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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?

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How much is a "knob"? – KatieK Aug 24 '10 at 16:17

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.

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I suspect it is a surface tension issue-the fat will float to the surface and disrupt the foam that forms there, but this is just speculation. I agree that it does reduce foaming. – Adam Shiemke Aug 24 '10 at 13:45
We need to develop some experiments to test these guesses. Surface tension sounds right to me too. – papin Aug 25 '10 at 1:01
There's another way of looking at this. The butter fats would rise to the top of the jar and form a thin layer across the top of the preserve. In days of yore, a dollop of butter or lard was melted over the surface of a preserve to "keep the air out". Might just be to improve the keeping qualities of the preserve. – klypos Jul 17 '12 at 23:20

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