Blackcurrant jelly disaster

Was the sugar properly dissolved before you started heating it up to jam setting point?

On my jam thermometer the temperature for jam is 110C, however, I have never seemed to manage to get it up to that temperature and have resorted to the wrinkle method again.
Did you sterilise the jars in the oven?
I usually pour the jam into the jars hot then seal straight away and hope to hear the 'pop' as the vacuum seal goes.

I wonder of there was something on the blackcurrants? Did you grow them or buy them?

Did you thoroughly sterilise the jars and whatever you sealed the jars with twist-lids/paper

I've used the same recipe for years - 1lb sugar to 1 pint juice boiled to 105c and then boiled until setting point. The jelly was vinegary when I opened it.

Very odd!

Just wondering how long you boiled it for and if you tried to reduce the sugar? Some bacteria produce spores which survive boiling but with normal sugar content of jam, they shouldn’t be able to grow.