The effects of shear and temperature history on the crystallization of chocolate

Experiments have been carried out on the tempering of chocolate using a temperature‐controlled shearing rig with a concentric cylinder geometry. This design maximizes uniformity of shear rate during tempering in contrast to most tempering devices where shear is often concentrated in a small part of the chocolate mass. Samples were subsequently cooled in a differential scanning calorimeter (DSC) to monitor how the sample crystallized, and then reheated to gain further information on the melting points of the polymorphs formed. The results can be interpreted using established theories on the crystallization mechanism. It was found that at least two polymorphic forms could be generated. The higher melting form predominated provided the shear rate was high enough, the temper time long enough, the rewarm temperature low enough, and the DSC scan rate slow enough. For parameters affecting the nucleation stage (temper time and shear rate), it was found that the transition was a sharp one, reflecting the notion that seed crystals need to grow past a threshold point in order to be stably formed. Raising the rewarm temperature had the effect of destroying seed nuclei, but this was a more gradual process. The bimodal nature observed of the melting points compared to the more spread‐out behavior of crystallization temperatures reflects the kinetic constraints found in crystallization which are not found on melting.