How to enhance formulation using rheology

Viscometers are used routinely to characterise food and drink products and offer simple, straightforward measurement for QC. They are also applied in formulation, but here rheology has far more to offer than simple viscosity measurements. Switching from a viscometer to a rheometer provides access to an extended range of test capabilities that can be used to accelerate and refine formulation to a more successful commercial solution.

Traditional barriers to fully exploiting the potential of rheology have been a lack of understanding regarding which test to apply and how to interpret the resulting data. Recent advances in the design and functionality of rheometers directly address these issues and make detailed, informative rheological studies more accessible. The following examples illustrate the potential rewards of exploiting these advances.

A more complete flow curve – extending viscosity measurements

A defining principle of rheological characterisation is that the measurement conditions applied should reflect those to which the product will be subjected during use. A failure to meet this condition results in the need for extrapolation beyond the measurement range and this can be problematic, as illustrated by Figure 1. The blue shaded window represents a typical measurement range for a rotational viscometer (Bohlin Visco 88, Malvern Instruments) and clearly highlights the inability of a viscometer to fully capture the viscosity profile of a sample, particularly at the extremes of the shear rate range.

In QC, this limitation may be unimportant, since it may be sufficient to simply verify consistency by testing against a single point specification within the accessible measurement range. However, the same is not true for formulation, where understanding how the material behaves under a wide range of conditions helps to ensure optimal performance under all circumstances.

For example, when a product is stored on a shelf and subjected only to the force of gravity, the resultant shear rate is extremely low, typically less than 10-3 s-1. Rheological behaviour measured under these conditions can be correlated with product stability, especially when the product is a suspension or an emulsion. A product that appears solidlike under such conditions (apparent yield stress) will present more resistance to settling than one that is more liquid-like and has a low zero shear viscosity. The ability to measure in the low shear rate region can, therefore, directly support the development of products with target stability. Measuring up into the higher shear rate range can be equally valuable for assessing dispensing characteristics when shear rates may be much higher. Modern rotational rheometers offer the torque and radial displacement sensitivity to measure down to these very low shear rates and stresses; they also have the gap accuracy and dynamic range to access much higher shear rates and stresses. This is exemplified in Figure 2 which shows data for ketchup measured over seven decades of shear rate and covering a viscosity range of 0.1 – 100,000 Pas. With a rotational viscometer only the bottom right hand corner of the chart would be accessible.

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