Flow Behavior of Chocolate Melts

The flow behavior of molten chocolate is a crucial parameter for many reasons. During production, the transport, filling, dipping, coating, and dosing steps depend on a well-defined viscosity and yield stress. Likewise, the properties of the final chocolate, like the look of its surface or its mouth feel, are directly related to the chocolate’s viscous behavior.

Testing the viscosity is therefore one of the standard QC test methods for any company producing chocolate or using chocolate for their own production, e.g. chocolate-coated cookies.

There are various instruments available to make viscosity testing in QC easier and more reliable. For example, the Thermo Scientific HAAKE Viscotester iQ makes it is possible to use smaller measuring geometries, which reduces sample volume, time for temperature equilibration, and cleaning effort. Also, smaller shear rates are accessible due to the improved sensitivity, which enhances the reliability of yield stress calculations with extrapolation methods like the Casson model.

A Look at a Preparation Method

Two chocolate samples, a milk chocolate and a dark chocolate, have been prepared according to ICA method 46 by putting chocolate pieces into glass containers, sealing the containers and leaving them in an oven at 52 degrees Celsius for between 45 and 60 minutes. Meanwhile, the cup and bob of the measuring geometry are preheated to 40 degrees Celsius in the Peltier temperature control unit of the Viscotester iQ.

For the tests done for this report, the CC25 DIN Ti measuring geometry has been selected. This small cylindrical system with only 16.1 milliliter sample volume fits into the Peltier cylinder temperature control.

The ICA test method 46 has been translated into a Thermo Scientific HAAKE RheoWin job. The shear rate profile is shown in Figure 1.

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Figure 1: Shear rate profile applied according to ICA method 46.

HAAKE RheoWin Job consists of three parts: sample conditioning, testing, and evaluation. The sample conditioning should always be part of the test method itself to ensure that it is not forgotten and always performed in the same way, which improves the reproducibility of the results. During the conditioning part the sample is kept at rest with the cylindrical upper part of the measuring geometry already in measuring position. During this time any mechanical stress caused by sample loading and closing the geometry should relax completely while at the same time, the whole sample should reach the temperature the test is going to be performed at.

In the final part, the data evaluation is performed automatically by HAAKE RheoWin. To calculate the yield stress of a chocolate melt, the traditional Casson model and the modern Windhab model can be selected from a long list of fit models. In a more simple approach, “Determination of Chocolate Viscosity” published in Journal of Texture Studies suggested to use the shear stress value at 5 1/s as the yield stress. If this method is preferred, a simple interpolation calculation in HAAKE RheoWin will do the job.

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Figure 2: Test results for a milk chocolate (open symbols) and a dark ­chocolate (filled symbols). The milk chocolate shows higher viscosity values (red curves), stronger thixotropy, and a higher yield stress. The extrapolation of flow curves (blue curves) to 0 1/s has been calculated according to Casson. The green vertical line at 5 1/s represents yield stress according to Servais.

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Table 1: Determination of yield stress based on the data from Figure 2 using different models.

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Figure 3: Viscosity curves of milk chocolate and dark chocolate at 40° C. The milk chocolate shows a significantly higher viscosity.