I used to either skim off the "scum" and discard and/or add about a tsp of butter to the jam - as you'd suggested, and is suggested in many recipes - but no longer. I've found that the "scum" is not necessarily a bad thing to be removed and discarded. No, it's actually frothy jam or jelly, and is perfectly fine to eat! These portions just don't look good in the jar, so what I do now is skim it off the top of the jam or jelly before I ladle them into jars, and put it in a container to refrigerate it and use it myself. I then finish jarring up the jam or jelly and process in the water bath canner as usual. Now, this is assuming you washed the fruit really well, and filtered the jelly juice well (like, 2 layers of cheese cloth) - if not, then the "scum" may contain things you don't want to consume, like fine sediment. However, most sediment is not harmful - gritty, but not harmful.
I used to either skim off the "scum" and discard and/or add about a tsp of butter to the jam - as you'd suggested, and is suggested in many recipes - but no longer. I've found that the "scum" is not necessarily a bad thing to be removed and discarded. No, it's actually frothy jam or jelly, and is perfectly fine to eat! These portions just don't look good in the jar, so what I do now is skim it off the top of the jam or jelly before I ladle them into jars, and put it in a container to refrigerate it and use it myself. I then finish jarring up the jam or jelly and process in the water bath canner as usual. Now, this is assuming you washed the fruit really well, and filtered the jelly juice well (like, 2 layers of cheese cloth) - if not, then the "scum" may contain things you don't want to consume, like fine sediment. However, most sediment is not harmful - gritty, but not harmful.