Flow properties of single origin chocolates: Effect of product formulation and particle size

Abstract: In small production facilities, such as those used to manufacture single origin chocolate, cocoa nibs are ground to reduce the particle size to levels below 20 microns. This is a very important step during chocolate production, since chocolates with particles greater than 20 microns will have a grainy texture and unacceptable mouthfeel. The objective of this research was to evaluate changes in viscosity during chocolate grinding as affected by chocolate formulation. Three different types of chocolates were evaluated: 70% dark, 50% milk, and a Swiss-style milk chocolate. Chocolates were processed in a stone grinder and samples were taken as a function of grinding time. For each timepoint, viscosity of the samples was measured using a rotational rheometer and fitted to the Casson model. Particle size was measured using a laser scattering instrument.

Results showed that yield stress increased linearly while the Casson viscosity decreased exponentially with grinding time. Particle size distribution of the chocolates showed a prominent bimodal distribution for short grinding times (~ 9 h) with small (~ 15 microns) and large (~100 microns) particles; with longer grinding time, the population of larger particles decreased. Yield stress values were higher for the 70% dark chocolate, but they were not very different between the two milk chocolates tested. The Casson viscosity was greatest for the 70% dark chocolate, followed by the 50% milk chocolate. The Swiss-style chocolate had the lowest Casson viscosity. Yield stress decreased exponentially with increased particle size, while Casson viscosity increased linearly with particle size. Changes of Casson viscosity with particle size were more evident for the dark chocolates compared to the milk ones.

These results are helpful for small chocolate producers that need better understanding on how the formulation and grinding of chocolate affect its flow behavior, which will ultimately affect chocolate handling during production.