Bloom development and associated fat migration were analyzed from a microstructural viewpoint. To do so, chocolate model bars were prepared by tempering a mixture made of sand particles (mean sizes of 120, 55 and 5 mu m) and cocoa butter (68% and 32% by mass, respectively). Model bars initial microstructure was characterized by means of confocal and electron microscopy, along with image analysis. Bloom was induced by storing model bars at 30 degrees C and was assessed by measuring the amount of migrated fat to the surface and the change in surface colour (whiteness index). Both measurements showed an initial rapid increase followed by a gradually decreasing gradient until reaching an asymptotic value. Importantly, it was determined that density change is a key factor in bloom development. Also, a higher fat migration rate was determined in small particle size bars. (c) 2006 Elsevier Ltd. All rights reserved.