Credit: © 2008 Neil Withers

The ability of a solid substance to exist in more than one crystalline form — a phenomenon known as polymorphism — is responsible for fat bloom in chocolate. Cocoa butter can adopt six different crystal forms and only one of these has the most desirable properties. Recrystallization to form other polymorphs can occur — leading to an unattractive dull grey coating — unless chocolate is processed and stored in carefully controlled conditions. Studying the microstructure of chocolate can help in understanding how fat bloom occurs, but the soft nature of chocolate can prevent the use of traditional analytical techniques.

Now Dérick Rousseau from Ryerson University in Toronto and Paul Smith from the Institute for Surface Chemistry in Stockholm have used environmental scanning electron microscopy to study how the structure of chocolate changes during prolonged storage1. It was found that the structure of the chocolate was non-homogeneous, with numerous air bubbles and pores. In both filled and plain chocolate, the growth of fat bloom structures was observed to start mainly at imperfections in the surface, with the rate of formation highest in the case of filled chocolates. The filled chocolate also became considerably softer over time because fats from the filling started to dissolve the chocolate shell.

Having obtained a greater understanding of the structure of chocolate, the next step will be to try to control the structure by including additives in the recipe or changing the processing techniques. If, for example, chocolate can be made without surface imperfections, then this may significantly improve its shelf life or ensure that it remains attractive with less strict storage conditions.