Churning is accomplished by a variety of mechanical devices that may take 15 minutes or a few seconds to damage the fat globules and form the initial grains of butter. The fat crystals formed during aging distort and weaken the globule membranes so that they rupture easily. When damaged globules collide with each other, the liquid portion of their fat flows together to make a continuous mass, and these grow as churning continues.
From the book On Food and Cooking (2nd edition) by Harold McGee. © 2004 Harold McGee.
By permission of Scribner, a division of Simon & Schuster, Inc.