 100 minutes it will take to kill bacteria in low acid foods. The most worrisome danger of canning is not killing C. botulinum spores. C. botulinum is a bacterium that thrives in the low oxygen, high moisture environment of the can. It releases a toxin that is among the most lethal toxins in the world. The toxin is odorless, colorless and tasteless. An amount equivalent to a quarter of a grain of sand (75 nanograms) is enough to kill a person. The good news is that, while the C. botulinum can survive boiling water, the toxin it produces breaks down with temperature. Heating canned food to a typical cooking temperature of 176°F (80°C) for 10 minutes before consumption can greatly reduce the risk of illness. Personally, I would make sure that the food was cooked enough to kill C. botulinum in the first place. So in a nutshell, canning is using heat and acid to reduce pathogens to a level that allows the food to be eaten years later (assuming proper storage - meaning cool temperatures and a good seal on the jar).