Cinnamon oil nanoemulsions by spontaneous emulsification: Formulation, characterization and antimicrobial activity

Highlights

• Coconut oil was used as the carrier oil to formulate cinnamon oil nanoemulsions.

• Cinnamon oil: Coconut oil at ratio of 6:4 provided the most stable nanoemulsions.

• Antimicrobial activity of nanoemulsions were enhanced by spontaneous emulsification.

Abstract

The goal of this study was to formulate stable cinnamon oil nanoemulsions (NEs) exhibiting high antimicrobial activity by using the low-energy approach: spontaneous emulsification (SE) and compare it with two high-energy methods. To prepare the nanoemulsions by SE, oil phase containing cinnamon oil (CO) and carrier oil (coconut oil (CNO)) at different ratios (2:8–10:0) and surfactant (Tween 80) at 10% (w/w) was titrated into an aqueous phase (distilled water). For antimicrobial activity, agar disc diffusion method with E. coli as the model microorganism was used. NEs were characterized by Dynamic Light Scattering (DLS) and Transmission Electron Microscopy (TEM). Both DLS and TEM gave parallel results and mean particle size were found as $\sim$ 100 nm for 6:4 (CO: CNO) oil phase composition. These NEs also showed high physical stability during one-month storage. NEs were also prepared by using two high-energy homogenization methods: microfluidization and ultrasonication. Ultrasonication and SE showed similar trends for mean particle size and microbial activity. Microfluidization resulted in the smallest mean particle size (p < 0.05) and antimicrobial activity was not effected from cinnamon oil concentration (p > 0.05).

Introduction

Essential oils have strong antimicrobial, antioxidant and antiradical activity due to terpenoid, aldehyde and phenolic constituents (Chang, Mclandsborough, & Mcclements, 2013). These bioactive compounds present in essential oils show strong antimicrobial activity against important food pathogens such as Escherichia coli, Listeria monocytogenes, Pseudomonas aeruginosa, Staphylococcus aureus, Helicobacter pylori and Salmonella Typhi (Ghosh et al., 2014, Sugumar et al., 2013). To prevent the microorganism proliferation and to substitute the synthetic antimicrobial compounds used in foods through natural ways, essential oils are considered as alternative antimicrobial additives (Bassolé and Juliani, 2012, Chang et al., 2013). For the dispersion of these lipid particles, colloidal delivery systems such as oil-in- water (O/W) emulsions or nanoemulsions are used to entrap the functional components into the aqueous based foods, beverages and packaging materials (Chang, McLandsborough, & McClements, 2012).

Nanoemulsions have droplet sizes ranging between 20 nm and 300 nm and to distinguish them from conventional emulsion, there is not an identified clear size range (Anton and Vandamme, 2009, Chang et al., 2013, Komaiko and McClements, 2015, Mahdi Jafari et al., 2006). From a physiochemical point of view, nanoemulsions (NEs) are thermodynamically unstable but kinetically stable systems. High kinetic stability can be obtained when preparation method, composition and component of the system is appropriately selected (Solans & Solé, 2012).

Fabrication of nanoemulsions are basically achieved by high and low energy approaches (Acosta, 2009, Tadros et al., 2004). Mechanical devices are used for high energy methods such as high-pressure homogenizers (Quintanilla-Carvajal et al., 2010), ultrasound generators (Maa & Hsu, 1999). By using mechanical devices, generation of intensive disruptive forces lead to formation of oil droplets while breaking up the water and oil phases (McClements, 2012). On the other hand, ultrafine droplet formation with low energy methods relies on the internal chemical energy of the system. Membrane emulsification (Sanguansri & Augustin, 2006), spontaneous emulsification (Bouchemal, Briançon, Perrier, & Fessi, 2004), solvent displacement (Yin, Chu, Kobayashi, & Nakajima, 2009), emulsion inversion point (Sadtler, Rondon-Gonzalez, Acrement, Choplin, & Marie, 2010) and phase inversion point (Shinoda & Saito, 1969) are some of the used low energy methods.

Spontaneous emulsification (SE) is a widely used technique to form nano-scaled particles. Without causing a change in the curvature of the surfactant, its movement from the dispersed phase to the continuous phase leads to the formation of a nanoemulsion (Solans & Solé, 2012). Surfactant to oil ratio (SOR), surfactant type and oil type influence the size of the droplets produced by this method. High amount of surfactant is required to stabilize the droplets formed. If the concentration is insufficient, a protective coating does not form and consequently particles collide with each other and droplet aggregation is observed (Komaiko & McClements, 2015). Requirement of the proper surfactant concentration to obtain small particle size strongly depends on the phase behavior of the surfactant-oil-water (SOW) system that is used to form the nanoemulsion since formation of ultrafine droplets by using SE occurs only at certain SOW compositions (Rao & McClements, 2011).

In this study, Tween 80 (Polysorbate 80) was used as the main surfactant. Tween 80 is a nonionic, single tail surfactant and is very commonly used as an emulsifier in foods and pharmaceutical products (Athas et al., 2014).

Due to high eugenol (4-allyl-2-methoxyphenol) and cinnamaldehyde (3-Phenyl-2-propenal) content, cinnamon leaf oil exhibits high antimicrobial and antifungal activities (Tzortzakis, 2009). There is recent study where cinnamon oil was encapsulated through spontaneous emulsification by using Tween 80 and medium chain TGs (Tian, Lei, Zhang, & Li, 2016).

In the current study, cinnamon leaf oil (C. zeylanicum) was used as the antimicrobial agent and rather than a pure medium chain triglyceride mixture, coconut oil (Cocos nucifera) was used as the carrier oil to formulate a stable nanoemulsion. In addition, the physical stability and antimicrobial activity of the nanoemulsions were investigated and compared with two high-energy methods: microfluidization and ultrasonication techniques. In this study, it was hypothesized that stable nanoemulsions that maintain high antimicrobial activity could be obtained by using coconut oil as a ripening inhibitor due to its high content of Medium Chain Fatty Acids (MCFAs) (>50 wt % of fatty acids) (Marten, Pfeuffer, & Schrezenmeir, 2006). Antimicrobial activity of cinnamon oil nanoemulsions was tested against a model bacteria strain: E. coli. ATCC 25922.