Fat bloom on chocolate confectionery systems - From core to surface

Abstract: Abstract Fat bloom on chocolate is a major problem for the confectionery industry since the unappetising appearance and negative sensory effects lead to rejection by customers. The presence of fat bloom on chocolate confectionery systems is usually connected to migration of liquid fat due to the difference in composition between filling triacylglycerols (TAGs) and cocoa butter TAGs. The filling TAGs migrate into the chocolate shell where they can dissolve cocoa butter crystals. Consequently, cocoa butter TAGs migrate to the surface followed by a re-crystallisation into the most stable polymorph ?1VI. Cocoa butter is the main fat in chocolate which can be considered as a composite material consisting of solid particles (i.e. cocoa particles, sugar crystals and in some cases milk solids) in a lipid continuous matrix of cocoa butter. The final quality of the product is highly dependent on the polymorphic forms of the cocoa butter TAGs in the fat phase and the distribution and size of the solid particles. In this thesis the migration of filling oil into model shells of cocoa butter and of chocolate has been investigated as well as the fat bloom development. This was implemented through the development of novel analytical methods, where optical profilometry and confocal Raman microscopy give information regarding the shell microstructure at and below the surface, and energy dispersive X-ray spectroscopy (EDS) provides the opportunity to follow the movement of brominated TAGs from the filling into the shell. By combining these techniques with established methods such as low vacuum scanning electron microscopy (LV SEM) and differential scanning calorimetry (DSC) a toolkit for the investigation of oil migration connected to surface microstructure development has been established. Imperfections, in form of pores and protrusions, at chocolate surfaces have been identified, confirming previous studies reporting these features. These imperfections were characterised using confocal Raman microscopy indicating that some protrusions are filled with fat and some are air-filled in conjunction with a fat shell, while the pores consist of air. These imperfections continued further into the chocolate shell, thus, it is suggested that they could be connected to oil migration and further to fat bloom development. The microstructure of model shells was shown to have a substantial impact on the TAG migration rate which was connected to fat bloom development. By applying seeding as pre-crystallisation technique to the shells the migration rates were decreased as well as the development of fat bloom crystals at the surface. In contrast, model pralines with poorly tempered shells indicated a higher oil migration rate and accelerated development of fat bloom. Furthermore, the presence of non-fat particles was shown to increase the migration rate and the fat bloom development. Additionally, the particle size of the non-fat particles proved to have an impact, where a smaller particle size gave rise to higher migration rates and thus, accelerated fat bloom development. The importance of controlled storage temperature was further demonstrated, where a minor increase in temperature from 20 to 23°C lead to substantially higher migration rates and accelerated fat bloom development. The mechanisms of oil migration in chocolate confectionery systems have mainly been referred to as molecular diffusion or capillary flow in literature. However, through results from the work of this thesis, convective flow is suggested to be an important contribution to the migration of filling oil in addition to molecular diffusion and capillary flow.