Popped Jars and cans

[jennyjj01]

Pectin forms a gel when it has the right concentration of sugar, water and acid. The correct acidity is just right to prevent botulism. This explains why my grandma would refuse to store poorly set jams and used them immediately.
If the set jam has turned to syrup, one of these factors has changed. For example water ingress or it may be because there is a microorganism growing in there reducing the acidity. Not necessarily a dangerous bug itself but this opens the door for any surviving botulism bugs to start multiplying.

Botulism is my main fear with my disrespect for BBE dates. Odourless, tasteless and potentially deadly.
Fortunately ( perhaps ) it's not common in commercially canned foods in this country. I understand that in the US it's more common in just a few less common, low acid foods such as home canned veg and fish.

Home-canned vegetables are the most common cause of botulism outbreaks in the United States. From 1996 to 2014, there were 210 outbreaks of foodborne botulism reported to CDC. Of the 145 outbreaks that were caused by home-prepared foods, 43 outbreaks, or 30%, were from home-canned vegetables.

[grenfell]

Pectin forms a gel when it has the right concentration of sugar, water and acid. The correct acidity is just right to prevent botulism. This explains why my grandma would refuse to store poorly set jams and used them immediately.
If the set jam has turned to syrup, one of these factors has changed. For example water ingress or it may be because there is a microorganism growing in there reducing the acidity. Not necessarily a dangerous bug itself but this opens the door for any surviving botulism bugs to start multiplying.

Sounds right enough. If I remember correctly it was rowanberry jelly that went runny. It's not really something that we eat a lot of so no huge loss. I did wonder if straining the skins out had any impact on the levels of pectin. MrsG prefers jams and the like without "bits" and so she stains it.

[GillyBee]

Americans still a have few people using Grandma's waterbath canned green beans (low acid) recipes and the like.
Examples of where it all went wrong include fresh garlic in oil, Inuit fermented fish made by memory (wrongly) and a French condiment maker who did not understand use of the "cold chain" for really preserved foods.
Producers who understand and take steps to reduce the risks very rarely have any problems.
To be safe from botulism in stored food, heat is not enough unless you can hit 125 Celcius. Hence commercial addition of "acidity regulators" to do many foods.

[jennyjj01]

From 1996 to 2014, there were 210 outbreaks of foodborne botulism reported to CDC. Of the 145 outbreaks that were caused by home-prepared foods, 43 outbreaks, or 30%, were from home-canned vegetables.

Just spare a thought for those numbers. A population of over 300 Million suffered just 210 outbreaks over 8 years. That alone made it a statistically very unlikely event. Of those, the majority were from home 'canning' and not commercially canned food gone bad. Among the US home canning population there would have been many that were doing it completely wrong, maybe even completely cold. And lets not forget the loons that 'canned' fermented fish
What I'm saying is that the odds of botulism from my tins of soup seem miniscule, even when I am reckless.