Flow Behavior of Chocolate Melts

In addition, a steady-state viscosity curve at 40 degrees Celsius has been recorded for both samples. Compared to transient viscosity data from shear rate ramps, the steady-state viscosity is independent from time-dependent effect and the slope of the shear rate ramp. For comparison of viscosity data, the steady-state viscosity is the best choice because it is independent of the instrument used and can be directly correlated with the shear rate applied.

The Results

A typical representation of the results from a test according to ICA method 46 is shown in Figure 2. The red curves depict the viscosity and the blue curves the shear stress. It clearly shows that the milk chocolate has the higher viscosity by a factor of two or more.

The viscosity curves for the increasing shear rate ramp and the decreasing shear rate ramp are almost identical for the dark chocolate. In contrast, the milk chocolate shows a pronounced thixotropic behavior with significant differences between the two viscosity curves.

Green parabolic curves extrapolating the flow curves to a shear rate of 0 1/s represent the Casson fit. The vertical green lines indicate where the interpolation according to Servais has been calculated. Results of the different methods to determine the yield stress of the two chocolate melts have been summarized in Table 1.

The first and probably most important result from Table 1 is the insight that even from the same data, different models give different results. Therefore, only yield stress values calculate with the same mathematical model can be compared.

Independent of the model chosen, the milk chocolate in this example shows the higher yield stress, the higher viscosity, and the stronger thixotropy.

Summary

In QC, the rheological characterization of chocolate mainly focuses on its viscosity and yield stress. Using an instrument that combines sensitivity and strength, like the HAAKE Viscotester iQ, can help to successfully test chocolate melts over a wide range of shear rates. The commonly accepted test method according to ICA method 46 can easily be performed using only a small sample. The same is true for steady-state viscosity curves.

Dr. Oldörp is a senior application specialist at Thermo Fisher Scientific in Karlsruhe, Germany. Reach him at [email protected].

References Furnished Upon Request