If I remember correctly, my mother cleaned empty jam bottles in boiling water with some soda (?), put them up side down on a clean tea towel to dry and fill them directly when the jam was ready and closed the jars immediately. It never failed.
But once she tried to make raspberry syrup and went moldy after a few weeks.
Re: Happy memory, brought to the surface by your comment
If I remember correctly, my mother cleaned empty jam bottles in boiling water with some soda (?), put them up side down on a clean tea towel to dry and fill them directly when the jam was ready and closed the jars immediately. It never failed.
But once she tried to make raspberry syrup and went moldy after a few weeks.