Influence of electrostatic interactions on the formation and stability of multilayer fish oil-in-water emulsions stabilized by whey protein-xanthan-locust bean complexes

https://doi.org/10.1016/j.jfoodeng.2019.109893Get rights and content

Highlights

  • The effect of NaCl on the stability of O/W emulsions stabilized by WPI-XG-LBG layers was investigated.

  • XG-LBG had the highest creaming stability at 0 mM and 5 mM NaCl at pH 7; while XG had the highest at 50 mM NaCl.

  • XG-LBG emulsions had the highest oxidative stability at every NaCl concentration at pH 7.

Abstract

The purpose of this research was to investigate the impact of electrostatic interactions on the stability of multilayered fish oil-in-water (O/W) emulsions stabilized by whey protein isolate (WPI)-xanthan (XG)-locust bean gum (LBG) complexes. Emulsions were prepared using the layer-by-layer deposition technique with salt concentrations (0, 5, and 50 mM NaCl) at pH below (pH 3) and above (pH 7) the isoelectric point of WPI. Results indicated that zeta potential at pH 3 resulted in positive values, whereas at pH 7 resulted in negative values, with the magnitude of the ζ-potentials increasing as the NaCl concentration increased. NaCl did not have any major impact on the particle size of the emulsions. XG emulsions had the highest viscosity at pH 3 regardless of time, though XG-LBG emulsions showed a significant increase at 0 and 5 mM NaCl over time. XG emulsions at pH 3 showed the highest viscosity at every salt concentration. At pH 7, XG-LBG emulsions had the highest viscosity results, yet decreased over time, indicating the negative salt effect the synergistic interaction between XG and LBG. With 0 mM and 5 mM NaCl at pH 7, XG-LBG emulsions had the highest creaming stability; while with 50 mM NaCl, XG emulsions had the highest creaming stability. For both the primary and secondary lipid oxidation tests, XG-LBG emulsions had the highest oxidative stability at every salt concentration at pH 7. These results have important implications in the design of biopolymer-based delivery systems for microencapsulating omega-3 polyunsaturated fatty acids for use in functional foods.

Introduction

Omega-3 polyunsaturated fatty acids (PUFA) are found in several plant and animal oils (e.g., fish oil), and are crucial in human growth and development throughout the life cycle as an important component of essentially all cell membranes (Ellulu et al., 2015; Simopoulos, 1991). As these fatty acids are not synthesized by the body, it is imperative that they are obtained through diet; however, it is difficult to include fish oils into food as they are highly susceptible to oxidation. Oxidation causes loss of fat-soluble vitamins, generation of off-flavors, palatability problems, and even production of toxins that cause foodborne illness (Arab-Tehrany et al., 2012; Shantha and Decker, 1994). Emulsions are commonly used in the food and beverage industry as delivery systems to protect and control the release of bioactive components (McClements et al., 2007). However, emulsions are thermodynamically unstable systems that break down over time through flocculation and coalescence, leading to creaming during storage. Successful development of delivery systems with omega-3 fatty acids depends on the properties of emulsion (e.g., droplet size, viscosity), the effect of environmental stresses (e.g., salt, pH), and the interaction between oil droplets and surface-active components.

The kinetic stability of emulsions can be enhanced using stabilizers such as emulsifiers and texture modifiers. Emulsion stability can be enhanced using multilayer adsorption at the interface. The formation of multilayer oil-in-water emulsions, utilizing the electrostatic layer-by-layer (LBL) deposition technique, consists of a multistep procedure where an oil and aqueous phase are first homogenized in the presence of a charged emulsifier (e.g., protein), after which consecutive layers of oppositely charged polyelectrolytes (e.g., polysaccharides) are added so that it adsorbs to the protein-coated oil droplets (Guzey and McClements, 2007). Many studies reported that multilayer emulsions provide better stability than conventional emulsions against environmental stresses such as pH, ionic strength, heating, and freezing (Guzey and McClements, 2006; Aoki et al., 2005; Ogawa et al., 2004).

