A common problem with unhomogenized cream is that it continues to separate in the carton: the fat globules slowly rise and concentrate further into a semisolid layer at the top. At refrigerator temperatures, the fat inside the globules forms solid crystals whose edges break through the protective globule membrane, and these slightly protruding fat crystals get stuck to each other and form microscopic butter grains.
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.