3.1.3. Cryo-Scanning Electron microscopic observation
(Place for figure 4)
Fig. 4. SEM observation of ultrasonic-treated and untreated 3% psyllium husk emulsion gel with different concentration of inulin. Dark arrow indicates length of 50 μm.
The SEM images (Fig. 4) revealed that inulin addition and ultrasonic treatment affected the morphology of the husk emulsion gel system. The microstructure of the emulsion gel without inulin was heterogeneous and porous. Droplets surface changed with increased inulin concentration from irregular to smooth and less cavities, suggesting that inulin was in a packed arrangement at the droplets interface, and could also form an interfacial layer coating the surface of oil droplets. Similar result was observed for an emulsion developed with zein/corn fiber gum complex (Zhu et al., 2019). Their study reported that the polysaccharide complex was adsorbed at the oil-water interface, and that complex also created a mechanical barrier to prevent interfacial film rupture. Polymers adsorption at the interface can enhance mechanical properties of the interfacial films, which can inhibit coalescence, therefore, bigger size droplets have weaker interfacial films (Mao & Miao, 2015),
Compared to the untreated emulsion, the droplets surface in ultrasonic-treated emulsions showed higher integrity of the coating layer. At 20% inulin, the dark area in the untreated emulsion gel (Fig. 4) showed that droplets were not completely covered by the inulin layer. On the other hand, the surface of the ultrasonic treated emulsion was very smooth, without gap, suggesting that the droplet is completely covered by the inulin layer at that concentration. Similarly, Xiong et al. (2019) reported that more emulsifier molecules were adsorbed on the surface of the oil droplets in the ultrasonic-treated xanthan gum emulsion compared to untreated emulsion. The increased particle adsorption on the surface of emulsion droplet by ultrasound apparently occurs from momentum transfer from the fluid to the particles and droplets caused by random cavitation events. The momentum transfer can overcome the stabilizing energy barriers that normally prohibit nanoparticles from spontaneously adhering to the oil-water interface (Lee et al., 2019).