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?

  • How much is a "knob"?
    – KatieK
    Commented Aug 24, 2010 at 16:17
  • @KatieK About the size of your thumb-tip. Commented Aug 28, 2022 at 21:37

3 Answers 3


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.

  • 3
    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. Commented Aug 24, 2010 at 13:45
  • We need to develop some experiments to test these guesses. Surface tension sounds right to me too.
    – papin
    Commented Aug 25, 2010 at 1:01
  • 1
    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
    Commented Jul 17, 2012 at 23:20
  • 1
    @klypos - yeah, but I think you'd need a dollop per jar, and added to the end, to achieve that barrier. A knob per pot (which may have several jar's worth), and well mixed in, I would not expect to separate into a useful barrier before cooling. Could be the butter was originally added for this purpose, and is still used in lesser quantities for anti-foaming or taste, texture reasons.
    – Megha
    Commented Jun 16, 2017 at 21:59

Butter def helps reduce foaming; the instructions in the older recipes state this. (And why I keep a copy of the older recipe pectin box insert.)


Butter reduces the foam. Skip the butter. After cooking jam but before placing in jars, take off heat and stir for 5 minutes. This reduces the foam and also helps the fruit pieces to disperse evenly into the jam. Win Win. Then place in jars and continue canning as usual. I read it in an canning book years ago sorry don’t know the name.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.