3.8 Discussion
We have shown that incorporating various aqueous phase ingredients interacted with GMO differently at the W-O interface, leading to improved W/O emulsion stability against phase separation for both TAG (in CO) and paraffin hydrocarbon (in MO)-based oils. The lowest stability was observed for the emulsions without any aqueous phase additives. Incorporation of either sodium chloride (S) or calcium chloride (Ca) showed significant improvement in emulsion sedimentation stability. Salt-induced improvement in W/O emulsion stability has been well studied, and many authors reported that the presence of salt in the aqueous phase is essential for efficient stabilization of W/O emulsions (Ganguly et al., 1992; Márquez et al., 2010; Opawale & Burgess, 1998; Scherze et al., 2006). It has been proposed that salt improves non-ionic emulsifier interfacial adsorption density, improving interfacial elasticity and emulsion stability (Aronson & Petko, 1993). Besides salts, we have also added organic acids such as ascorbic acid (AA) and citric acid (CA) into the aqueous phase, which also showed significant improvement in W/O emulsion stability compared to the emulsions without any additives. It has been proposed that an ideal emulsifier for W/O emulsion must form a strong hydrogen bond with water with its polar head group (Villamagna et al., 1995). However, in canola oil (CO), GMO form stronger hydrogen bond with >C=O groups of TAG glycerol polar heads compared to the -OH groups of water, leading to an eventual disruption of GMO molecules from the W-O interface towards the oil phase (Ghosh et al., 2011). The addition of AA or CA with multiple >C=O groups in the aqueous phase provided stronger hydrogen bonds with interfacial GMO towards the aqueous phase, thereby providing improved stability to the W/O emulsions. Such interaction of GMO with ascorbic acid in water has previously been reported by Bitan-Cherbakovsky et al. (2009). Improvement in W/O emulsion stability in the presence of AA and CA was also observed for mineral oil (MO). Interestingly, a synergistic improvement in emulsion stability was observed when both S and AA or S and CA were added to the aqueous phase. Such synergistic effect in increasing W/O emulsion stability was not reported before. Such differences in emulsion stability can also be seen from their average droplet size. For CO-emulsions, the smallest droplet size was observed when both S and AA or S and CA were present in the aqueous phase. Comparing CO and MO, a much smaller average droplet size and higher sedimentation stability were observed for MO-emulsions compared to CO-emulsions. This could be due to a lack of interactions between GMO and MO, leading to a favourable presence of GMO at the W-O interface than when GMO was present in CO. Such difference in W/O emulsion stability based on the continuous oil phase has also been reported before (Bus et al., 1990; Ghosh & Rousseau, 2011). For example, Bus et al. (1990) showed that if the emulsifier does not form any hydrogen bond with the oil (as in the case of GMO in MO), they will be more available to form hydrogen bonds with water, and their stability will be higher compared to a hydrogen-bonding oil such as CO.
We have also investigated the presence of polysaccharide pectin (LMP) with numerous >C=O groups and -OH groups on the stability of W/O emulsions. Our initial hypothesis was that the presence of a large number of hydrogen bond-forming groups in the aqueous phase would provide extensive stability to GMO at the W-O interface. We found the highest emulsion stability in the presence of LMP in the aqueous phase. Moreover, the hydrogen bonding ability of LMP with GMO was able to overcome the destabilization effect of CO on interfacial GMO, so that when LMP was present, no significant difference in the stability of the W/CO and W/MO emulsions was observed. However, the presence of Ca and LMP led to lower emulsion stability, which was due to the interaction of Ca with LMP making it less available to hydrogen bond with interfacial GMO. The effect of pectin on improving W/O emulsion stability has recently been reported for W/CO emulsion-based low-fat tablespread by Romero-Peña and Ghosh (2021).
The microstructure of the emulsions revealed extensive aggregations of water droplets and the formation of a droplet network in both oils. It is known that water droplets in W/O emulsion generally have a smaller energy barrier against flocculation (Kent & Saunders, 2001), which could also facilitate droplet aggregation. It can be said that the extensive water droplet network was eventually responsible for the higher stability of the W/O emulsions against sedimentation. For example, water droplets containing LMP aggregated massively, more than any other additives, which could be related to the interaction between GMO and LMP and interfacial strengthening of water droplets surfaces leading to the highest emulsion stability (Romero-Peña & Ghosh, 2021).
From the viscosity index, it was observed that while the water droplet network structure was protected and indices remained ≥ 1 for all MO-emulsions, for most of the CO-emulsions viscosity, dropped with storage time, indicating breakdown of droplet network with time. Viscoelastic behaviour of all emulsions showed weak gel-like properties. Interestingly, all MO-emulsions showed a faster drop in storage moduli with strain and lower crossover strain than the CO-emulsions. A similar indication of weaker droplet network structure for MO-emulsions was also observed when the emulsions were subjected to accelerated gravitation, where all MO-emulsion showed faster sedimentation velocities due to the faster breakdown of water droplet network in MO than that in CO. However, with LMP in the aqueous phase, such a difference in sedimentation velocity was smaller, which could be due to the enhancement of the droplet network in the presence of LMP. More research is needed to further understand the mechanism behind the difference in water droplet network strength in these two different oils
The excess of polar molecules in CO reduced the interfacial tension to a minimum which did not change in the presence of any aqueous phase ingredients. However, this mechanism was not enough to stabilize the CO emulsions without additives (S0) due to the stronger hydrogen bonds between GMO and TAG glycerol polar head groups of CO, which pulled the GMO out of the interface towards the oil phase in an emulsified state (Bus et al., 1990; Ghosh et al., 2011). It can be said that the interfacial tension determined using a platinum plate in a static setup was not able to simulate the dynamics in play at the water droplet interface in the presence of various aqueous phase additives and the oil phase. When GMO was present in MO, interfacial tension was the lowest without any aqueous phase additive, further supporting the hypothesis of lack of hydrogen bonding of GMO in MO leading to their free movement towards the interface. Interestingly, although the interfacial tension of GMO in MO was higher than GMO in CO in the presence of various aqueous phase additives, the droplet size of the final emulsions was significantly smaller for MO-emulsions than CO-emulsions. This apparent discrepancy could be explained by partial droplet destabilization in CO due to GMO’s disruption towards the oil phase. Finally, LMP molecules demonstrated better stability of GMO and similar interfacial tension in both oils due to its numerous >C=O groups that could form hydrogen bonds with interfacial GMO. Although the interfacial tension in the presence of LMP was not significantly different compared to AA, CA or their mixture with S, LMP provided a stronger interface which allowed higher stability of the droplets in an aggregated state (Ford & Furmidge, 1966), thereby improving emulsion stability and rheology. However, adding Ca ions with LMP led to the formation of weak gel in the aqueous phase, which hindered GMO adsorption at the O-W interface and reduced emulsion stability.