Analyzing Product Texture Using Oscillatory Testing on a Rotational Rheometer

Formulation of products with the correct sensory as well as functional attributes poses a high level of difficulty. Specifically, the sensory attribute relies to a great degree on user feedback which can be achieved only after spending significant time and effort. In addition such feedback cannot always be easily interpreted in context of the material properties and the resultant rheological data.

In order to apply rheology as a tool for assessing product texture, it is important that the rheological test that is best suited for the specific application and the most suitable parameters for the test are recognized. For example, the process of application and rubbing of skin cream is one that involves high shear rate and is best assessed using steady shear test at a suitable shear rate. In contrast, in-the-pot texture, which is related to the underlying microstructure, can be best assessed by using creep test or oscillatory test.

Material texture under slight deformations can be assessed by using a simple test, such as an oscillation amplitude sweep that can provide significant information regarding sample stiffness, structural strength, springiness and deformation. Stiffness is measured using the complex modulus G*, where a higher value of G* indicates a stiffer structure. Conversely, the degree of elasticity and hence, the springiness of a structure, is denoted by the phase angle δ. As illustrated in Figure 1, a simple plot of G* vs δ represents this information.

Diagram showing simple interpretation of a G* vs δ in terms of material properties.

Figure 1. Diagram showing simple interpretation of a G* vs δ in terms of material properties.

The yield strain and yield stress are other information that can be obtained from such a test. Here, the yield strain and yield stress respectively denote the extent of structural deformation and the structural strength. A plot of elastic stress σ’, i.e. stress associated with the elastic (or storage) modulus G’, vs strain can provide this information. As illustrated in Figure 2, the values of strain and stress measured at the yield point denote the yield strain and yield stress, respectively, where the yield point is a peak in the elastic stress.

Diagram showing how an amplitude sweep can be used to determine the yield stress and strain.

Figure 2. Diagram showing how an amplitude sweep can be used to determine the yield stress and strain.

The way in which a material responds to slight shear deformations before beginning of the macroscopic flow can be evaluated by combining all the aforementioned information.

Experimental Procedure

  • A number of different products were assessed to indicate the differences between these products with respect to their textural properties.
  • A Kinexus rotational rheometer equipped with a roughened parallel plate measuring system and a Peltier plate cartridge was employed for making rotational rheometer measurements by using standard pre-configured sequences of the rSpace software.
  • To make sure that both the samples underwent a controllable and consistent loading protocol, a standard loading sequence was employed.
  • Unless specified, all rheological measurements were carried out at a temperature of 25°C.
  • The measurement involved execution of a strain controlled amplitude sweep beyond the material’s yield strain and automatic analysis of the data to obtain δ and G* values within the linear region and to obtain the yield stress and yield strain values on the basis of the peak in the elastic stress (σ ’).

Results and Discussion

Figure 3 illustrates the comparison of a wide range of products based on their elasticity and relative stiffness at a frequency of 1Hz.

Chart showing a G* vs δ plot for a number of food and personal care products at 25°C.

Figure 3. Chart showing a G* vs δ plot for a number of food and personal care products at 25°C.

The chart indicates that most of the samples are primarily elastic and have phase angles lower than 45°. On the contrary, the samples exhibit differing degrees of stiffness. For example, the body butter is 25 times stiffer, i.e. has a higher modulus, than the body lotion, and the hair gum is nearly 100 times stiffer. In contrast, the shower cream has a primarily fluid-like texture with a comparatively low stiffness and a phase angle of nearly 90°. When compared to the body butter which has a G* value of 8000Pa, the shower cream has a G* value of merely 23Pa.

Temperature has a predominant effect on the texture of butter. A highly elastic and stiff structure results due to fat crystallization at lower temperatures, i.e. during storage in refrigerator. In contrast, due to the melting of the fat matrix at room temperature, a less elastic and softer structure is formed, which is more similar to the texture of the body butter and the toothpaste.

The corresponding yield strain and yield stress values for different products are listed in Table 1. From the table, it can be noted that yield stress fundamentally denotes the stress that is needed to begin the network structure breakdown.

Table 1. Results from peak analysis of stress-strain curves.

Sample Yield Stress (Pa) Yield Strain (%)
Mayonnaise 11.26 1.79
Toothpaste 1.86 0.057
Body Butter 15.87 0.81
Body Lotion 2.24 2.63
Shower Cream 10.18 27.22
Hair Styling Gum 11.12 0.15
Butter (5°C) 34000 1.06
Butter (25°C) 1.12 0.096

In the case of viscoelastic fluids that do not have a network structure, i.e. fluids with δ value > 45°, yield stress refers to the stress that is needed to begin the significant flow, i.e. shear thinning.

Comparison between the body lotion and the body butter clearly shows that a higher stress is required to break down the structure of the body butter, which is obvious during product usage given the fact that body butter requires greater force to make it flow. As the body lotion possesses a higher yield strain, it deforms more before thinning, indicating a more ductile/less brittle structure. The yield strain and yield stress of the highly elastic mayonnaise are greater, thus resulting in the ‘rubbery texture’ when stored in the jar.

Even though the body wash exhibits a high critical strain and stress, in contrast to mayonnaise, it does not have a network structure, i.e. its d value is >45°. These critical values correspond to the deformation and stress that a material can endure before the flow is notably increased. Such values can at times correspond to the product stringiness or the extent of filament formation.

At refrigerator temperature, butter has a higher yield stress and can be difficult to spread. On the contrary, a notable reduction in yield stress is observed at 25°C due to melting of the crystalline fat matrix. Of interest is the fact that at such higher temperatures, butter is more brittle, which is denoted by the smaller yield strain.

Conclusion

Significant information regarding textural property of materials, e.g. stiffness, structural strength, springiness and brittleness, can be obtained by performing the oscillation amplitude sweep test. A material’s appearance and the way in which it responds to slight deformations can be assessed by measuring the parameters related to these textural properties. This technique is greatly helpful in the characterization and comparison of the material properties.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.

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