## 3 Results and Discussion  
### Impact of Surfactant Type and Concentration of Emulsion Stability  
Previous studies have shown that the concentration of certain types of surfactant may impact the rate of Ostwald ripening in an emulsion [15]. This effect is mainly attributed to the ability of non-adsorbed surfactants to form micelles that can solubilize and transport non-polar molecules between the oil droplets. Excess ionic surfactants typically have little impact on the rate of Ostwald ripening [16; 17], which may be because the strong electrostatic repulsion between the oil droplets and surfactant micelles keeps them apart. Conversely, excess nonionic surfactants may accelerate Ostwald ripening [18], possibly because these micelles can transfer oil molecules between droplets. Therefore, to investigate the impact of the type of surfactant used to create emulsions and ester gum on the Ostwald ripening of orange oil emulsions, it is important to minimize the effect of the surfactant micelles on the Ostwald ripening of orange oil emulsions. Based on our previous study, the concentrations of P100 SE, P23 LE, P20 SE, and P10 SE were determined to produce medium chain triglyceride emulsions where there was little non-adsorbed surfactant present and they were 1.00, 1.79, 2.93, and 3.17 mM for P10 SE, P20 SE, and P100 SE, respectively [14]. The initial droplet size of all the emulsions was not significantly different (Table 1), suggesting that all the surfactants were effective at forming small oil droplets during homogenization. Droplet growth was observed immediately after homogenization in all of the emulsions (Figure 2). The observed droplet growth may be due to coalescence and/or Ostwald ripening. The size of the droplets in the P100 SE-stabilized emulsions increased progressively throughout storage leading to a relatively large final mean particle diameter, whereas the size of the droplets in the other emulsions leveled off after 1-day storage leading to much smaller mean particle diameters. The droplet growth profile of the P100 SE-stabilized orange oil emulsion was very similar to the finding in the previous studies, which highlights its susceptibility to Ostwald ripening [14]. However, the droplet growth profiles of P10 SE-, P20 SE-, and P23 LE-stabilized emulsions differed from the previous findings [14]. The extent of droplet growth did depend on the nature of the surfactant used. The influence of surfactant type on droplet growth may have been due to their impact on surface properties, such as interfacial tension, thickness, rigidity, or permeability [1]. Ostwald ripening theory predicts that the rate of droplet growth should increase as the oil-water interfacial tension increases, because the free energy change associated with droplet curvature differences become greater [4]. For this reason, the interfacial tensions of oil-water interfaces saturated with the different surfactants were measured: 0.205\({}^{\text{d}}\), 0.302\({}^{\text{c}}\), 0.385\({}^{\text{b}}\), and 0.589\({}^{\text{a}}\) mN/m for P10 SE, P20 SE, P23 LE, and P100 SE, respectively. Here, the values with different superscripts were found to be significantly different using [PERSON]'s multiple range test (\(p<0.05\)). Although orange oil is quite polar, these interfacial tension values seemed smaller than the values reported in the previous study [19]. It was possibly due to the various compositions of commercial orange oils [20] and it may be because the interfacial tension measurement method was different from the previous studies [19].