**2. Emulsifying Properties**

Two aspects of emulsifiers are commonly studied in literature during the consideration of their emulsifying potential. These aspects include emulsifying activity and stability. Basic information regarding their emulsifying potential can be acquired by the determination of their efficacy to reduce the interfacial tension of emulsion [1]. Typically, interfacial tension is measured as a function of the increasing level of emulsifier and then calculated the surface tension versus concentration profile of emulsifier. However, the electrical properties of emulsifiers may have an impact on the stability and performance of an emulsion. Although various methods are available to measure the electrical properties of emulsifiers, microelectrophoresis is considered the simplest and most widely used method. The important aspect of emulsifying properties includes their emulsifying activity, which can be assessed by two comparatively indexes: emulsifying capacities and emulsifying activity index. The emulsifying capacity of an emulsifier is described as the maximum oil content that can be emulsified in an aqueous medium containing a specific amount of the emulsifier without reverting or breaking down the emulsion, while the emulsifying activity index is described as the amount of oil that can be emulsified per unit emulsifier.

Various analytical techniques and protocols are present for the investigation of emulsion stability in different aspects [2]. Among all, the emulsion stability index (ESI) established by Pearce and Kinsella [3], is a widely used and simplest method based on the analysis of the turbidity of the diluted emulsion at a specific wavelength. The phenomena affecting the emulsion stability mainly include coalescence, flocculation, creaming, sedimentation, Ostwald ripening, and phase inversion (Figure 1). Density-driven gravitation separation is the most easily observed emulsion instability phenomena in food emulsion storage, including the upward movement induced creaming and the downward movement induced sedimentation. Coalescence is the merging process of small emulsion droplets into larger emulsion droplets, resulting in the formation of a distinct layer of oil on the surface (oiling-off). Flocculation phenomena are also derived from the aggregation of emulsion droplets, while the droplets would not merge but remain as individual droplets. Phase inversion is a process that the dispersed phase becomes the continuous phase and vice versa, which has also been applied as an emulsification method to make fine emulsions. Ostwald ripening is the growth of one emulsion droplet at the expense of the smaller one due to the difference in chemical potential of the emulsion droplets. The destabilization of the soy-based stabilized emulsions is usually affected by the nature of the molecules (three-dimensional structure, hydrophobicity, charge, solubility, molecular flexibility) and conditions of the environment (e.g., pH and ionic strength) [4].

**Figure 1.** Mechanism for the destabilization of emulsion system [5].
