# Critical Review of Techniques for Food Emulsion Characterization
## 3 Techniques for Emulsion Characterization
### Rheological Analysis of Emulsions
The elastic modulus represents the stored energy in the emulsion, reflecting its ability to recover after deformation, while the viscous modulus characterizes the energy dissipation and flow resistance. These moduli are crucial in understanding the stability and texture of emulsions, especially in applications where elastic or viscous properties are essential. The phase shift between stress and strain in oscillatory shear tests provides information about the nature of the interactions within the emulsions, such as droplet collisions, interfacial interactions, and structural rearrangements. Understanding these phase shifts helps predict the stability and structural changes of emulsions under different processing and storage conditions. Oscillatory shear behavior can reveal shear-thinning properties in emulsions, where the viscosity decreases under increasing stress amplitudes. This shear sensitivity is vital in industrial applications, such as food  
Figure 10: Classification of rheometers based on geometry of measurement cell: spindle, concentric cylinder, parallel plate (plate–plate), cone–cone, cone–plate, double cone–plate. and pharmaceutical processing, as it affects the pourability, spreadability, and mouthfeel of emulsion-based products. Across measurements of oscillatory shear behavior, we could underline the amplitude, frequency, or temperature sweep test. The amplitude sweep test can be performed by varying the oscillatory stress (e.g., in the range of 0.01-100 Pa) at a fixed frequency (e.g., 1 Hz). This measurement can show a linear viscoelastic region (LVR), in which \(G^{\prime}\) (storage modulus) and \(G^{\prime\prime}\) (viscous modulus) are almost constant and, later, a nonlinear region, in which \(G^{\prime}\) and \(G^{\prime\prime}\) start to change [132]. The oscillatory stress value, where \(G^{\prime}\) sharply decreases, is defined as the critical oscillatory stress, also known as the limiting value of oscillatory stress (OSL). Obtaining the OSL value is crucial, as it indicates the maximum deformation that a system can withstand without structural breakdown [133]. Here, we can investigate how the composition or processing of emulsions can impact their strength and rigidity during storage at a defined time, temperature, or other conditions of our choice. During the frequency sweep test, the viscous and elastic behavior of emulsions can be monitored [133]. During storage, we can test the emulsions within a defined experimental frequency range (e.g., 0.01-10 Hz) and observe if the samples display gel-like behavior, resembling a solid rather than a liquid, or the other way around. Deformation can be interpreted as elastic and recoverable. In order to determine the frequency dependence of the \(G^{\prime}\) parameter (\(n^{\prime}\)), we can apply a power-law relation. Parameter \(n^{\prime}\) close to 1 indicates that the system behaves as a viscous gel, whereas a low \(n^{\prime}\) parameter shows characteristics of elastic gels. To determine the sensitivity of the sample structure to thermal changes, a temperature sweep test can be conducted. In this test, we can observe the changes in the elastic behavior of the emulsion with a temperature increase, such as melting or transformation of the liquid state into a gel state (sol-gel transition), which can be related to the composition of the emulsifier [134; 135]. To accurately reflect the rheological behavior of emulsions, their rheological properties should be measured under conditions mimicking real-life environments in terms of temperature, pressure, mechanical stress, shear rate, etc. For example, the assessment of emulsion long-term stability should be performed under creeping flow conditions that simulate storage environments. Creeping flow (also known as Stokes flow) is defined as a non-turbulent flow, in which fluid flow velocity is very low and the Reynolds number is below 1 [136]. Under such conditions, inertial effects are negligible, while the dominant role is played by the viscous forces and viscous resistance.