# Critical Review of Techniques for Food Emulsion Characterization
## 2 Emulsion Formation and Stability
Meanwhile, their heads point outward toward the continuous phase, forming a protective layer around the droplets. In water-in-oil (W/O) emulsions, where water droplets are dispersed in the oil phase, the orientation of emulsifier molecules is reversed: non-polar tails are oriented outward toward the oil phase, while polar heads are directed inward toward the center of the water droplets. Oil-in-water emulsions are known as standard emulsions [2], while water-in-oil emulsions are termed reverse or inverted emulsions. When an emulsion is additionally dispersed into another continuous phase, it is referred to as a multiple emulsion or an emulsified emulsion. An instance of a multiple emulsion is oil-water-oil (O/W/O), which includes small oil droplets scattered in larger water droplets suspended in the continuous oil phase. The stability of emulsions refers to their capability to withstand alterations in physico-chemical properties and droplet size over time [37]. Emulsion stability is a crucial factor in various applications, including in the food, cosmetic, and pharmaceutical industries, as it ensures the desired physicochemical properties and shelf-life of the product. There are several destabilization mechanisms responsible for the separation of emulsions (Figure 1), including flocculation, coalescence, Ostwald ripening [29], sedimentation, and creaming, which may occur simultaneously or separately [38]. One of the key emulsion destabilization mechanisms is coalescence, which occurs when the droplets of the dispersed phase come into contact and merge to form largerdroplets. The fusion of droplets can happen due to Brownian motion, collisions, and attractive forces between the droplets. Coalescence causes the thin liquid layer separating individual droplets to break. As two droplets approach, the layer becomes thinner until it reaches a stable thickness determined by surface forces. The resulting force, known as disjoining pressure, involves different interactions, such as repulsive electrostatic forces, van der Waals interactions, and repulsive steric forces, depending on the properties of the surfaces involved. It is worth noting that, in stable emulsions, as droplets draw closer together, disjoining pressure increases; however, beyond a critical distance and disjoining pressure, the interfacial layer is compromised, and coalescence begins [39]. The interfacial tension between the dispersed and continuous phases and the viscosity of both phases are crucial parameters determining emulsion resistance to coalescence. In emulsions where the dispersed phase is partially crystallized, the droplets can undergo partial coalescence (called also arrested coalescence). During partial coalescence, droplets begin to merge, but before coalescence is completed, a smaller daughter droplet is formed. Partial coalescence may occur again in daughter droplets creating the next generations of daughter droplets. Another important mechanism in emulsion destabilization is flocculation, where the droplets group together to form larger clusters or flocs due to attractive forces between them, including van der Waals forces [40]. The rate of flocculation is related to the strength of the van der Waals attraction, which depends on the droplet radius and effective Hamaker constant (dictated by the interactive van der Waals energy and the distance of separation between two droplets), as well as the electrostatic or steric repulsion between the droplets. The rate of flocculation can be predicted from a frequency factor that determines how often drops encounter each other, and a probability factor indicating how long they stay together. Usually, flocculation leads to enhanced creaming (i.e., the movement of the dispersed phase to the upper layer of the emulsion), because flocs rise faster than individual drops due to their larger effective radius. However, exceptions occur in concentrated emulsions, where gel-like network structures can have a stabilizing influence.