Jammer

By experiment.

You send out flyers with a note "I will be testing a jammer at time date, Please send SMS to mobile number if you are affected.

It all depends on the ratio of sugar to fruit in the mix.

Another thing to consider is the pectin, because that affects setting time.

And the acidity of the fruit also varies, as well as the permeability of the real impingus.

Here is the type of range our jammer uses

Without knowing more details than given, it is difficult to assist you further .....