Hollow out the Siam pumpkin and cut the pulp in small. Cut into cubes. Also cut the apples into small cubes. Mix the fruit cubes with the preserving sugar in a large saucepan and let stand for about 10 minutes, stirring once in between. Bring everything to a boil and only when the mixture has boiled for about 2 minutes, add the multivitamin juice and the lemon juice. Let simmer for about 4 minutes in total.
Just before the end, stir in the tip of the cinnamon knife.
Pour into twist-off glasses and turn over immediately. The amount makes about 10 smaller glasses.
Note: You can certainly use other fruit juices. I just tried it because the jam just seemed too light to me and probably wouldn`t have looked that appetizing. The Siam pumpkin has a white pulp, like the apple, that`s why I came up with the multivitamin juice.