The amount of sweetener you're going to use for jam from 2 lb of fruits needs to equate to 2 cups (500 g) of sugar.
Wash and dry the fruits, removing the pits (if they have any). Blend them to a puree using a food grinder and pour it into a suitable container with a wide bottom.
Pour in the water and preservative, stir and put everything on the stove. Let it boil 5 min. and add the sweetener, along with the rum.
Let the jam boil for another 5 min. and remove from the stove. It may seem runny but it will thicken after it cools.
Distribute it into jars immediately, close them and flip them upside down. Leave them until they cool well.