# Critical Review of Techniques for Food Emulsion Characterization
## 3 Techniques for Emulsion Characterization
### Rheological Analysis of Emulsions
Due to their high molecular weight, they exhibit a thickening effect and create a network that enhances emulsion stability by increasing its viscosity and the thickness of interfacial film [123]. As a result, hydrocolloid-stabilized emulsions display improved long-term stability compared to emulsions stabilized with proteins and small molecule surfactants [124]. Texture modifiers also improve the textural and functional attributes of the emulsion, such as gelation mechanism, gel strength, and fracture properties. Other parameters that significantly impact the rheological behavior of emulsions include the concentration of the dispersed phase, the size, and distribution of the dispersed droplets, as well as the presence of solid particles [125]. At higher volume ratios, emulsions typically exhibit non-Newtonian behavior. Smaller droplets tend to decrease the viscosity of emulsions due to reduced interfacial area and improved packing efficiency. The reduction in viscosity is attributed to the decreased resistance to flow exhibited by the smaller droplets. Conversely, larger droplets lead to increased viscosity due to enhanced droplet interactions and a higher dispersed phase volume. The interactions between larger droplets result in higher resistance to flow, thus increasing the viscosity of the emulsion [126]. Furthermore, the presence of solid particles in particle-stabilized emulsions, also known as Pickering emulsions, such as colloidal particles or nanoparticles, can significantly alter the rheology of the emulsion, influencing its flow behavior and overall texture. These particles can act as fillers or thickeners, contributing to increased emulsion viscosity and improved stability. The rheology of emulsions is also strongly related to the structure, thickness, and elasticity of the adsorption layer at the interface between the dispersed and the continuous phase. The properties of the interface influence droplet size distribution, coalescence, and flocculation [37]. The interface layer shields the internal liquid from velocity gradients across the continuous phase. A thick and elastic interfacial layer creates strong steric hindrance, which is key for the stability of the emulsion [88]. Studies comparing the thickness of the interfacial films in emulsions stabilized by different types of surfactants revealed that the thickest interfacial layers are formed by hydrocolloids and solid particles (10 nm to several mm and 5-10 nm, respectively) followed by proteins (1-5 nm), while the thinnest layers are generated by low molecular weight surfactants (0.5-1 nm) [123]. Moreover, emulsions stabilized by multilayer adsorption films exhibit greater stability due to higher thickness compared to single-layer films [127]. The interfacial layer thickness is linked also to the concentration of the emulsifying agent. In general, at higher emulsifier concentrations, the adsorption layer at the interface becomes thicker and more rigid, leading to increased viscosity and elastic behavior of the emulsion. Another crucial aspect affecting emulsion rheological behavior and stability is the elasticity of the interfacial layer. More elastic interfacial films prevent the aggregation and coalescence of the droplets, resulting in improved long-term stability of the emulsion [128]. On the other hand, a less elastic interfacial layer may intensify droplet coalescence and flocculation, thus reducing stability and increasing the viscosity of the emulsion. Higher elasticity of the adsorption layer contributes also to the viscoelastic behavior of emulsions. A more elastic adsorption layer can give the emulsion viscoelastic properties, such as shear-thinning. Conversely, if the relative rigidity of the interface is high, the rheological behavior of the emulsion may mimic a dispersion of solid particles [129]. The flow properties of emulsions can be divided into linear and non-linear [119].