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

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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.
Emulsions, Water-in-oil-in-water, Food technology