Rheology is a branch of physics that measures the deformation or flow of matter when force is applied. The rheological behaviour of a material, such as liquids and soft solids, can indicate whether they are viscoelastic.
Deformation is defined as the change in shape or size of a material when an external force is applied to it. Deformation measurements can be conducted using a rheometer.
There are two types of deformation that occur and can be measured: elastic and viscous. Elastic deformation is reversible — imagine a rubber ball returning to its original shape after you squeeze it. In contrast, viscous deformation is irreversible and the material flow will continue even when the force is removed. Here we might imagine melted cheese oozing out of a sandwich, being acted upon by gravity and the pressure from the slices of bread.
Rheologists measure the deformation of a material by monitoring its response to a known force. To do this, they use instruments called rheometers that apply controlled stress, controlled deformation or controlled rate, and measure the material’s response. Rheometers can measure both the viscous and elastic properties of non-Newtonian fluids, the flow behaviour of which have a non-linear relationship between shear stress and shear rate.
The textural properties of food products are an essential part of consumer satisfaction. Expectations about the way food will behave in someone’s mouth must be met to deliver a pleasant culinary experience. This is particularly true in recent times where vegan substitutes are being developed to provide plant-based alternatives to staples across the food industry.
Dairy cheese textures, for example, can range from creamy to solid depending on the type — consumers of vegan alternatives expect this range to be replicated. To grow their consumer bases, non-dairy cheese manufacturers are attempting to engineer vegan cheese with a similar appearance, feel and behaviour to its dairy equivalent. As part of this process, rheologists can quantify the viscoelastic properties of cheese and alternative formulations.
Additionally, modern rheometers are equipped with normal force sensors and can perform texture analysis tests. In the context of cheese, this might mean quantifying its bite or cutting properties.
Amplitude sweeps employ oscillatory rheology to determine the LVR (linear viscoelastic region), which describes the range of deformation in which a material exhibits a linear relationship between applied stress and the resulting deformation. Beyond the upper limit of the LVR, the physical and structural stability of the test subject will be compromised.
Amplitude sweeps are performed by applying an increasing, oscillating amplitude to a material. The amplitude, expressed as percentage strain or stress, increases until the upper limit of the LVR is reached. This is usually defined as a 5% G’ (storage modulus) decrease, after which point the material structure is being damaged.
Temperature sweeps can be conducted in either rotational or oscillatory modes and are used to measure rheological changes in food as a function of temperature while a constant stress or strain is applied. For our purposes, the data collected is complex viscosity (η*) and the phase angle (δ), a measure of the phase difference between the input strain signal and output stress response.
Axial relaxation tests are used to observe a material’s ability to relax or recover from deformation over time. These tests involve applying an axial force to the material and measuring the resulting deformation over time.
Using a Thermo Scientific HAAKE MARS iQ rheometer, the rheological characteristics of cheese vs vegan cheese were found to differ significantly.
Sample 35 mm discs of both types of cheese were placed between the parallel plates, which were serrated to prevent slippage. A parallel plate geometry was selected to enable adaptation to the thickness of the discs of cheese. Amplitude sweeps, temperature sweeps and axial relaxation procedures were used to investigate the rheological properties of the two cheese products.
The amplitude sweeps performed using the Thermo Scientific HAAKE Mars iQ Air rheometer were conducted at 37 °C over a strain range from 0.01% to 100% at a frequency of 1 Hz. The results highlight an interesting difference in the behaviour of the two food products.
Figure 1 – Results from the amplitude sweeps at 37 °C plotted over the deformation γ in %. Credit: ThermoFisher Scientific ®
The phase angle, δ, is a relative measure of a material’s viscosity and elasticity. Closer to 0° indicates elastic behaviour, whereas closer to 90° indicates viscous behaviour. At 37 °C, the vegan substitute showed a plateau value for the phase angle at approx. 2°, demonstrating that under these conditions it is more elastic than the dairy-based cheese product. Under the same conditions, this exhibited softer behaviour and shows a plateau value of 24.8°.
The temperature sweeps conducted with the Thermo Scientific HAAKE Mars iQ Air rheometer within the linear viscoelastic range yield results over a wide temperature range of 5 to 90 °C. During the measurement, the rheometer applied a constant oscillation with 1% strain and 1 Hz to the sample while the temperature was increased with a heating rate of 2 °C/min.
Figure 2 – Development of viscosity and phase angle during heating up to 90 °C.
Using δ as a measure for the viscoelastic properties, the vegan substitute cheese (average δ=29.3°) appears to behave less elastically than the regular cheese (average δ=20.5°) at the starting temperature of 5 °C.
However, at higher temperatures, the differing behaviour from the protein-based structure of the milk-based cheese and the starch-based structure of the vegan alternative becomes apparent. The δ-values of the dairy product increased with temperature, and showed a melting point at 48 °C. On the other hand, the δ-values of the vegan alternative showed little variation above 27 °C but decreased at around 45 °C.
Cheese melting characterisation showed that, at higher temperatures, the milk-based cheese had more liquid-like characteristics. By contrast, the vegan cheese did not ‘melt’ but became a more elastic gel.
Figure 1 – Results from the amplitude sweeps at 37 °C plotted over the deformation γ in %. Credit: ThermoFisher Scientific ®
Figure 2 – Development of viscosity and phase angle during heating up to 90 °C
The axial relaxation tests carried out using the Thermo Scientific HAAKE Mars iQ Air rheometer were conducted at 37 °C. The 35 mm discs of cheese were placed on the lower plate and the upper plate was lowered until an axial force of 3 N was detected, although this was reduced to 0.7 N for the milk-based cheese to stop it from being squeezed out of the gap.
A waiting time of ten minutes was employed for thermal equilibrium and mechanical relaxation. The difference between the two food products was significant: the axial force in the vegan substitute cheese relaxed to 1.6 N, 53% of the initially applied force, whereas in the milk-based cheese the axial force dropped to just 0.05 N.
Rheological investigations such as this are essential in quality control for the food industry. The measurements performed on the Thermo Scientific HAAKE MARS iQ Air rheometer can be used to quantify the texture of cheese, either to control the quality of milk-based cheese or to better match the texture of vegan substitutes to the original.
Figure 3 – Axial relaxation of both samples after being exposed to 3 N at 37 °C.
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