What is the butter for in a jam recipe? I made plum jam at the weekend. The recipe I had (from my Good Housekeeping cookbook) wanted me to simmer the plums in water, add sugar and a knob of butter, then boil until a set was reached. 

I realised too late that I was out of butter, so I quickly looked up another jam recipe online and discovered what seemed like a 50/50 split between recipes with and without the knob of butter. 

I made it without and it came out beautifully - clear, well-textured, lovely flavour. So what was the knob of butter meant to add?
The usual explanation given is that adding butter to the fruit and sugar before you cook it will reduce (or even eliminate) the foaming.

My guess is that the small amount of proteins in the fruits create the foam.  As you heat the fruit, the proteins open up into strands that get tangled up and help stabilize the bubbles into a foam. Adding the butter (a fat) helps prevent this tangling.