This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
From: Anna Welborne. My dad always told me it kept the foam quantity down.
That seems to be pretty much true, as I tried leaving it out of the
strawberries this last summer, and had more foam. [BTW, for a beginner,
cutting down on the foam is helpful. Less foamy jam gives a more accurate
reading for your candy thermometer; too much foam is hard for a beginner
From: Eric Decker. Let the jam rest after cooking. Now the scum can be
easily skimmed off. Adding butter will foul the flavour to a degree you
may or may not like.