Evaluating the Flow Behavior of Molten Chocolate

Introduction

Chocolate is a concentrated suspension with particles such as sugar, cocoa and milk suspended in a continuous fat matrix. The rheological properties of molten chocolate are important in food processing for obtaining high-quality products with well-defined texture [1]. Different formulations in chocolate such as fat content, particle size and distribution, moisture or emulsifiers content all have a big influence on the chocolate’s flow behavior [2-3]. Molten chocolate behaves in a non-Newtonian manner with a yield stress. Controlling a desirable viscosity and yield stress in a molten chocolate formulation is critically important in chocolate processing as well as in applications. Over the past years, there have been numerous literature articles that discuss the analysis of rheological properties of molten chocolate using different methods [4-6].

The International Confectionery Association (ICA) introduced a standard method (ICA method 46) for evaluating the viscosity and yield stress of molten chocolate [7]. This standard recommends testing the viscosity of molten chocolate at a temperature of 40 °C. The test procedure can be either a continuous flow ramp from 2-50 s^-1 within 3 minutes or a flow step from 2-50 s^-1 at 16 seconds per step (Figure 1). Prior to the measurement, the molten chocolate is pre-sheared at 40 °C with a shear rate of 5 s^-1 for 5 minutes. The measurement results are then analyzed using the Casson flow model (equation 1).

Where σ_y is the yield stress and k is the Casson plastic viscosity [5].

√σ=√(σ_y )+ √(kγ).     (1)

The most commonly used geometries for ICA testing are concentric cylinders with cups and rotors (Figure 2). This cup/rotor device requires large amount of sample and longer temperature equilibration times.

The TA Instruments Discovery Hybrid Rheometer can be equipped with an Upper Peltier Plate (UPP) temperature control system, which is used in combination with the lower Peltier plate (Figure 3). This device provides the most uniform temperature control across the sample. The platinum resistance thermometer (PRT) is placed in direct contact with the testing plate to get the most accurate measure of sample temperature. In this application note, a 40mm sandblasted parallel plate geometry was used in conjunction with the UPP to conduct ICA method 46 testing on 2 molten dark chocolates. The Parallel plate geometry uses much less sample compared with the cup/bob geometry, approximately 1-2mL versus 8-20mL. Also, the sandblasted roughened surface texture helps to effectively prevent sample slippage at the interface.

Experimental

Two dark chocolates with 70% and 95% cocoa content, respectively, were purchased from a grocery store. The samples were equilibrated and pre-melted at 55 °C for one hour before any measurements. A TA Instruments Discovery Hybrid Rheometer with the UPP temperature control system was used for this measurement. The testing geometry was a 40mm sandblasted parallel plate. At a measurement gap of 1mm, the required sample volume is 1.26ml. The ICA method 46 test protocol was translated and programed in TRIOS software and is shown in figure 4. In the first step, molten chocolate was pre-sheared at 40 °C with a shear rate of 5 s^-1 for 5 minutes. Then in the second step, the shear rate was ramped up from 2 s ^-1 to 50 s^-1 in 3 minutes. After that, the shear rate was held constant at 50 s^-1 for 1 minute. Then in the last step, the shear rate was ramped down from 50 s^-1 to 2 s^-1 in another 3 minutes. The ICA stepwise method can also be setup in TRIOS and the testing results are identical (data not shown in this apps note).

Table 1. Flow analysis summary on 2 dark chocolates

% of Cocoa content	Yield stress (Pa)	Casson viscosity (pa.s)	Normalized thixotropy (10-5 1/s)
70	2.67	0.33	8.82
95	0.61	0.28	8.52

 

In recent years, researchers have noticed the limitation of using the ICA test method. First, the recommended shear rate range is limited. Many food processing applications often require the chocolate viscosity information in a much broader shear rate range than the ICA test outlined. The low shear viscosity (e.g. 0.01 s-1) helps correlate to formulation stability, and the high shear viscosity helps guide specific applications needs. Also, even with the stepwise ICA method, the test time at each individual shear rate (i.e. 16 seconds) may not be long enough to ensure that the sample has reached equilibration. This is especially the case when testing high viscosity chocolate formulations under lower shear rates. TA TRIOS software has a steady state sensing algorithm, which helps users to accurately collect steady state equilibration viscosity data with a wide range of shear rate (shown in figure 7).

Figure 8 shows the steady state flow test results of 2 dark chocolate samples at shear rates from 0.01 s^-1 to 500 s^-1. The measurement results fit the Carreau viscosity model well (equation 2).

Where η₀ is the zero shear viscosity, η_∞ is the infinite rate viscosity, k is the consistency, and n is the rate index. The results show that molten dark chocolate with less cocoa content has higher viscosity and exhibits more shear thinning behavior compared with the chocolate with greater cocoa content.