Until recently (in the ‘humans making jam’ scheme of things, at least) melted paraffin wax was commonly used for this. It’s now considered unsafe.
Also works well for other "liquidy" things people usually put into jars.
We’re just making a gasket here, no?
I thought I'd try for a light syrup so I used 1/3 of the sugar - some recipes in the 1939 Kerr canning cookbook that I have called for that, with paraffin on top. Plus, I decided to reuse some of the previously used lids, being too cheap that specific year. They turned out beautiful to look at and went into my pantry
Well, about a month later, I was awoken in the middle of the night by explosion from downstairs. 2 jars exploded, most of those jars were fermenting, some had really nasty organic growth scum on it, and the cleanup was quite terrible. I think it was the lack of sugar, not the lids, but lid reuse didn't help.
Key takeaway - always use the right amount of sugar (1:1, even if it seems like you're pouring white death on), and never reuse lids.
Also..this is why I always preserve fruit in alcohol. Why not?
Botulism actually dies when in contact with oxygen. So you can only die to Botulism if you properly seal a can. Actually... you don't die to Botulism, but to Botulism's toxins. As soon as you open a can, Botulism dies. The leftover toxins however will kill you.
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You seal a can to protect it from all other infectious diseases: E. Coli, Black Mold, etc. etc. This stuff can kill you as well.
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"Normal" diseases die at boiling temperatures. So when you boil a can, its sufficient to kill off E. Coli, Black Mold, etc. etc. However, Botulism dies at higher temperatures... you need a pressure canner to raise the boiling point above 212F (!!!) to kill Botulism spores.
So the high-temperature is to kill Botulism. The sealing is to then keep all other diseases out after you've decontaminated the food.