Improved oxidative barrier properties of emulsions stabilized by silica–polymer microparticles for enhanced stability of encapsulants

Highlights

• Electrostatic complexation of silica nanoparticles and ε-polylysine formed microparticles.

• Microparticle stabilized emulsions showed lower permeation of peroxyl radicals.

• Microparticles quenched peroxyl radicals more effectively than silica nanoparticles.

• Retinol was more stable in microparticle stabilized emulsions.

Abstract

The materials encapsulated within oil-in-water emulsions are prone to oxidation due to the permeation of oxidative species across the oil–water interface and into the lipid phase. Thus, the oxidative barrier properties of the interfacial layer are pivotal in reducing oxidation within emulsified oils. To enhance these barrier properties, we explored an approach of stabilizing emulsions using ‘silica–polymer microparticles’. We hypothesize that these microparticles will enhance the barrier properties of emulsion interfaces by mechanisms such as higher interfacial thickness and quenching of oxidative species before they permeate into the emulsions. Silica–ε-polylysine (Si–EPL) microparticles were synthesized by electrostatic aggregation of anionic silica nanoparticles and cationic ε-polylysine in the aqueous phase. Formation of Si–EPL microparticles was validated using particle size, ζ-potential and scanning electron microscopy measurements. These microparticles were subsequently used for emulsion stabilization. Emulsions stabilized by silica nanoparticles alone were used as control. Oxidative barrier properties were determined by measuring the rate of permeation of peroxyl radicals from the aqueous to the oil phase of the emulsion using fluorescence based methods. The rate of permeation of peroxyl radicals was significantly lower in emulsions stabilized by Si–EPL microparticles compared to that stabilized by silica nanoparticles. One of the mechanisms responsible for the observed effect was enhanced quenching of peroxyl radical by Si–EPL microparticles before they can permeate inside the oil phase. To further validate the results, stability of a model bioactive compound, retinol, encapsulated in these emulsions was compared. Consistent with peroxyl radical permeation measurements, emulsion stabilized by Si–EPL microparticles significantly improved the oxidative stability of retinol compared to that stabilized by silica nanoparticles alone. Thus, by engineering the physical properties of the interfacial layers, the oxidation of the encapsulants in emulsions can be controlled.

Introduction

Oil-in-water emulsion based encapsulation systems are widely used in food, pharmaceutical and biomedical industry for the delivery of hydrophobic bioactive compounds, vitamins, flavors and antibacterial compounds (Donsi et al., 2011, Frelichowska et al., 2009, McClements et al., 2007, Tikekar et al., 2013). Many of these compounds are susceptible to oxidation induced degradation (Gonnet, Lethaut, & Boury, 2010). These oxidation reactions in emulsions are often initiated by free radicals localized at the interface of the emulsions (Berton-Carabin et al., 2014, Mosca et al., 2013, Tikekar and Nitin, 2011, Tikekar and Nitin, 2012). The free radical species can permeate across the interfacial barrier and induce oxidation of the encapsulated compounds. Based on this understanding, it is evident that the barrier properties of interfacial material have a significant role in limiting the permeation of these oxidative species and thus improving the stability of the encapsulated material.

Previous studies have evaluated the role of layer-by-layer coatings to improve the oxidative barrier properties of the interface and reduce oxidation of encapsulants (Chen et al., 2011, Lim et al., 2014, Shchukina and Shchukin, 2012). The results of these studies demonstrated that higher interfacial thickness and or the electrostatic shielding of metal ions from the interface were largely responsible for limiting the oxidation of lipids. Despite the success of this approach in improving oxidative stability of lipids, the layer-by-layer assembly process can induce destabilization in the emulsion due to bridging flocculation. Furthermore, layer-by-layer coating of colloidal particles is a multi-step process involving intermediate separation steps that may not be compatible with the industrial scale processing and manufacturing (McClements et al., 2007). Another strategy includes localization of antioxidant molecules at the emulsion interface through either chemical modifications or conjugation with surface active materials (Astete et al., 2011, Di Mattia et al., 2009, Yuji et al., 2007). However, this approach may not be cost-effective for bulk food applications and modified compounds may need additional regulatory approvals prior to their applications in food. Therefore, there is a need to develop new, simple and scalable strategies to improve the barrier properties of emulsion based encapsulation systems.

To address these challenges, in this study we explored a new approach of stabilizing emulsion based encapsulation systems by ‘silica–polymer microparticles’. These microparticles were generated by electrostatic complexation of cationic small molecular weight polymer, ε-polylysine (MW ~ 5000) (Yu, Huang, & Huang, 2010), with anionic silica nanoparticles. The underlying hypothesis of the proposed approach was that larger size of microparticles and their enhanced ability to quench oxidative species before they permeate into the oil phase will result in enhanced barrier properties of the emulsion droplets. A previous study that used a similar approach where emulsions were stabilized by aggregates formed from anionic silica nanoparticles (Ludox HS-30) and cationic surfactant (hexadecyltrimethylammonium bromide), found improved stability of emulsion against phase separation (Binks, Rodrigues, & Frith, 2007). Similar results have been reported by other studies where emulsions stabilized using nanoparticles complexed with other amphiphilic small molecules enhanced the stabilization of emulsions (Akartuna et al., 2008, Pichot et al., 2010). However, these studies did not investigate the effect of stabilizing emulsions with complex microparticles on enhancing the oxidative stability of materials encapsulated within these emulsions.

The objective of this study was to compare the oxidative barrier properties of emulsions stabilized by microparticles with those stabilized by anionic silica nanoparticles alone. The oxidative barrier properties were then correlated with the stability of a model encapsulant, retinol (Vitamin A), a carotenoid that is known to have health benefits (Abdel-Aal & Akhtar, 2006) but a significant susceptibility to oxidation (Eskandar, Simovic, & Prestidge, 2009)