[Wh1zz]

I am looking at a whole jar of plum jam made by my grandpa with 0 added sugar.

[throwaway_tech]

Then that is not jam...jam is defined as fruit with added sugar to act as a preservative.

The question is what did you granpa use as a preservative? The preservative is going to be the difference between say: jam, marmalade, chutney, etc...