So, what does all of this stuff have to do with pressure-cooking? Clostridium botulinum is a soil-living anaerobic bacterium, which is a spore-former, to boot. As a vegetative cell, this bacterium can generate a protein (named botulism toxin) which is toxic to humans - only a few-millionths of a gram (there are 454 grams in one pound), can kill a human! (paralysis leading to respiratory failure and/or cardiac arrest). So, green-beans grow in the soil, we pick them or buy them at a market, and, even though we wash them, some C. botulinum spores can adhere to the beans. As the beans are prepared for canning, if the beans are not pressure-cooked, once the beans are inside the sealed container (jar or can), very soon, conditions inside the can will remove all of the oxygen that was there initially, and no more can get in (conditions become perfect for C. botulinum, i.e., completely anaerobic). Since there are nutrients in the can, the bacterial spores will convert to the vegetative form, and begin to make toxin, which stays inside the can. If a real can is "bloated", or in any way looks "suspicious", never even open it! Many of these kinds of bacteria make gas, too (not oxygen). You can get enough toxin on your fingers to kill you - even though properly cooking the food would destroy the toxin.
Therefore, we always pressure-cook such things, before sealing them into a container for storage, because, spores are killed at 121 degrees-Celsius. Since 100 degrees-Celsius is the same as 212 degrees-Farenheit, and we all know that water boils at this temperature at sea-level (one-atmosphere pressure), how do we get the temperature to 121 degrees Celsius, so that we know we have killed all of the spores? Ahhh.... why not put the water under pressure? Then, the temperature can climb ABOVE 100 degrees C, and the water won't boil away! The pressure-cooker is designed to maintain 15 pounds/square-inch (psi) - the amount of pressure necessary to allow the temperature of water to reach 121 degrees-C (that's what that little valve is for at the top that keeps releasing steam... as the pressure rises above 15 psi, the release-valve releases...)
Of course, all "canned" foods are not pressure-cooked before-hand. Whether to pressure-cook or not, also depends on the type of food we are storing - if the conditions are very acidic (pickles) for example, or very, very, sugary, some foods won't need to be treated by pressure-cooking. To be safe, always check requirements - ask your County Agricultural Extension Agent (USDA) about the safest practice.
One important note: It is best that children less than one
year of age not be fed any honey. There is not a great risk, but there
is some risk, in giving less-than one year-olds honey, since C.
botulinum spores may be in the honey (not completely clear, though..
may just somehow have been ingested). For unknown reasons, C.
botulinum can live for awhile in the intestines of these very young
children, and produce enough toxin to harm them. This particular form of
botulism poisoning (associated with ingestion of honey) is relatively rare
(about 100 cases per year), and not yet well-understood. But, this
condition is known, and represents the majority of botulism cases in the
U.S.; therefore, just to be on the safe side, one should avoid this