TN NOTE: THIS IS NOT A YURI STORY
Also, I'm making strawberry, cherry, blueberry and blackberry jam once a year, sugar can be added and it's a good preservative but there's enough of pectin in those fruits for about the same storage time.
The trick is to fry or boil fruits just a bit longer in slightly lower temperature to thicken it, sterilized jar and temperature will pasteurise the jam anyway.