High acid foods kill bacteria (most importantly botulism toxin) at a lower temperature. That's why it is okay to water bath can high acid foods. At 212*F, the acid in the food kills off any dangerous microorganisms. I think the ph has to be below 4.6.
Good reasoning, but unfortunately not quite accurate and I'm not being critical at all. I just think it is important to have more accurate information so that it can be passed on.
High-acid foods just prevent most bacterias from growing back. It does not kill them. General cooking will kill the active toxins including the Botulism "toxin", but the Boiling Water canning process itself is what kills the "spores" that create the toxins, except for the Botulism spore.
The Botulism spore is heat resistant and takes a temperature of 240F for a period of time to destroy it. But since it can't breed in a high-acid/sugar environment it is no worry for that stuff, but low-acid foods must be Pressure Canned to kill it or you have a potential toxin problem, if it should exist. Cases are rare, but they do exist, and since it kills.....well, being on the safe side is recommended.
Yes, the pH borderline is 4.6. At or below is acidic. Tomatoes run about 4.3 to 4.90 and you never know what an individual tomato is going to be, so you can see where the problem is. Then if you toss in some other low-acid foods like onions and peppers in a good salsa, well, you see the direction its going. Here's some links for more info:
IS734 Botulism (Botulism)
US FDA/CFSAN - Approximate pH of Foods and Food Products (pH of Foods)
And you are very correct about our parents and grandparents not knowing about Botulism. That's a recent discovery by our technology and research over the years. They "knew" something was there because people were getting sick and dying, but they didn't know what it was. They got in to the habit of boiling all food coming out of a can/jar for 10 minutes, which seemed to eliminate the problem. Some of you may even remember that practice. Now you know why.
As stated above, normal cooking kills the "toxins", including the Botulism toxin. So if you have a questionable jar, just boil the food after you open it for about 10 minutes and any toxin will be destroyed and it will be safe to eat.
It may be confusing, but there's a lot of chemistry and science going on here. If you can things by today's rules, enjoy it and have fun.