# Critical Review of Techniques for Food Emulsion Characterization
## 4 Emulsion Fractionation and Characterization Methods
### Field-Flow Fractionation (FFF)  
Field-flow fractionation (FFF) techniques allow separation of emulsions into multiple (typically 10 to 15) different fractions. However, the true potential of these platforms lies in their ability to perform multiple downstream analyses by incorporating detectors that can provide multiparametric information on separated fractions. The detectors most commonly incorporated into those systems include UV-Vis spectrophotometers, differential refractive index (dRI) detectors, fluorescence detectors (FLD), fluorescence correlation spectroscopy (FCS) detectors, ICP inductively coupled plasma mass spectrometers (ICP-MS), ICP optical emission spectrometers, DLS detectors, and multi-angle light scattering (MALS) detectors [224]. For example, [PERSON] et al. [225] used FFF to investigate the size and compositional heterogeneity of droplets in a polydisperse commercial lipid emulsion (Intralipid) used intravenously as a source of calories and essential fatty acids in patients requiring parenteral nutrition. The emulsion was fractionated into 13 monodisperse droplet fractions, which were then thoroughly investigated in terms of droplet size, content of triglycerides, phosphatidylcholine, and penclomedine and aqueous-entrapped volume (defined as volume of water or aqueous phase entrapped between droplets). Figure 15: Physicochemical conditions at various regions of the human gastrointestinal tract. Picture of the human body was obtained from [[http://en.wikipedia.org/wiki/Digestive_tract](http://en.wikipedia.org/wiki/Digestive_tract)]([http://en.wikipedia.org/wiki/Digestive_tract](http://en.wikipedia.org/wiki/Digestive_tract)) accessed on 2 October 2023. Field-flow fractionation (FFF) utilizes the laminar flow of a carrier liquid through a separation channel and an external physical field to fractionate emulsions [226]. The FFF devices typically consist of a carrier liquid reservoir, an injector, a separation channel, a pump, and a detector. To some degree, FFF devices resemble HPLC (high-performance liquid chromatography) instruments, as they operate based on the elution of different sample components at different rates. However, in contrast to HPLC, FFF is a non-chromatographic method, as the separation channel does not contain a stationary phase [227]. During the FFF, the analyzed emulsion sample is introduced into a separation channel. The pump is used to generate a laminar flow of the liquid through the channel, which is then subjected to an external field applied perpendicularly to the channel symmetry axis. The external field is directed toward the channel wall, referred to as the \"accumulation wall\", and allows the separation of droplets and/or other components present in the emulsion according to their size, density, mass, charge, shape, surface properties, or other physicochemical properties. The flow profile in the separation channel is parabolic (Poiseuille flow) [228], with the highest velocity at the center of the channel and decreasing gradually towards the walls, where it reaches zero (Figure 16). When subjected to an external field, emulsion droplets flowing through the separation channel are pushed toward the accumulation wall, while simultaneously being moved back toward the channel center by the diffusion counterforce. This establishes an equilibrium position, leading to the separation of droplets along the carrier flow streamline [8]. Small droplets with a higher diffusion rate tend to migrate further away from the accumulation wall toward the faster-flowing central region of the parabolic flow, while larger droplets move towards slower-flowing streamlines near the channel wall. As a result, smaller droplets exhibit lower retention times, which implies that they are eluted earlier from the separation channel than larger droplets. The retention behavior of droplets can be described using the retention ratio (\(R\)), which is calculated as void time (\(t_{0}\)) divided by analyte retention time (\(t_{r}\)).