Jam if made at home must be put into pre boiled jam jar's and still hot. To keep the top of the jam free of fungus wet the top of the jam with a little rum.
A jam includes the flesh of the fruit (a jelly includes only the juice); technically, both jams and jellies are kinds of preserves, but people often think of a preserve as including the whole fruit...
Jams are made from juice and pulp.
Jellies are made from juice that the pulp has been strained from, so it is clear.
Preserves are made from the whole fruit or crushed fruit so that they have...
Because there is so much sugar in jam, it is so concentrated that it creates osmotic pressure that draws the water out of any microorganisms that might get in it. In other words, it dries them out.