Formation and stability of food-grade water-in-oil-in-water emulsions : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Food Technology, Riddet Institute, Massey University, Palmerston North, New Zealand
The inherent thermodynamic instability of water-oil-water (W/O/W) emulsions
restricts their applications in food systems. The objective of this study was to develop
food grade W/O/W emulsions with both high encapsulation efficiency (EE) of
entrapped compounds and long term stability using minimal concentrations of
emulsifiers. Emulsions were characterised by phase separation, confocal microscopy,
droplet sizing and EEs of macromolecules such as poly R-478 dye (a water-soluble
dye) and small molecules such as sorbitol, NaCl and sodium ascorbate (SA).
Emulsions were prepared using soybean oil, polyglycerol ester of polyricinoleic acid
(PGPR) as the emulsifier for the primary water-in-oil (W/O) emulsions and sodium
caseinate (NaCN) as the sole emulsifier for the secondary W/O/W emulsions, in both
buffered (0.1 M sodium phosphate buffer) and non-buffered (distilled water) systems.
The concentration of PGPR had a significant effect on the EE of W/O/W emulsions.
In non-buffered systems, the PGPR concentration could be reduced to 2% (w/v) to
obtain an EE > 90%, whereas, in buffered systems, 4% (w/v) PGPR was required to
maintain a similar EE. The ionic environment created in buffered systems had an
effect on the emulsifying ability of PGPR. However, the concentration of PGPR in the
internal aqueous phase could be reduced to 2% (w/v) without affecting the EE and the
stability of the W/O/W emulsion by partially replacing the PGPR with 0.5% (w/v)
NaCN, added to the aqueous phase of the primary W/O emulsion. The results indicate
that there may be a possible synergistic effect between PGPR and NaCN, thus
allowing the formulation of double emulsions with reduced surfactant concentration.
A modified gum arabic (Acacia (sen) SUPER GUM™) was investigated as an
emulsifier to aid in the stabilisation of W/O/W emulsions. SUPER GUM™ at a range
of concentrations was added as an emulsifier to replace NaCN in the external aqueous
phase of W/O/W emulsions. The addition of 10% (w/v) SUPER GUM™ to the
external aqueous phase allowed the PGPR concentration to be further reduced to 0.5%
(w/v) while maintaining an EE > 90%. W/O/W emulsions stabilised with SUPER
GUM™ were also found to be stable over a wide pH range, in the internal or external aqueous phase, compared with W/O/W emulsions stabilised with NaCN; therefore,
they may be suitable for applications over a wide range of pH values, as may occur
during ingestion or incorporation into different food systems.
The encapsulation of small molecular compounds (sorbitol, NaCl or SA) in the
internal aqueous phase had a significant effect on the stability of W/O/W emulsions,
because osmotic pressure gradients were created by the addition of these compounds.
With the addition of 0.5 M sorbitol to the internal aqueous phase, the osmotic pressure
induced a water flux from the external aqueous phase to the internal aqueous phase,
resulting in swelling of the internal water droplets. The resulting enlarged water
droplets were more susceptible to coalescence and were expelled to the external
aqueous phase. The EE of poly R-478 dye decreased significantly, mainly because of
the expelling of internal water droplets. In contrast, the pulsed field gradient nuclear
magnetic resonance (PFG-NMR) technique showed that sorbitol diffused through the
oil phase; however, there was no evidence that sorbitol had an adverse effect on the
emulsifying ability of PGPR.
The encapsulation of 0.5 M NaCl not only created an osmotic pressure gradient
between the internal and external aqueous phases, but also reduced the emulsifying
ability of PGPR, as phase separation occurred in W/O emulsions. The release of NaCl
from W/O/W emulsions took place quickly; this was mainly due to diffusion of NaCl
through the oil phase, because it was observed that release via expelling of internal
water droplets took much longer to occur. The release of NaCl from W/O/W
emulsions could be reduced by increasing the PGPR concentration.
For identically formulated W/O/W emulsions, the release of encapsulated SA was slower than the release of NaCl. Both diffusion and expelling of internal water
droplets contributed to the release, depending on the experimental conditions. The
release of SA could be slowed by adding macromolecules such as 0.5% (w/v) NaCN
to the internal aqueous phase, by increasing the viscosity of the internal aqueous
phase by the addition of 2% (w/v) carboxy methyl cellulose or by using a rotor-stator
homogeniser instead of a high-pressure homogeniser to prepare the W/O/W
emulsions. In addition, the replacement of 0.5% (w/v) NaCN with 10% (w/v) SUPER GUM™ improved the EE of SA, possibly by acting as a rigid barrier on the surface of
the oil droplets and therefore blocking both the diffusion of SA to the external
aqueous phase and the expelling of internal water droplets.
Content removed from thesis due to copyright restrictions: Su, J., Flanagan, J., Hemar, Y., & Singh, H. (2006). Synergistic effects of polyglycerol ester of polyricinoleic acid and sodium caseinate on the stabilisation of water-oil-water emulsions. Food Hydrocolloids, 20, 261-268. doi: 10.1016/j.foodhyd.2004.03.010. Su, J., Flanagan, J., & Singh, H. (2007). Improving encapsulation efficiency and stability of water-in-oil-in-water emulsions using a modified gum arabic (Acacia (sen) SUPER GUM TM). Food Hydrocolloids, 22, 112-120. doi: 10.1016/j.foodhyd.2007.03.005. Hindmarsh, J. P., Su, J., Flanagan, J., & Singh, H. (2005). PFG-NMR analysis of intercompartment exchange and inner droplet size distribution of W/O/W emulsions. Langmuir, 21, 9076-9084. Flanagan, J., Su, J., O'Brien, C., O'Riordan, B., Singh, H., & Dunne, C. (2008). Microencapsulating properties of Acacia (sen) SUPER GUM TM. Foods & Food Ingredients Journal of Japan, 213(3), 1-6.