Good ol' Jamsetta comes to the rescue 
What is Jamsetta actually made from? Maybe we could make our own. It's pectin isn't it? Is it something like citric acid?? 
Nay, my Jams book tells me a little about pectin and citric acid. Bottom line is, I wouldn't bother making my own version of Jamsetta. Berry fruits are low in pectin and without adding it, the jam has problems setting. My Jams book says "Pectin is a carbohydrate present in certain fruits and vegetables which, when combined with sugar, helps a jam, jelly or marmalade to set or jell. Commercial pectin helps when there is not enough natural pectin present".
And, "Citric acid is commonly found in most fruits, especially limes and lemons. Commercial citric acid helps to accentuate the acid flavour of the fruit; it does not act as a preservative."
My book notes that jam will not set if there is an imbalance of pectin and acid, or if insufficient evaporation during the cooking process. It is recommended to add lemon juice to such fruits as berries as it contains both pectin and acid, and a good balance of both.
There is 1/2 cup lemon juice to only 2 punnets of strawberries in the recipe I use, and I think that helps. Obviously the Jamsetta is crucial. I don't know how you could fabricate it at home.
HTH
Miss G
