Influence of emulsion interfacial membrane characteristics on Ostwald ripening in a model emulsion
Introduction
Orange oil obtained from citrus peel is a flavoring agent that is widely used in many food products (Perez-Cacho and Rouseff, 2008, Tan, 2004). Orange oil is a complex mixture consisting of more than 200 components, with the main ones being limonene and linalool, which are classified as terpenes (Vora, Matthews, Crandall, & Cook, 1983). Although not present in large quantities, several components found in orange oil, particularly phytochemicals, show health promoting effects including anti-carcinogenic and anti-inflammatory activities (Manthey and Bendele, 2008, Xiao et al., 2009). For this reason, there has been increasing interest in orange oil not only as a flavoring agent but also as a source of health-promoting components. One of the most commercially convenient methods of incorporating orange oil into a wide variety of food products is the use of the emulsified form, which consists of small orange oil droplets dispersed within an aqueous solution (Given, 2009, Qian et al., 2011, Tan, 2004). By emulsifying the orange oil in an aqueous solution, its dispersity in water is improved and the bioavailability of the health-promoting components could be enhanced during passage through the gastrointestinal track (McClements, 2013).
Emulsions in soft drinks and beverages are stabilized using emulsifiers, which are amphiphilic and surface-active molecules that rapidly adsorb to the surface of the oil droplets that are created during homogenization (McClements, 2005). Emulsifiers decrease the amount of energy required to convert large-sized oil droplets to smaller-sized ones by reducing the interfacial tension between the oil and aqueous phases (Silva et al., 2011). Since emulsifiers form a protective steric and electrostatic barrier at the interfacial membrane between oil and aqueous phases surrounding the oil droplets, they stabilize the oil droplets against aggregation during product transport, storage, and utilization and could also protect the encapsulated components from harsh conditions in the aqueous phase (Chen et al., 2010, Faraji et al., 2004, Qian et al., 2012). However, emulsions are not always stable, with physical destabilization via gravitational separation, flocculation, coalescence, and/or Ostwald ripening being observed in many emulsions. Ostwald ripening in particular is a major problem in emulsions where the oil phase has a relatively high solubility in water, which is the case for many flavor oils (McClements et al., 2012, Wooster et al., 2008). Ostwald ripening is a process whereby the large-sized droplets in an emulsion grow at the expense of the smaller-sized droplets due to molecular diffusion of oil molecules through the aqueous phase separating the droplets (Kabalnov and Shchukin, 1992, Taylor, 1998). Since the Laplace pressures of larger- and smaller-sized droplets are different, smaller-sized oil droplets have higher local oil solubility than larger ones, resulted in a Kelvin effect which drives Ostwald ripening (Capek, 2004, Wooster et al., 2008). Therefore, the prevention of Ostwald ripening is an important challenge for the development of food products containing flavor and/or essential oils. To prevent and/or retard the Ostwald ripening of emulsified flavor oils, emulsions are generally stabilized using amphiphilic polysaccharides such as gum arabic or hydrophobically modified starch, which can create thick interfacial membranes (Chanamai and McClements, 2002, Garti, 1999, Tan, 2004). Another method of preventing and/or retarding the Ostwald ripening of flavor oil-in-water emulsions is the addition of highly hydrophobic triglyceride molecules to the oil phase (McClements et al., 2012). Although ester gum was usually used as a weighting agent in oil-in-water emulsions, its addition could also help to inhibit Ostwald ripening in orange oil emulsions (Lim et al., 2011).
As stated above, there are many reports showing that Ostwald ripening of flavor oil emulsions could be prevent by incorporating the highly hydrophobic molecules such as vegetable oils and ester gum to the oil phase prior to homogenization. However, since almost the previous findings focused on the evaluation of the effect of the incorporation of highly hydrophobic molecules on the Ostwald ripening of flavor oil emulsions, there is a still lack of information regarding the effect of the structural properties of emulsifiers on the rate and extent of Ostwald ripening in orange oil-in-water emulsions. Therefore, in this study we examined the influence of droplet interface characteristics on the stability of orange oil-in-water emulsions to Ostwald ripening. In particular, we examined how the sizes of the hydrophilic and hydrophobic groups on the emulsifiers affected the rate and extent of Ostwald ripening in orange oil-in-water emulsions over time. We also determined the effect of micelles on the size increment of orange oil droplets in emulsions by preparing orange oil emulsions at various concentrations of emulsifiers. Moreover, we evaluated the dependency of the ability of highly hydrophobic triglycerides in the oil phase to prevent the Ostwald ripening of orange oil emulsions on the structural properties of emulsifiers and the presence of micelles.
Section snippets
Materials
Orange oil and polyoxyethylene alkyl ether-type surfactants (polyoxyethylene 10 stearyl ether (S10), polyoxyethylene 20 stearyl ether (S20), polyoxyethylene 23 lauryl ether (L23), and polyoxyethylene 100 stearyl ether (S100)) were purchased from Sigma-Aldrich (St. Louis. MO, USA). Polyoxyethylene alkyl ether-type surfactants are nonionic surfactants containing polyoxyethylene chains as hydrophilic groups and n-alkyl chains as hydrophobic groups. A medium chain triglyceride (Delios S) was
Results and discussion
Based on previous reports, micelles formed with unabsorbed emulsifier molecules promote the depletion flocculation of emulsion droplets when they exceed a particular concentration (McClements, 1994, Wulff-Pérez et al., 2009), and are therefore considered a reason for emulsion destabilization that cannot be ignored. Thus, in order to eliminate the effect of micelles formed with unabsorbed emulsifiers on the rate and extent of Ostwald ripening, the minimum emulsifier concentration (MEC) required
Conclusion
The results obtained in this study show that the structural characteristics of emulsifiers could be an important factor in the stability of flavor oil emulsions to Ostwald ripening. Our findings suggest that the density and thickness of the droplet interfacial membranes in flavor oil emulsions could be an important determinant in the stability of flavor oil emulsions to Ostwald ripening. Moreover, our work shows that the ability of corn oil to inhibit Ostwald ripening highly depends on the
Acknowledgements
We gratefully acknowledge financial support by the High Value-Added Food Technology Development Program (313021-3) funded by the Ministry of Agriculture, Food and Rural Affairs, and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1B03930215), Republic of Korea.
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Sung Won Han and Ha Youn Song contributed equally to this work as first authors.