# Critical Review of Techniques for Food Emulsion Characterization
## 2 Emulsion Formation and Stability
(e.g., chitosan). Emulsion pH level governs the ionization of surface groups, which impacts the density of droplet surface charge [60]. By adjusting the solution pH, it is possible to tailor the thickness and integrity of the interfacial layer [61] and control how much ionizable molecules adsorb to droplet surfaces. For instance, anionic pectin will not attach to anionic \(\beta\)-lactoglobulin-stabilized droplets at neutral pH (pH = 7) due to electrostatic repulsion between polysaccharide molecules and droplet surfaces but will do so at lower pH levels of around 3 when polysaccharide molecules and droplets are oppositely charged [62] At the same time, pH-driven changes in the thickness of the interfacial layer can strongly impact emulsion stability by altering the strength of steric repulsion and van der Waals attraction between droplets. Protein emulsifiers are particularly effective in manipulating droplet charge by adjusting the pH of the emulsion. Protein molecules have a positive charge below their isoelectric point (pJ) and a negative charge above it. By adjusting the solution pH, it becomes possible to tune the electrostatic interactions between droplets. Moreover, since different proteins have varying isoelectric points, selecting proteins with the necessary electrical properties at the desired solution pH allows for altering the magnitude of electrical repulsion between the droplets [58]. The change in temperature impacts various factors, including the viscosity of the phases, interfacial tension, surfactant adsorption to droplets, solubility, and diffusivity of the droplets in the continuous phase [63]. These changes indirectly affect emulsion stability. A rise in temperature decreases interfacial tension and viscosity, facilitating emulsion formation. However, high temperatures can reduce emulsion stability due to increased dispersed phase solubility in the continuous phase and accelerated Ostwald ripening [43]. Sudden temperature variations are particularly detrimental to emulsion stability [63]. Conversely, increasing the viscosity of the continuous phase can enhance emulsion resistance against separation by slowing down gravitational separation processes, like sedimentation, while decreasing droplet collisions and coalescence frequency [59; 64]. Emulsion stability can also be enhanced by reducing droplet size and the polydispersity index (PDI). In general, emulsions exhibit greater stability when the droplets and PDI are small, due to diminished attraction between droplets and increased viscosity, resulting in reduced coalescence [37]. In polydisperse systems, smaller droplets show greater solubility in the continuous phase compared to larger ones. With an increase in emulsion polydispersity, differences in solubility and chemical potential between droplets intensify, accelerating Ostwald ripening. Another crucial factor for emulsion stability is the concentration of droplets. In O/W emulsions, an increase in droplet concentration has been shown to improve their stability and reduce creaming rate, which can be attributed to droplet crowding effects, higher packing density, and stronger inter-droplet interactions [65]. However, the strength of these interactions largely depends on the surface charge of the droplets, since electrically charged droplets cannot get as close to each other as uncharged ones. Efforts have been made to enhance control over emulsion stability through the use of emulsifiers that respond to specific stimuli, such as light (e.g., azobenzene-modified emulsifiers), pH (e.g., poly(methacrylic acid-co-methyl methacrylate-co-7-(4-vinylbenzyloxyl)-4-methylcoumarin emulsifier), or salts (e.g., zwitterionic emulsifiers) [64]. These stimuli-responsive emulsifiers enable reversible stabilization and destabilization of emulsions in response to physical or chemical changes, offering potential benefits for on-demand site-specific drug delivery.