The International Confectionery Association (ICA) and other industry bodies set forth a wide range of protocols and methodologies designed to guide food manufacturers toward best practices in the production of chocolates. Among the protocols is ICA Method 46, which pertains to testing chocolate viscosity. The latest advancements in rheological instrumentation have enabled food manufacturers to test viscosity in chocolate products faster and more reliably than previously, while meeting the requirements of ICA protocols.
Improving quality control
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 product—e.g., its surface appearance and mouthfeel—are directly related to the chocolate’s viscous behavior. Testing the viscosity is therefore one of the standard QC test methods for manufacturers that produce chocolate or use chocolate to produce related confectionery items, such as chocolate coated cookies.
Viscosity testing in QC is easier and more reliable using the Thermo Scientific HAAKE Viscotester iQ rheometer (Thermo Fisher Scientific, Waltham, MA), which features functionality that has not traditionally been available in a viscometer. The rheometer was designed for QC applications, in particular. Due to improved sensitivity, the latest technology enables food producers to use smaller measuring geometries, which reduces sample volume, time for temperature equilibration, and cleaning effort. Also, smaller shear rates are achieved due to improved sensitivity, which increases the reliability of yield stress calculations with extrapolation methods like the Casson model.
Viscosity analysis: A case study
Two chocolate samples—milk chocolate and dark chocolate—were prepared according to ICA Method 46. Chocolate pieces were placed into glass containers, which were sealed and heated in an oven at 52 °C for 45–60 minutes. The cup and bob of the measuring geometry were preheated to 40 °C in the Peltier temperature control unit of the Viscotester iQ rheometer.
The CC25 DIN Ti measuring geometry was used for the testing. This small, cylindrical system with a sample volume of only 16.1 mL fits into the Peltier cylinder temperature control and is easy to disassemble and clean.
The test method was taken from ICA Method 46 and was translated into a Thermo Scientific HAAKE RheoWin job. The shear rate profile is shown in Figure 1.
Figure 1 – Shear rate profile applied according to ICA Method 46. The numbers 5–9 represent the job element number of the HAAKE RheoWin job shown in Figure 2.The HAAKE RheoWin procedure (Figure 2) consists of three parts: sample conditioning, testing, and evaluation. Sample conditioning should always be part of the test method to ensure that it is always performed the same way, which improves reproducibility of the results. In the conditioning phase (job elements 1–4), the sample was kept at rest, with the cylindrical upper part of the measuring geometry in the measuring position. During this time, any mechanical stress caused by sample loading and closing the geometry should relax completely. At the same time, the entire sample should reach the temperature at which the test will be performed.
Figure 2 – HAAKE RheoWin procedure used to run the test according to ICA Method 46. The test consists of sample conditioning (elements 1–4), rheological testing (elements 5–9), and data evaluation (elements 10–13).In the final part of the experiment (job elements 11–13), data evaluation was performed automatically by the 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 simpler approach, as discussed in a 2003 report,1 it was suggested to use the shear stress value at 5 s-1 as the yield stress. If this method is preferred, a simple interpolation calculation will work.
A steady-state viscosity curve at 40 °C was recorded for both samples (see Figure 3). Compared to transient viscosity data from shear rate ramps, the steady-state viscosity was independent of 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.
Figure 3 – Viscosity curves of milk chocolate and dark chocolate at 40 °C. The milk chocolate shows significantly higher viscosity.A typical representation of the results from a test according to ICA Method 46 is shown in Figure 4. 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.
Figure 4 – 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 the flow curves (blue curves) to 0 s-1 has been calculated according to Casson. The green vertical line at 5 s-1 represents the yield stress according to Ref. 1.The viscosity curves for the increasing shear rate ramp and 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. The green parabolic curves extrapolating the flow curves to a shear rate of 0 s-1 represent the Casson fit. The vertical green lines indicate where the interpolation according to the report in Ref. 1 has been calculated. The results of the different methods to determine the yield stress of the two chocolate melts are summarized in Table 1.
Table 1 – Determination of yield stress based on the data from Figure 4 using different models 
Importantly, Table 1 shows that different models yield different results, even from the same data. Therefore, only yield stress values calculated 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.
Conclusion
In quality control, the rheological characterization of chocolate focuses mainly on its viscosity and yield stress. Compact rheometers that offer the right combination of sensitivity and strength are able to successfully test chocolate melts over a wide range of shear rates. Using only a small sample, the commonly accepted test method according to ICA Method 46 can be performed easily, and the same is true for steady-state viscosity curves. The high quality of the results is a good basis for reliable data analysis with a variety of available methods and models.
Reference
- Servais C.; Ranc, H. et al. Determination of chocolate viscosity. J. Texture Studies Dec. 2003, 34(5–6), 467–97.
Klaus Oldoerp is senior application specialist, chemical analysis, Thermo Fisher Scientific, 168 3rd Ave., Waltham, MA 02451, U.S.A.; tel.: 781-622-1000; e-mail: [email protected]; www.thermofisher.com