# Critical Review of Techniques for Food Emulsion Characterization
## 3 Techniques for Emulsion Characterization
### Rheological Analysis of Emulsions
The linear viscoelastic properties are measured within the range of stress, strain, and shear rates, in which the measured properties depend only on frequency and time (as well as temperature and pressure). These properties include the frequency-dependent storage and loss moduli, \(G^{\prime}\) and \(G^{\prime\prime}\), the various combinations of these such as complex modulus \(G^{\star}\), the phase lag, and the time-dependent creep compliance. The storage modulus \(G^{\prime}\) serves as an indicator of the elastic component within the viscoelastic behavior, effectively capturing the solid-state characteristics of the sample. In contrast, the loss modulus as \(G^{\prime\prime}\), delineates the viscous component of the viscoelastic behavior, reflective of the liquid-state properties of the sample. \(G^{\prime}\) symbolizes the stored deformation energy, while \(G^{\prime\prime}\) characterizes the deformation energy lost (dissipated) through internal friction when flowing. Viscoelastic solids with \(G^{\prime}>G^{\prime\prime}\) have a higher storage modulus than loss modulus, whereas viscoelastic liquids with \(G^{\prime\prime}>G^{\prime}\) have a higher loss modulus than storage modulus. While, typically, these properties may not apply to high-stress technical applications, they are valuable for evaluating the microstructure and stability of emulsions [119]. The non-linear characteristics of emulsions include properties that are influenced by the applied stress, strain, or shear rate. These properties include the non-linear variations of the moduli and compliances, mentioned above, and viscosity. Viscosity is one of the most basic parameters describing the rheological properties of emulsions, which enables distinction between whether the analyzed sample is a classic (O/W) emulsion or reverse (W/O) emulsion. In general, low viscosity indicates O/W emulsion, while high viscosity indicates W/O emulsion [113]. The viscosity of emulsions can be measured with various types of viscometers, such as capillary, rotational, falling-ball, oscillatory, torque, and interfacial viscometers [33]. Based on the geometry of the measurement cell, rheometers can be divided into several categories including spindle, concentric cylinder, parallel plate (plate-plate), cone-cone, cone-plate, and vane spindle apparatuses, depicted in Figure 10[129]. Dynamic shear rheometers, such as the Brookfield rotational rheometer, are the most frequently used devices for simple single-speed viscosity measurements of emulsions. This type of viscometer applies shearing deformation force and measures viscosity based on the torque required to rotate a spindle immersed in the fluid, which increases proportionally to the viscosity of the analyzed sample. However, single-speed viscosity measurements are not sufficient to characterize the rheology of emulsions, the majority of which are complex non-Newtonian liquids, whose viscosity decreases as the applied shear rate increases or _vice versa_. As a result, emulsions require a much more thorough rheological analysis, including a comprehensive assessment of their oscillatory shear behavior. Oscillatory shear tests involve subjecting emulsions to alternating shear stresses to study their responses under dynamic conditions [130]. This type of analysis provides valuable insights into the viscoelastic behavior of emulsions, including their ability to deform and flow under various stress conditions. Dynamic oscillatory shear tests involve subjecting a material to sinusoidal deformation and measuring the resulting mechanical response over time. Most emulsions, when subjected to shear stress, display rheological properties characteristic of both fluids and solids [118]. As a result, during oscillatory shear tests, emulsions typically exhibit a range of dynamic responses, such as elastic (storage) and viscous (loss) moduli, phase shifts, and shear thinning behavior [131].