Comparative study of the stability of multiple emulsions containing a gelled or aqueous internal phase
Graphical abstract
Introduction
There has been a long standing interest in developing multiple emulsions in food (Dickinson, 2011, Dickinson et al., 1991, Matsumoto, 1986) and pharmaceutical applications (Davis and Walker, 1987, Degim and Celebi, 2007). The interest stems from their unique structure, in that for Water-in-Oil-in-Water (WOW) emulsions, the dispersed oil droplets themselves contain aqueous phase droplets which can be used to carry guest molecules or simply contribute to the overall dispersed phase volume. WOW emulsions have been the main focus of research, particularly for food applications, and have been studied because of their added functionality in terms of reducing fat content in foods (Lobato-Calleros et al., 2009, Lobato-Calleros et al., 2008), encapsulation of volatiles(Dickinson, Evison, Gramshaw, & Schwope, 1994), delivering bioactive compounds (Owusu, Zhu, & Dickinson, 1992), and taste masking of drugs and other compounds (Sonawane et al., 2010, Vaziri and Warburton, 1994). Although they have great potential in these applications, the biggest barrier to their use is their poor stability (Dickinson, 2011) which arises from the presence of the internal water droplets. In the preparation of double emulsions 2 types of emulsifiers are required, firstly a lipophilic emulsifier with a low Hydrophile–Lipophile Balance (low HLB) emulsifier soluble in the oil phase is required to stabilise the primary, internal water-in-oil (WO) emulsion droplets, and secondly, a hydrophilic (high HLB) emulsifier is needed in the external aqueous phase to stabilise the secondary oil droplets. The primary water droplets require a low HLB emulsifier, and are sensitive to coalescence both with other droplets, but also with the outer continuous phase (Dickinson, 2011, Dickinson et al., 1994), thus losing the whole basis of their added functionality. WOW emulsions can also lose water to the outer phase through mass transport driven by osmotic and chemical potential differences between the inner and outer water phases (Dickinson, 2011, Pawlik et al., 2010). Because of the specific physical requirements of the low HLB emulsifier to stabilise the water droplets, it is beneficial for the emulsifier to possess a large, steric hydrophobic group, however, only a limited number of food approved emulsifiers are suitable. From the various emulsifiers tested, only a few showed any WO stabilising properties, and only polyglycerolpolyricinoleate (PGPR) imparted significant stability to the WO emulsions over a range of conditions, agreeing with other studies of WO emulsion stability (Ushikubo & Cunha, 2014).
The second stage of multiple emulsion formation is the emulsification of the WO emulsion into the continuous aqueous phase. This requires a high HLB emulsifier to provide a stabilising layer around the oil droplets. The functionality of these emulsifiers is the same as for stabilising conventional oil in water (OW) emulsions. Commonly used high HLB emulsifiers include proteins such as caseins and whey proteins, or hydrophilic low molecular weight emulsifiers such as polysorbates and lecithins.
The more complex structure of WOW emulsions means that their kinetic stability is limited (Sapei, Naqvi, & Rousseau, 2012) as well as changes in morphology. Different approaches have been used to solve these stability issues, including careful choice of emulsifiers (Akhtar & Dickinson, 2001) and the use of steric stabilizers, fat crystals and protein-polysaccharide mixtures (Sapei et al., 2012). Several studies have shown that incorporating a polymer or protein in the internal water droplets, perhaps to gel the internal water droplets, thus forming a gel-in-oil-in-water (GOW) emulsion, can improve stability (Benichou et al., 2004, Dickinson, 2011). These include proteins such as sodium caseinate, whey protein and gelatin (Hemar et al., 2010, Su et al., 2006, Surh et al., 2007), and polymers such as xanthan (Evison, Dickinson, Owusu-Apenten, & Williams, 1995). Incorporation of NaCl and polymer or glucose in the primary emulsion (Pawlik et al., 2010, Sapei et al., 2012), and levels of PGPR in the oil phase or addition thickener in the external aqueous phase have been shown to improve stability (Benichou et al., 2004, Dickinson, 2011). These approaches can sometimes improve not only the stability of the WOW emulsions, but also their resilience to change, particularly during processing, when the pH, osmotic, and chemical potential of the exterior aqueous phase can be subject to change.
In this manuscript we aim to demonstrate which are the key parameters controlling the stability and functionality of multiple emulsions. These include the low and high HLB emulsifiers, the effect of temperature and salt, and specifically the effect of gelling the internal water droplets using polymers requiring calcium or heat induced gelation, and investigating their stability under a range of conditions commonly found during food processing. This approach will hopefully improve their application in real systems, particularly for reduced fat food applications.
Section snippets
Materials
All materials were food grade and were used as received. Sodium Alginate (Flavicans HV) and a citric acid ester of monoglyceride (CITREM LR10, diglyceride enriched) were obtained from Danisco A/S (Copenhagen, Denmark). Sunflower oil and Vege-gel (carrageenan and locust bean gum) (Dr. Oetker) were purchased from a local supermarket (Norwich, UK). Lecithin (Optima Healthcare lecithin from soya beans) was purchased from a local food supplier (Holland and Barrett). The emulsifier polyglycerol
Preparation of WO emulsions – selection of emulsifiers
The first stage in the process was to screen a wide range of low HLB emulsifiers and their ability to stabilise the primary water droplets in the WO emulsion. The stability of the water droplets are critical for the overall functionality of the multiple emulsions, therefore only stable WO emulsions would be able to form part of a stable WOW or GOW multiple emulsion. Fig. 1 demonstrates some typical observations/outcomes of WO emulsions showing coalescence, flocculation or stable, well dispersed
Conclusions
Stabilising WOW emulsions using PGPR as the low HLB emulsifier provides excellent stability to the emulsions against shear, temperature treatment and storage. The stability against salt concentration is improved significantly by gelling the initial WO droplets with sodium alginate using added calcium during the homogenisation stage. This also allowed a reduction in PGPR concentration to stabilise the WO droplets. Therefore by gelling the internal water droplets with alginate significantly
Acknowledgement
The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 289397 (TeRiFiQ project). The authors would like to acknowledge the Department of the Environment, Food and Rural Affairs for funding through LINK project FQI02 and the Biotechnology and Biological Sciences Research Council for funding through the Institute Strategic Programme Grant to the Institute of Food Research.
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