Chocolate Tempering in a Rheometer: Monitoring Rheological Properties During and After Crystallization of Cocoa Butter

The typical quality attributes of chocolate, such as its glossy appearance, hard and brittle texture, sharp melting, and fat bloom resistance, are determined by the process conditions under which solidification occurs. It is therefore reasonable to test the success of this process from their solid form through thermal, textural, and/or optical analysis. However, the solidification process of chocolate consists of a dynamic pre-crystallization step and a subsequent static crystallization step that relies on the crystallization of cocoa butter in its matrix. Each of these crystallization steps has a different mechanism and target. Additionally, the flow parameters such as apparent viscosity and yield stress during each crystallization step change and may affect the subsequent process steps. This work evaluates the possibility of performing rheological tests to follow changes in the flow behavior of dark chocolate during and immediately following pre-crystallization, as well as during solidification. Different tempering protocols, consisting of distinct time and temperature profiles, were performed by manually stirring and scraping the molten chocolate from a glass beaker in temperature-controlled water baths. Selected protocols were mimicked in the rheometer equipped with sandblasted parallel plates by performing a rotational test with temperature ramps to monitor changes in flow behavior. Key experimental settings including gap size, shear rate, pre-conditioning temperature, and residential time are discussed in this paper. Thermal and shear effects attributed to the evolution of apparent viscosity can be monitored within different phases of pre-crystallization. The flow curves of tempered chocolates from corresponding manual and rheometer tempering protocols, as well as the melting profiles upon their solidification, were comparable, reproducible, and indicating well-tempered chocolate characteristics. Furthermore, it was possible to follow up static crystallization in the rheometer after tempering. Thus, rheometrical techniques are proven to be a useful tool to monitor flow behavior changes in different phases of the solidification step in chocolate manufacturing and may provide important rheological data of molten, pre-crystallized, and solid chocolate.