3.3 Emulsion microstructure
All emulsions formed water droplets dispersed in the continuous oil phase as a free and aggregated network. The control CO emulsions without any additive (S0) showed a few larger droplets with some aggregation as most droplets were destabilized phase-separated (Figure 3A). The incorporation of S0.125 showed the initial appearance of extensive water droplet network formation (dark irregular-shape matter in the image) (Figure 3B). Such droplet aggregation also appeared with AA or CA addition into the aqueous phase (Figure 3C, D and G, H). When S0.125% was incorporated with AA (Figure 3E, F) and CA (Figure 3I, J), a more extensive water droplet network structure was identified, where free droplets were hard to locate in the continuous phase. It can be said that the water droplet network’s extension was responsible for their higher emulsion height (Figure 1 and 2). Similar water droplet aggregation was also observed for W/O emulsions stabilized with Span 80 and lecithin emulsifiers (Ushikubo & Cunha, 2014).
MO-emulsions without any aqueous phase additive (S0) showed water droplets well distributed in the continuous phase (Figure 4A). Similar to CO emulsion, the microstructure of MO-emulsions with S0.125 (Figure 4B), AA (Figure 4C, D) or CA (Figure 4G, H) also demonstrated a water droplet network formation. The addition of S0.125% with AA or CA in MO-emulsions developed an extensive water droplets network structure similar to CO-emulsions.
The microstructure of emulsions formed with LMP and Ca are presented in Figure 5. CO-emulsions without Ca (S0, Figure 5 I-A) and with Ca (Figure 5 I-B) showed dispersed water droplets with some degree of droplet coalescence. The incorporation of LMP formed an extensive water droplets network throughout the emulsion, making it challenging to capture the emulsion’s clear images (Figure 5 I-C, I-D). However, the mixture of LMP and Ca illustrated a different structure related to gel-formation due to the interaction between calcium ions and the carboxyl groups in the pectin molecules (Yuliarti & Othman, 2018). The structure formed by LMP-Ca in the aqueous phase can coexist with the water droplet network (Figure 5 I-E, I-F). All MO-emulsions, in the presence of LMP (with or without Ca) (Figure 5 II-B, II-A), also formed an extensive water droplets network compared to the emulsions without LMP (Figure 5 II-C, II-D). Comparing the CO and MO-emulsions with LMP, it can be said that the presence of MO led to a finer droplet network that was better able to provide emulsion stability against phase separation. Overall, CO and MO-emulsions with LMP (Figure 5) showed a more expanded water droplets network of smaller water droplets than the emulsions with S, AA, CA, or their mixture (Figures 3 and 4). Such droplet networks could be responsible for increasing emulsion stability against phase separation reported in Figures 1 and 2.