Effect of particle size distribution on rheological properties of chocolate

This study focused on the influence of the particle size distribution (PSD) of ingredients used for chocolate preparation (cocoa powder and sugar) on viscosity of model chocolates with different fat contents. Model chocolates with varying PSDs and two particle size ratios (cocoa particles : sugar of 1 : 4 and 1 : 6) were prepared by mixing highly defatted cocoa powder as a fine fraction and sugar as coarse fraction in several proportions. Samples containing spherical quartz beads were included as a reference to gain insight into the role of particle properties. Comparing chocolate samples with the same composition, but different particle size ratio, the largest difference in viscosity was found at a proportion of coarse fraction of 0.6. This shows that at this proportion of coarse particles, the highest maximum packing fraction was reached, which is in accordance with theoretical predictions. However, for the investigated chocolate samples, the lowest viscosity values were obtained at a proportion of the coarse fraction of 0.8 or 1. This could be attributed to parameters other than PSD, such as a decrease in surface area with an increasing amount of coarse particles. Also the morphology and surface properties were shown to influence viscosity. Compared to cocoa and sugar particles, spherical and smooth quartz beads led to an improved particle packing, and therefore a lower viscosity. The addition of lecithin led to a decrease in viscosity due to a decrease in particle-particle interactions, particularly for hydrophilic sugar particles. The knowledge obtained in this study provides possible approaches on how to reduce fat content of chocolates.