# Critical Review of Techniques for Food Emulsion Characterization
## 4 Emulsion Fractionation and Characterization Methods
### Field-Flow Fractionation (FFF)
In addition, the ability to separate delicate particles without causing damage makes it an attractive choice for industries dealing with sensitive compounds such as the pharmaceutical sector. Nonetheless, when employing EIFFF, it is imperative to acknowledge the constraints associated with the generation of electrical double layers around emulsion droplets. The presence of ionic constituents within emulsions induces the formation of an electrical double layer, diminishing the applied electrical field and potentially leading to constrained droplet separation and poor resolution [224]. This phenomenon is exacerbated by increasing the ionic strength of the continuous phase. Consequently, the separation efficiency in EIFFF is markedly influenced by the composition of the emulsion. EIFFF is most effective in the case of emulsions with low ionic strength continuous phases, such as emulsions based on deionized water, but is unsuitable for emulsions containing high concentrations of ions or charged molecules. Enhancement of the effective field across the separation channel can be achieved by elevating the applied voltage. Higher voltages prolong particle retention in the channel, thereby augmenting separation efficiency. However, meticulous control of the applied voltage is imperative, as excessive voltage may induce electrophoresis of the continuous phase and electrolysis. Electrolysis may generate gaseous products [242], which can ultimately lead to the formation of bubbles that interfere with the sample's parabolic flow profile, detrimentally affecting the separation process. To mitigate this effect in oil-in-water (O/W) emulsions, utilizing water as the continuous phase, a voltage below 2.0 V should be employed. Additionally, to minimize the formation of an electrical double layer, a cyclical EIFFF method involving the application of an alternating current electrical field across the channel may be applied. Another issue during EIFFF is related to changes in the conductivity of emulsion samples resulting from atmospheric carbon dioxide dissolution in the aqueous phase, which can contribute to irreproducible separations. Some researchers have proposed using buffer solutions at very low concentrations (micro-molar levels) to prevent this problem. One of the primary factors in achieving greater precision with EIFFF separation lies also in the manufacturing approach. While older systems were typically fabricated by macro-machining, the development of a new generation of high-precision miniaturized electrical field-flow fractionation (\(\upmu\)-EFFF) systems has been made possible through the utilization of micro-machining technologies [240]. The reduced separation channel height has been shown to improve resolution, leading micro-EIFFFs to be favorably compared with macro-EIFFF systems, particularly in terms of separation time and required sample volumes. Overall, field-flow fractionation offers several advantages over other separation methods, including the ability to operate at low pressure, reducing shear stress on the samples, and no need for sample preparation before separation. Additionally, FFF can be coupled with various detectors, such as UV-Vis spectrophotometers, ICP mass spectrometers, ICP optical emission spectrometers, and multi-angle light scattering detectors, making it a versatile technique for analytical applications. FFF also provides more accurate and consistent measurements of emulsion droplet size than methods based on light scattering, such as DLS, as it takes into account the density and shape of the particles, whereas DLS only measures the hydrodynamic size of the particles. As a result, it can be used to analyze and separate droplets ranging in size from 10 nm up to several \(\upmu\)m and thus is better suited to study highly polydisperse emulsions [243]. The undeniable advantage of this method lies also in the very small sample volume required for analysis, usually between 1 and 20 \(\upmu\)L) [8].