Richad Mabley's recipe for a blackerry junket is as follows:

"a junket made from nothing other than blackberry juice. Remove the juice from the very ripest blackberries with the help of a juice extractor, or by pressing them through several layers of muslin. Then simply allow the thick dark juice to stand undisturbed in a warm room. Do not stir or cool the juice, or add anything to it. After a few hours it will have set to the consistency of a ligh junket, and can be eaten with cream and sweet biscuits"

I have made this and it works beautifully (i highly recommend it). What makes it set? Would i work with an other fruit?