I can’t believe how quickly I could turn off-cream into butter. I have one of those stick blenders, with a whisk attachment. I emptied the old cream (it was probably about 100mls) into a long beaker and whisked until the cream became blobs of yellow butter. I pressed the butter with my fingers to get out the buttermilk, and tipped that out. I then added some cold water into the beaker and patted the butter again. More buttermilk came out; I repeated this again until the water ran clear.
I added about 1/2 teaspoon of table salt; more might need to be added if you use more cream. I patted this around with my fingers, then tipped the butter into my butter dish. Hey Presto!
I think it took me about 5 minutes to make the butter, and to wash up the beaker. Too simple, and quite delicious.
The science behind turning white, smelly soured cream into beautiful, yellow, delicious butter is as follows: Cream consists of butter fat in microscopic globules. These globules are surrounded by membranes made of fatty acid emulsifies and proteins. Butter is produced by agitating cream which damages these membranes and allows the milk fats to conjoin thus separating from the other parts of the cream which is called the buttermilk.