Nick Maclaren wrote:

: The reason that some jams, jellies etc. don't keep very long is that
: they have a low concentration of sugar and acid.

Indeed.

I generally like to keep added sugar levels to a minimum, the "hot seal"
method works better as a general method.

: Because of this, the normal problem with home-made acid fruit preserves
: is mould on the surface, and that can just be scraped off. If they are
: low on sugar, they may ferment (acetobacter, saccharomyces or lactobacter
: are the main ones). The last makes things smell like sick, but none
: will harm you.

: I regularly eat jams and jellies over 5 years old, and have eaten ones
: over 10 years old.

The oldest I have is 1995 - it looks fine.

: This Christmas, we ate japonica cheese that had been
: open to the air since October 2001. It was excellent. But the key to
: all of them is high acid and not low sugar.

But there's a world of a difference between a fruit cheese and a fine
fruit jelly :-)