Structure–fat migration relationships during storage of cocoa butter model bars: Bloom development and possible mechanisms Pablo Altimiras a , Leo Pyle b , Pedro Bouchon a, * a Departmento de Ingenierı ´a Quı ´mica y Bioprocesos, Pontificia, Universidad Cato ´ lica de Chile, Santiago 690441, Chile b School of Chemical Engineering and Analytical Science, The University of Manchester, M60 1QD, UK Received 26 May 2006; received in revised form 20 June 2006; accepted 22 June 2006 Available online 22 August 2006 Abstract 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 lm) 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 anal- ysis. Bloom was induced by storing model bars at 30 °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 gra- dient 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. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Bloom; Chocolate; Fat migration; Microstructure 1. Introduction The commercial title of chocolate is reserved for products that contain almost exclusively cocoa butter. This fat is con- stituted of triacylglycerols that crystallize at different tem- peratures and rates; between 1% and 2% of the fat is all saturated (SSS) and melts at a much higher temperature than the symmetrical mono-unsaturated fraction (SOS) that comprises approximately 80% of the fat. In turn, the way in which the SOS fraction crystallizes determines most of choc- olate attributes. This fraction can crystallize in six different forms, a property known as polymorphism, but only one of these forms (form V or b 2 ) gives the gloss and snap upon breaking which is appreciated by customers and desired by confectioners (Beckett, 2000). Form V is induced during tempering (that is pre-crystallization) and improper temper- ing leads to formation of an unstable polymorph known as form IV or b 0 1 , which is one of the causes of chocolate bloom. Fat bloom is the whitish haze formed on the surface of chocolate. This physical phenomenon is caused by the dis- persion of light on the small fat crystals (P5 lm) that are formed on the surface and impacts both, visual appearance and textural attributes (Hartel, 1999). In effect, bloom is a mayor quality problem for the chocolate industry after products leave the factory (Briones & Aguilera, 2005). The main causes behind fat bloom have been identified, however, how this phenomenon takes place is not clearly understood. Most theories point out that fat migration to the surface plays a determinant role. Beckett (2000) explains that fat bloom may occur due to insufficient for- mation of stable polymorphs (form V) in cocoa butter dur- ing tempering, leaving a liquid fat fraction that is propelled to the surface, particularly if chocolate has cracks and cre- vices (Hartel, 1999). Also, form IV to form V transforma- tion releases some latent heat, which can enhance fat migration. Another cause of fat bloom is associated with a polymorphic crystalline transition from form V to VI during a long period of storage. As before, latent heat is released during this transformation, however changes 0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.06.022 * Corresponding author. Tel.: +56 2 3547962; fax: +56 2 3545803. E-mail address: pbouchon@ing.puc.cl (P. Bouchon). www.elsevier.com/locate/jfoodeng Journal of Food Engineering 80 (2007) 600–610