Use lemon vinegar to cook jam
I do not cook jam according to plan. If I get cheap fruit, I'll go, but almost never have lemons in the house. That's why I tried the following: I bought lemon vinegar (Citroessenz from Surig) and have been working very successfully for years.
Instead of adding fresh lemon for the preservation of the jam, always add the lemon vinegar.
I've never had a jam in over 5 years I even have some of 2012 to stand.
The amount you have to try for yourself, I always preferred to take something more. The jam is even heartier with it.
On the bottle, which looks like Essigessenz, depends on a small manual. You can use the lemon vinegar for many other things.