The role of particle size distribution of suspended solids in defining the flow properties of milk chocolate

The flow characteristics of molten milk chocolate, like most dense suspensions, are a function of the maximum packing fraction (φ_m) of the suspended solids. Milk chocolates were prepared from mixtures of two relatively narrow coarse (d_4,3 = 17 μm) and fine (d_4,3 = 8.5 μm) size distributions of non-fat solids to contain 0, 25, 50, 75, and 100% of the coarse component. Solids bed density (a measure of maximum packing fraction) correlated with the apparent viscosity of milk chocolates (r = -0.98). The highest value for solids bed density and lowest value for apparent viscosity were measured for size distributions with 75% of solids from the coarse fraction. Casson yield value correlated with d_4,3 (r = -0.98) and specific surface area (r = 0.93). The maximum packing fraction of binary mixtures of two distinct sizes of non-fat chocolate solids attained a maximum value for a specific volume ratio of the components. The maximum in φ_m corresponded with the minimum viscosity in molten chocolate at a given solids volume concentration.