Sugar helps to thicken the fruit jam and helps the jam to ‘’set’’. There is a specific ‘’Jam Sugar’’ available for making jams which consist of sugar (thickener), pectin (key ingredients in how jam gets its firm texture), and citric acid (helps the jam to set). Good news is that you don’t need this to make a good homemade jam. Coarse-grain white granulated sugar or even caster sugar will do an excellent job too.