# Critical Review of Techniques for Food Emulsion Characterization
## 3 Techniques for Emulsion Characterization
### Droplet Size Analysis  
Droplet size is one of the most vital characteristics of emulsions, determining their stability and physicochemical properties. Emulsion droplet size and size distribution are typically analyzed by dynamic light scattering [95], small-angle X-ray scattering, ultrasonic spectrometry, or electrical pulse counting techniques [29; 37] (Figure 8). The analysis is typically performed using fully automated particle size analyzers, which allow measurements of large numbers of droplets within several minutes or less. There are two main categories of commercially available light scattering instruments for particle size analysis: dynamic and static light scattering devices [96]. Dynamic light scattering (DLS) devices measure the intensity fluctuation of light scattered by the emulsion droplets by assuming that smaller particles move faster than larger ones and create a higher rate of intensity fluctuation. When a monochromatic light beam passes through an emulsion, the light is scattered by the droplets undergoing the Brownian motion at a certain angle [83]. The random movement of emulsion droplets causes rapid fluctuations in the intensity of scattered light, which are dependent on droplet size. The percentage and angle of backscattered light are recorded by a detector [97] and converted into the photocount (intensity)-time correlation, which is further used to calculate droplet size, size distribution, concentration, and polydispersity index (PDI). The polydispersity index describes the uniformity of droplet size and can take values between 0 and 1 with 0 corresponding to completely monodisperse systems [83]. Figure 7: An interfacial film in emulsion stabilized with \(\beta\)-lactoglobulin visualized by atomic force microscopy (AMF). The images show the gradual displacement of \(\beta\)-lactoglobulin molecules from the interfacial film at an air-water interface by the progressive addition of the water-soluble surfactant Tween 20. The images were obtained by spreading the protein at the interface and then adding the surfactant to the bulk phase. Adapted with permission from [92]. Copyright © 2004 Elsevier Ltd. Dynamic light scattering devices generally operate using a specific, fixed, or variable scattering angle and mathematically convert the fluctuation intensity into particle size distribution (PSD). This approach is utilized for particles ranging from 3 nm to 5 um in size. The static light scattering instruments operate based on [PERSON]'s light scattering theory and utilize parameters, such as particle refractive index and shape, to determine the size of particles measuring from 100 nm to 1000 um. Small-angle X-ray scattering (SAXS) techniques are based on a similar principle as the DLS method, but instead of a light beam, they use a monochromatic beam of X-rays that are scattered by emulsion droplets in a size-dependent manner. The results from droplet size analysis are typically depicted on histograms (Figure 9). In the case of stable emulsions (Figure 9A), the histogram shows one peak, which represents a population of small droplets that are relatively uniform in size. Droplet sizes remain within a relatively narrow range (1-10 um). In contrast, in unstable emulsion separating through coalescence (Figure 9B), two populations of droplets, small and large (represented by two peaks on the histogram) are present at the same time. In addition, the uniformity of the droplet size decreases, which is clearly visible on the histogram as the widening of both peaks, corresponding to the range of droplet sizes becoming wider. The polydispersity index serves as a crucial indicator of emulsion stability, as it directly correlates with the uniformity of droplet sizes. A lower PDI value indicates a more uniform distribution of droplet sizes and higher emulsion stability, whereas an increase in PDI reflects destabilization of the emulsion leading to high variation in droplet size. In emulsions where coalescence and flocculation occur, the droplet size varies widely, leading to a higher PDI.