Old time canners, such as those who canned using methods prior to the 1970's frequently question the need to use a water bath canner to process the sealed jars, particularly for jams and jellies. They also often use paraffin as a sealing wax. So... what's wrong with this? Well, let's look at what they did (and some still do...)
Here's the olde tyme method:
The next generation updated the process once they heard about the Ball ring and lid method. Unfortunately, many of them never read the instructions nor took a course ion food safety..
Have you ever melt paraffin on your stove? If you have, I can say with certainty that at some point you've burned yourself and probably started a small fire. Candles are made of wax. It burns. And doesn't reliably form a seal. And if it does, the seal breaks easily. And then you pick wax out of the jam. Seriously?
Even in the second method (inversion), the jars full of presumably "hot" jam are exposed to the atmosphere and THEN sealed and inverted. So, let's play microbiologist for one second. Air contains... spores! Yes, inactive little "seeds" of things like clostridium botulinium, commonly known as botulism. These spores land on the surface of the "hot" jam. How hot? Well certainly no higher than 212 F (100 C), the boiling point of water, and probably far cooler, especially on the surface.
What temperature do you think it takes to substantially kill or reduce the number of spores? Would you have guess above 240 F? And even then it may take 20 minutes or more at that temperature. Commercial processers use high pressure canning machines to reach far higher temperatures, long enough to accomplish this.
Granny and her mother had a lot of jars spoil, certainly a lot more than someone using a jar, lid and ring and a water bath. Not to mention burns and kitchen fires. And a fair amount of jam that is unusable because it has wax floating in it. Illness and fatalities? Hard to say. Probably not many because jams and jellies inherently prohibit the growth of most bacteria due to their high sugar and acid content. But if these methods are used on things like corn, beans, tomatoes... that's far more dangerous and there are many documented cases of fatal food poisoning from these when canned like this at home.
. It's actually quite simple:
The USDA and many, many universities have warnings against the use of this method (see the bottom of this page for references). Here's a typical statement, from the University of Georgia:
"An old out-dated method of canning - the open-kettle method - is now considered unsafe. In this method, foods were heated in a kettle, then poured into jars, and a lid was placed on the jar. No processing was done. With this method there was often spoilage, because bacteria, yeasts, and molds that contaminated the food when the jars were filled were not killed by further processing. The growth of these microorganisms, in addition to spoiling the food, often caused lids that did seal to later come unsealed. This method resulted in a very real danger of botulism."
"Note. Jelly jars and paraffin are no longer recommended. An incomplete seal with paraffin and the absence of a heat treatment may result in mold growth and toxin production in the jelly. Persons continuing to use the paraffin / no water bath method should be aware of the potential health risk."
This is just a small sampling of the many authorities who concur that the only safe home canning methods are the water bath canner (for jams and acidic fruits and vegetables) and the pressure canner (for low acid fruits and vegetables, meats, and dairy). Click on the links to see their articles.