International Viscosity Test Methods for Chocolate

Method ICA 46 from the International Confectionery Association prescribes the primary protocol used in Europe for testing chocolate viscosity. The National Confectioners Association in the USA defines a similar test for use in the Americas. Standards organisations have asked if it is possible or practical to incorporate the various methods for chocolate viscosity under one harmonised specification. Despite discussions toward this end, attempts to bring methodologies together into one single all-purpose test procedure have not been realised. What reasons prevent the world from standardising on one way to measure the product that is universally loved in all countries?

Chocolate formulations are a suspension of solid particles from a combination of milk powder, cocoa powder and sugar blended together in a liquid fat phase referred to as ‘cocoa butter’. The ability to process this mixture successfully and produce a quality final product depends in part on the Rheology of the formulation. Rheology is the science which characterises the flow properties and quantifies parameters such as yield stress, viscosity vs. shear rate, viscosity vs. temperature, and creep behaviour. The chocolate chemist must evaluate each formulation for these properties and ensure that they fall within established ranges for acceptable behaviour.
The instrument used to measure chocolate is a rotational type viscometer or rheometer with Searle (rotating spindle) or Couette (rotating chamber) geometry. The measurement system is referred to as ‘coaxial cylinder’ or ‘concentric cylinder’ (Figure 1). The spindle choice and instrument torque measuring capacity must be appropriate for the viscosity range of the sample. Standards organisations recommend that the spindle and chamber surfaces be made of polished steel. Some companies choose to use a spindle with fluted or serrated edges to ensure contact of the particle-laden sample with the rotating surface of the spindle. Because there is some variability in the equipment selected to make the rheological measurements, it is even more important that the test method be standardised if possible.
Shear rate is the single most important control parameter for viscosity evaluation of chocolate. Figure 2 shows mathematically how shear rate is calculated based on the geometry of the spindle/chamber measurement system. Viscosity behaviour for chocolate formulations typically shows a decrease as the shear rate increases. This type of flow performance is referred to as ‘shear thinning’ and has been given the scientific word ‘pseudoplastic’. Depending on the combination of ingredients and other additives in each batch of chocolate, the degree of shear thinning behaviour can vary substantially. Therefore, monitoring performance by making viscosity measurements in Quality Control is a most necessary control procedure.
Choice of spindle/chamber geometry will affect the viscosity readings obtained. A DIN measurement system used in Europe with a pointed end to the spindle (Figure 1) may be used such that the ratio of spindle to chamber diameter is not less than 0.85, giving a gap distance that will guarantee free migration of particles within the sample (more than 10 times the largest particle diameter). However, the popular Brookfield SC4-27 spindle (Figure 3), commonly used in the Americas and many other countries to measure chocolate viscosity, is below the 0.85 value. Nonetheless, measured viscosity values seem to correlate with the DIN geometry when measurements are made at similar shear rates.
In the actual test procedure the sample is pre-sheared and brought to temperature equilibrium. Uniform distribution of sample in the coaxial cylinder geometry annulus is the objective. Measurements of flow properties are carried out at 40°C using a rotational type viscometer/rheometer applying shear rates over the range of 2 to 50 s-1. Viscosity is measured throughout the shear rate range. A mathematical model can be used to analyse the data.
Figure 4 shows chocolate viscosity test data on a sample tempered at 40°C using an up/down shear rate ramp that varies from 2 to 50 s?¹. Viscosity decreases from almost 35 Pa.s (35,000 cP) to under 10 Pa.s (10,000 cP) as the shear rate increases. (Pa.s = Pascal second; cP = centipoises; both are scientific units of measurement for viscosity.) Decreasing the shear rate thereafter shows that the viscosity does not recover right away. There is some time sensitivity to the shearing action which results in a temporary loss of structure in the chocolate.
Figure 5 shows a similar viscosity test on chocolate using the NCA method. Note again that viscosity decreases with increasing shear rate, changing from slightly above 20,000 cP at 2 s?¹ to under 5,000 cP at 35 s?¹. Flow behaviour is analogous in both cases and gives the definitive information needed by the formulation chemist to confirm that the mixture is acceptable.
Whether methods need harmonisation may not be the critical question. The viscosity flow data appears adequate to make the pass/fail determination in both cases. Math models, such as Casson, are then used to evaluate the viscosity data and provide a calculation for the yield stress and plastic index. Yield stress is the resistance to initial movement or flow, measured in Pascals, while plastic index is the degree of shear thinning behaviour, i.e. how fast the viscosity decreases as a function of increasing shear rate.
Given that chocolate processors in Europe have an established history of success using methods like ICA 46, there is every reason to continue with this practice. By the same token, American confectionery manufacturers will continue to use the NCA method. Viscosity data can be compared in a straightforward manner because the graphs clearly show flow behaviour vs. shear rate. The underlying science in both cases is similar, therefore the results will always be comparable.
About the author: Robert G. McGregor, is the General Manager, Global Marketing for Brookfield Engineering Laboratories, Inc. He holds M.S. and B.S. degrees in mechanical engineering from MIT in Cambridge, Mass.