Mixtures of different proteins and polysaccharides are usually used to fabricate biopolymer-based delivery systems. In the current study, stabilized multilayer emulsions were created as delivery systems for the fish oil. Whey protein isolate (WPI) is a surface-active protein product containing mainly α-lactalbumin and β-lactoglobulin. These proteins contain functional groups that allow WPI-stabilized emulsions to act as an antioxidant system by scavenging free radicals, thereby inhibiting lipid oxidation (Berton-Carabin et al., 2014; Sun et al., 2007; Elias et al., 2005; Gordon, 2001). At pH 3, below its isoelectric point (pI) of pH 4.7–5.2 (Charoen et al., 2011; Demetriades et al., 1997), WPI has a very net positive charge; while at the isoelectric point, the protein has a net charge of zero and the potential for protein-protein interaction is at its highest point. As the pH is increased up to pH 7, the net charge on the protein will shift to a negative charge. Xanthan gum (XG) is an anionic polysaccharide that displays different pH-dependent interactions with the WPI. At a low pH, XG and WPI form complexes that prevent coalescence of the oil droplets; alternatively, at a high pH the biopolymers repel one another. Locust bean gum (LBG), a non-ionic galactomannan, is not influenced by variations in pH value and salt concentration. It does provide enhanced viscosity to emulsions containing other polysaccharides, such as XG or carrageenan (Barak and Mudgil, 2014; Goycoolea et al., 1995). XG and LBG have a synergistic relationship, which has been extensively researched (Owens et al., 2018; Khouryieh et al., 2015; Bresolin et al., 1997; Tako et al., 1984). A synergistic interaction occurs between XG and LBG, which results in substantially enhanced viscosity or gelation. This phenomenon is due to the intermolecular binding that may occur between the sidechains of XG in the helical form and the backbone of the LBG (Khouryieh et al., 2007). When added to the emulsion, these two polysaccharides synergism has been shown to increase the adsorption of the WPI to the oil droplet at the O/W interface, and thus, inhibiting coalescence and creaming (Khouryieh et al., 2015).

Various studies have investigated the antioxidative effect of salt on O/W emulsions containing different oils, polysaccharides, and transition metal ions (Berton-Carabin et al., 2014; Nielsen et al., 2013; Laguerre et al., 2007; McClements, 2004; McClements and Decker, 2000). Positively charged salt ions (e.g. sodium or calcium) aid the protein and polysaccharide(s) in repelling positively charged transition metals that may come near the oil droplet interface; thus, the positively charged salt ion benefits the oxidative stability of the emulsion (Nielsen et al., 2013). In this study, multilayered oil-in-water emulsions containing menhaden droplets were fabricated. The main objective of this research was to study the influence of electrostatic interactions on the formation and stability of multilayer oil-in-water emulsions stabilized by WPI-XG-LBG complexes. The impact of adding XG and LBG on the stability of WPI-coated O/W emulsions as function of salt was investigated to determine whether WPI-stabilized emulsions containing XG-LBG mixtures would provide better physical and oxidative stability than emulsions containing either XG or LBG alone. The effect of NaCl salt content (0, 5, and 50 mM) at pH below (pH 3) and above (pH 7) the isoelectric point of WPI was studied.

Section snippets

Materials

Menhaden oil (14:0 Myristic acid 6–9%, 16:0 Palmitic acid 15–20%, 16:1 palmitoelic acid 9–14%, 18:1 oleic acid 5–12%, 18:2 linoleic acid <3, 20:4 arachidonic acid <3%, 18:4 octadecatetraenoic acid 2–4%, 20:5 eocosapentanoic acid 10–15% and 22:6 docosahexaenoic acid 8–15%, as provided by the manufacturer), xanthan gum, locust bean gum, and sodium azide were purchased from Sigma-Aldrich Co. (St Louis, MO, USA). Sodium Chloride was purchased from Thermo Fisher Scientific (Waltham, MA, USA), and

Effect of ionic strength and pH on droplet charge

Regardless of the NaCl concentrations, all emulsions at pH 3 resulted in positive ζ-potentials, while all emulsions at pH 7 demonstrated negative ζ-potential values (Fig. 1). The positive ζ-potential values indicate the dominance of the WPI at the oil-water interface because the surface charge values mirror the positive net charge of the WPI, resulting from pH 3 being below the pI of the WPI. Similar results were reported by Long et al. (2013). The reason for WPI dominating the ζ-potential in

Conclusion

The results indicated that the addition of NaCl had a significant impact on the physical and chemical stabilities of the O/W emulsions depending on the type of biopolymer and pH of the emulsion. The addition of NaCl affected the physical stability of WPI-stabilized emulsions containing XG-LBG mixtures or XG alone; however, it had a limited effect on those containing only LBG or WPI. With 0 mM NaCl at pH 3, emulsions containing XG alone were the most stable among all gum types. At pH 7,

Author contributions section

Kristen Griffin carried out the experiments and took the lead in writing the manuscript. Hanna Khouryieh conceived and planned the experiments, supervised the project, conducted the statistical analysis, contributed to the interpretation of the results and final manuscript.

Acknowledgment

This research was supported by grants from the United States Department of Agriculture's National Institute of Food and Agriculture (Grant #. 11281827) and Kentucky NSF EPSCoR (No. 1355438).

References (37)

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