Lemon oil nanoemulsions fabricated with sodium caseinate and Tween 20 using phase inversion temperature method
Introduction
Nanoemulsions are dispersions with droplets smaller than 200 nm in diameter (McClements, 2011). The reduced droplet dimension in nanoemulsions makes the thermal energy of droplets significant when comparing to the gravitational energy and possibly attractive colloidal forces. Therefore, nanoemulsions are able to stabilize oil droplets against instability mechanisms of gravitational sedimentation and droplet aggregation (McClements and Rao, 2011). The reduced droplet dimension also weakens the scattering of visible light and thus improves optical clarity of emulsions, which makes it possible to use nanoemulsions in transparent products (McClements, 2011).
Oil-in-water (O/W) nanoemulsions can be prepared by two groups of methods that differ in the amount of mechanical energy input (Anandharamakrishnan, 2014). In high-energy methods such as high pressure valve homogenization, microfluidization, and sonication, a substantial amount of mechanical energy is used to break up oil droplets, which increases capital and operating costs (Anandharamakrishnan, 2014, Yang et al., 2012). It is difficult to produce fine oil droplets when the dispersed-to-continuous phase viscosity ratio is not suitable and disruptive forces are weaker than restoring interfacial forces (McClements and Rao, 2011). Moreover, “over-processing” caused by high pressures and long emulsification times can take place when proteins used as emulsifiers are denatured, resulting in the increased droplet size (Jafari et al., 2007). Conversely, low-energy methods do not require intense mechanical energy but utilize curvatures of interfacial films being a function of compositions and environmental conditions, which makes it possible for the inversion of dispersed and continuous phases to prepare nanoemulsions (Wang et al., 2008). High concentrations of synthetic nonionic small molecule surfactants (SMSs) such as Tween® (Rao and McClements, 2011) and Span® (Yu et al., 2012) family surfactants are commonly used to form nanoemulsions using low-energy methods that usually involve a phase inversion upon changes in composition (PIC) (Yu et al., 2012) or temperature (PIT) (Rao and McClements, 2010). Proteins and polysaccharides are considered to be infeasible to form nanoemulsions in PIC and PIT methods, because they are not able to change the curvature of interfacial films due to their surface charge and relatively large molecular weight (Maestro et al., 2008). To reduce the amount of synthetic SMSs used in PIT nanoemulsion fabrication, combinations of synthetic and natural emulsifiers may be an option, which has not been studied for SMSs and biopolymers.
Sodium caseinate (NaCas) is a well-known protein emulsifier. The emulsifying property of NaCas is derived from its high contents of hydrophobic amino acids such as proline, tyrosine, and tryptophan, which makes caseins naturally occurring amphiphilic block copolymers (Pan et al., 2013). Main surface-active components of NaCas are αs1- and β-caseins (Horne, 2003). αs1-Casein has hydrophobic blocks at both ends and another in the middle, while β-casein has one phosphoseryl cluster (hydrophilic region) and one hydrophobic region (Horne, 2003). Similar to synthetic block copolymers, NaCas adsorbed on oil droplets can take the “train-loop-tail” conformation, which provides steric repulsion, together with electrostatic repulsion provided by charged amino acid residues, to stabilize oil droplets against aggregation (Surh et al., 2006). The excellent emulsifying and stabilizing properties of NaCas make it a popular emulsifier in the food industry.
