# Critical Review of Techniques for Food Emulsion Characterization
## 3 Techniques for Emulsion Characterization
### Droplet Size Analysis
Tracking changes in emulsion PDI over time can provide valuable insights into the emulsion's performance and stability under different manufacturing, processing, and storage conditions. Figure 8: Evaluation of emulsion droplet size, size distribution, and stability using ultrasonic spectrometry (**upper panel**) and light scattering (**bottom panel**). Although DLS is capable of measuring droplets with sizes down to 3 nm, its reliability in assessing polydisperse samples may be significantly compromised [98; 99; 100]. This results from the fact that the light scattering intensity fluctuation detected by DLS is theoretically linked to the sixth power of the droplet size According to manufacturers of DLS instruments, a sample is considered monodisperse if its polydispersity index (PDI) is below 0.1, moderately polydisperse within the range of 0.1-0.4, and polydisperse if the PDI exceeds 0.4. When the polydispersity index (PDI) in DLS is elevated, and the direct method indicates a non-normal particle size distribution, the reliability of particle size measurements is significantly diminished. Under such circumstances, it becomes feasible to conduct high-resolution liquid sample measurements using a combination of Multi-Angle Light Scattering (MALS) and DLS with size-exclusion chromatography (SEC) or field flow fractionation (FFF) [101; 102]. However, these measurements rely on the assumption that the separation process does not impact the particles of interest. As will be discussed later, FFF necessitates intricate measurement conditions, and particles may potentially interact with the membrane or channel walls. Some particle size analyzers have the capability of performing automatic analysis of emulsion stability over time [97]. An example of such an instrument is the Turbiscan(r), which operates based on the principles of multiple light scattering and permits the assessment of phase separation phenomena, such as creaming or sedimentation [103; 104]. Figure 9: Histograms showing droplet size distribution in stable emulsion (**A**) and unstable emulsion separating through coalescence (**B**). Adapted with permission from [66]. Copyright © 2023 Springer Nature. The operation of this device relies on vertical scanning of the emulsion, enabling the real-time detection of changes in backscattering (BS) and transmission (T) intensities over time. During static multiple light scattering, a light source with a wavelength of 850 nm is directed onto the emulsion sample, and both backscattering and transmission signals are collected [105]. The acquired signals are associated with droplet concentration and size, as described by the Mie theory [106]. By repeatedly performing these measurements with a suitable frequency, the instrument allows for the continuous monitoring of physical stability, which provides insight into the development of emulsion instabilities. Despite being some of the most fundamental tools for emulsion characterization and stability assessment, particle size analyzers relying on dynamic light scattering have significant shortcomings. The first is reduced accuracy of measurement at high droplet concentrations, resulting from the occurrence of multiple scattering effects. Multiple scattering is a common problem that causes overestimation of droplet size. As a result, emulsion samples with high droplet concentrations need to be diluted before measurement [21]. However, dilution of emulsions may induce destabilization processes, including flocculation and coalescence, which in turn makes the results unreflective of the original sample. Furthermore, the calculation of droplet size in these devices is most often based on the assumption that the shape of the droplets is spherical, which in real-life samples may not often be the case. For emulsions containing droplets deviating in shape from the sphere, this assumption may lead to significant under- or overestimation of droplet size. Numerous studies have shown that there are significant discrepancies between data obtained from DLS and microscopic analysis.