Runny Jam!

They may wash the fruit before freezing which would get rid of some of the natural pectin on the skin or it may just be a breakdown of cells caused by freezing soft fruit.
I'm going to try using the frozen forest mix but I'll add fresh lemon and cooking apple  (grated skin included) and hope it works......

They may wash the fruit before freezing which would get rid of some of the natural pectin on the skin or it may just be a breakdown of cells caused by freezing soft fruit.
I'm going to try using the frozen forest mix but I'll add fresh lemon and cooking apple  (grated skin included) and hope it works......

Don't forget that you can make your own apple pectin to keep in the freezer, do a pectin test on the cooked fruit, and then add enough apple pectin until you a get a blobby jelly clot in the meths. I use 5 fl oz and test.

If you pop any apple peelings and apple cores into the freezer until you have a goodly amount, just cover them with water and simmer until soft. Leave to strain in a jelly bag overnight and bag up the juice in 5 fl oz amounts and pop them into the freezer you can avoid any non setting mishaps.
Also saves time in the long run. I also pop a goodly spoonful of the jam mixture in to a teacup, after I think it has come to a set and leave it in the fridge while I have a well earned cuppa.

I then check that it really has cooked to a set and then bring the jam back to a boil and pot.

Jam is sneaky stuff, sometimes it tries to fool you into thinking it has come to a set when it hasn't. Been caught out like that before

Just thought I'd add my most recent experience of making this (my favourite) jam.  Prior to the 'runny jam' episode that prompted the original post, I'd only ever made it with fresh blueberries, so presume it was the fact they were frozen that caused the problem.  This time, I therefore allowed the fruit to defrost in the bags and drained the juices off before putting them in the bowl to macerate with the sugar.  Although I still needed to boil it for longer than usual, it did set without recourse to Certo/additional pectin so I was much happier (as I always feel that's cheating!!). 

I would just say that Mrs Ball's tip about leaving a dollop in the fridge for five minutes or so before taking recourse to additional boiling is a good one - if I'd just gone on the chilled saucer/wrinkle test, I'd have boiled it for longer.

So, thanks Mrs B - so glad I asked!!