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?