Utilizing distinct properties of SMSs and proteins to prepare emulsions has been studied using high-energy methods (A Bos and van Vliet, 2001). SMSs have a better packing efficiency, can lower the surface free energy, and thereby reduce droplet dimension more effectively than proteins (Dickinson, 1998). This has been demonstrated for smaller droplets in emulsions prepared with the combination of polyoxyethylene (20) sorbitan monolaurate (Tween 20) and β-lactoglobulin than those using β-lactoglobulin alone (Li and McClements, 2013). The competitive adsorption of proteins and SMSs on O/W interfaces has been studied extensively (Dickinson et al., 2003, Dickinson et al., 1999, Euston et al., 1995). Because only one-third of the available interfacial area is taken up by the train segments of NaCas at the monolayer saturation coverage (Dickinson, 1998), it is possible for SMSs to adsorb at the interface and displace the adsorbed NaCas (Bos and van Vliet, 2001). It is also possible for the two groups of emulsifiers to co-exist on the interface to form a stable film (Tual et al., 2006). The exact physical event is decided by many factors, such as oil type (Courthaudon et al., 1991c), surface charge of SMSs (Chen and Dickinson, 1995), molecular structures of both SMSs and protein (Courthaudon et al., 1991b, Dickinson and Tanai, 1992), and SMS-to-protein molar ratio (Rs-p) (Courthaudon et al., 1991a). It was reported that when NaCas and Tween 60 were used together to prepare soya oil nanoemulsions, NaCas was present on the interface even at a Rs-p of 100 (Euston et al., 1995). In another study using dioxyethylene glycol n-dodecylether (C12E2) and β-casein to prepare n-hexadecane nanoemulsions, 60% of β-casein adsorbed at the interface at a Rs-p of 100 (Courthaudon et al., 1991a). Therefore, it may also be possible to prepare nanoemulsions co-emulsified and stabilized by SMSs and NaCas using the PIT method.
In the present study, the objective was to fabricate lemon oil nanoemulsions with NaCas and Tween 20 or polyoxyethylene (20) sorbitan monooleate (Tween 80) using the PIT method. Lemon oil was selected because it is a widely-used flavoring agent. Tween® family surfactants with a polyethylene head group and different hydrophobic fatty acid tails have been used to prepare nanoemulsions and microemulsions using the PIT method (Rao and McClements, 2011). Tween 20 has been studied to form microemulsions of peppermint oil with lecithin using the PIT method (Chen et al., 2015). Our hypothesis is that the O/W interface can be co-adsorbed with Tween 20/80 and NaCas, and phase inversions during thermal treatments will enable the formation of nanoemulsions with both surfactants at the interface. In addition to identifying nanoemulsion preparation conditions, properties of nanoemulsions were studied.
Section snippets
Materials
Lemon oil produced by cold pressing was a product from Now Foods Company (Bloomingdale, IL, USA). The oil contained approximately 70% limonene and 2.2–3.8% aldehydes, according to the supplier. NaCas and sodium chloride (purity >99.5%) were products of Sigma–Aldrich Corp. (St. Louis, MO, USA). Tween 20, Tween 80, hexane, and acetone were purchased from Fisher Scientific (Pittsburgh, PA, USA).
Preparation of nanoemulsions
NaCas and Tween 20 or Tween 80 were dissolved in deionized water at concentrations of 2% w/v and 0–1.2%
Difference between Tween 20 and Tween 80
The first group of samples with 2% w/v lemon oil was prepared with 2% w/v NaCas alone, 0.2% w/v Tween 80 or Tween 20 alone, and their combinations by heating coarse emulsions at 90 °C for 1 h. Samples prepared with NaCas–Tween combinations showed better clarity than those prepared with an individual emulsifier (Fig. 1A). The sample prepared with the NaCas–Tween 20 combination was clearer than that with the NaCas–Tween 80 combination. Emulsions prepared with an individual surfactant were all
Conclusions
In conclusion, lemon oil nanoemulsions can be formed with the combinations of Tween 20 and NaCas using the PIT method. The emulsion turbidity and droplet size reduced significantly using the combination when compared with those prepared with each emulsifier individually. An increase in Tween 20 concentration resulted in a smaller amount of NaCas on the interface, but NaCas was present on the interface in all treatments. The addition of NaCas enabled the PIT between 80 °C and 90 °C. The presence
Acknowledgments
This work was supported by the University of Tennessee and the USDA National Institute of Food and Agriculture Hatch Project 223984. We are grateful to Dr. Barry Bruce for use of ultracentrifuge and Meng Li for assisting with the experiments.
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