Yogurt Monograph Series: Rheology
Daniel Wilbanks, Technical Contributor
To determine the textural properties of a material we must first probe it. For example, a foreign substance resting in a bowl could be fluid or solid. We would need to shake or touch the bowl to determine how it responds – i.e., to see if it “flows” or not. Rheometers similarly measure responses from a material by probing it, which is performed by a geometry in an oscillating or rotating motion (Figure 1). Most foods are complex and possess both fluid (viscous) and solid (elastic) properties and are referred to as viscoelastic materials. Rheology, then, is the study of deformation and flow.
Rotational rheology probes the sample in a clockwise or counter-clockwise motion and is often used to determine the viscosity of a material. Viscosity (η) is the resistance to deformation (flow) and is itself a ratio of the stress required to maintain a particular speed, or shear rate of the geometry (Equation 1). For many viscoelastic materials the viscosity is not a material constant, but changes based on how much stress is applied to the sample and for how long. For instance, the harder and longer yogurt is stirred, the thinner the texture becomes as the gel breaks or yields.
Shear thinning materials like yogurt cannot be adequately described by a single viscosity value – as an example try to determine which material in Figure 2 has the highest viscosity. Shear rate sweeps, as demonstrated in Figure 2, can be performed to describe a range of viscosities from very low to very high shear rates. A mathematical model can then be built to describe the viscosity of a material across the range of shear rates tested, or a single viscosity value can be reported for a particular shear rate if desired. Generally, the viscosity at low shear rates (η0) describe the material under low strain (at rest), while higher shear rates ~ 50 /sec (η50) relate to the mouthfeel.
For the flow curves shown in Figure 2, yogurt and whipped cream exhibit similarly high viscosities at low shear rates (η0). Think about yogurt and whipped cream if left undisturbed; both products at rest exhibit solid-like behavior indicative of a very high viscosity. However, the structure of whipped cream is weaker than yogurt and its viscosity deteriorates – or thins – more rapidly under stress. The degree to which the viscosity thins under shear forces can be quantified from math models obtained via flow curves. If you’ve ever consumed whipped cream, you’ve likely experienced its thin mouthfeel compared to yogurt. In contrast, honey exhibits a constant viscosity at a particular temperature. Viscous stabilizers such as starch, xanthan gum, and galactomannans reduce the shear thinning index of yogurt, leading to a higher viscosity under high shear.
The yield stress of a gel, which is the minimum force required to induce flow, can also be calculated from flow curves. By understanding the desired performance of a product, such as the texture of yogurt on the spoon (η0) or the mouthfeel (η50), product developers can use rheology to measure the textural properties of yogurt and adjust their formulation or processing parameters to meet those specifications.
This article originally appeared in the Spring 2023 issue of the Dairy Pipeline.