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.