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).