Grinding spray-dried milk powder near the glass transition temperature

The fine grinding of chocolate is typically accomplished on five-roll mills. Chocolate manufacturers consider milk powder, a component of milk chocolate, difficult to grind. Spray-dried milk powders comprise a glassy lactose matrix in which fat globules, air vacuoles and protein are entrapped. The glassy-rubbery transition in commercial milk powders usually lies between 60-70C, depending on the moisture content. A mixture of 60% wt/wt commercial whole milk powder, T_g∼60C, and 40% wt/wt cocoa butter was ground in a three-roll refiner at temperatures of 40, 50, 60, 70 and 75C. Below T_g the particles exhibited brittle fracture, while above T_g plastic deformation was evident and particles became highly asymmetric. The amount of fat liberated from the lactose matrix, so-called free fat, particle density, and mean particle size increased with grinding temperature. However, the Casson yield value and plastic viscosity of finished "white chocolate" coatings, manufactured to a constant free fat content, increased with grinding temperature, suggesting an influence of particle shape on flow behavior.