When your jam mix is boiling, you may notice a frothy scum forming on the surface. This is formed from the impurities in the fruit and the sugar, which rise to the surface and then move towards the edges of the preserving pan. 
Although the scum is sterile after a long boiling, it may include bits of leaf and insect so isn’t an attractive bonus and should be removed before you transfer the jam to jars. 
Don’t try to remove the scum during the boiling as you will always remove some jam along with the scum, and repeated de-scummings will deplete your jam supply. Wait until the jam is ready to transfer to the jars, and do one de-scumming just before bottling starts. 
If despite your best efforts a little scum remains, adding a small knob of butter to the jam mixture and stirring well will usually remove the final traces. 

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