Why is heat so vital to home canning? I could lecture for hours about this topic, but I'm just going to make a few strong points here.
Heat is one factor in killing harmful microorganisms in canning. The bacteria I worry most about is Clostridium botulinum (botulism). This bacteria does not need oxygen to survive, so creating vacuum in a canning jar will not kill it. But it CAN be killed by high heat. Adding acid is important, too, but that's another subject.
Let's take green beans. Beans are low acid, meaning the pH is more than 4.6. Botulism is most prevalent in low acid foods. To preserve green beans safely, you must can them under a high temperature. You must use a pressure canner to achieve a high enough temperature. (Water bath canning is only for highly acidic foods with a pH below 4.6.)
Water you put in a pressure canner boils first, just like a water bath. As the water boils, it produces steam. In a water bath, the steam leaks out of the pot. But in a pressure canner, the steam is not released as easily, and thus pressure builds inside the pot. As pressure builds, the temperature rises - much higher than in a water bath. Thus the harmful bacteria is killed.
It is important to process foods in a pressure canner for the number of minutes and the pounds of pressure as listed in your recipe. For safe canning recipes, here is a great link to the National Center for Home Food Preservation. Or if you want hands-on canning classes, consider registering for our next class, April 17th and 18th, 6:30 - 9:30 in the evenings.
Thanks, and have a safe day!