Optimization of Orange Oil Nanoemulsion Formation by Isothermal ...

Article pubs.acs.org/JAFC

Optimization of Orange Oil Nanoemulsion Formation by Isothermal Low-Energy Methods: Influence of the Oil Phase, Surfactant, and Temperature Yuhua Chang†,‡ and David Julian McClements*,† †

Department of Food Science, University of Massachusetts, Amherst, Massachusetts 01003, United States College of Food Engineering and Nutritional Science, Shaanxi Normal University, Xi’an, Shaanxi 710062, People’s Republic of China

‡

ABSTRACT: Nanoemulsions are particularly suitable as a platform in the development of delivery systems for lipophilic functional agents. This study shows that transparent orange oil nanoemulsions can be fabricated using an isothermal low-energy method (spontaneous emulsification), which offers the advantage of fabricating flavor oil delivery systems using rapid and simple processing operations. Orange oil nanoemulsions were formed spontaneously by titration of a mixture of orange oil, carrier oil [medium-chain triglyceride (MCT)], and non-ionic surfactant (Tween) into an aqueous solution (5 mM citrate buffer at pH 3.5) with continuous stirring. The oil/emulsion ratio content was kept constant (10 wt %), while the surfactant/emulsion ratio (SER) was varied (2.5−20 wt %). Oil-phase composition (orange oil/MCT ratio), SER, and surfactant type all had an appreciable effect on nanoemulsion formation and stability. Transparent nanoemulsions could be formed under certain conditions: 20% surfactant (Tween 40, 60, or 80) and 10% oil phase (4−6% orange oil + 6−4% MCT). Surfactant type and oil-phase composition also affected the thermal stability of the nanoemulsions. Most of the nanoemulsions broke down after thermal cycling (from 20 to 90 °C and back to 20 °C); however, one system remained transparent after thermal cycling: 20% Tween 80, 5% orange oil, and 5% MCT. The mean droplet size of these nanoemulsions increased over time, but the droplet growth rate was reduced appreciably after dilution. These results have important implications for the design and utilization of nanoemulsions as delivery systems in the food and other industries. KEYWORDS: nanoemulsions, emulsions, spontaneous emulsification, encapsulation, delivery, low-energy homogenization, essential oil, orange oil, flavor

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and biological properties.10−13 The size of the droplets in nanoemulsions is often much smaller than the wavelength of light (d ≪ λ), and therefore, they do not scatter light strongly, making them transparent or only slightly turbid. They can therefore be used to incorporate lipophilic bioactive compounds into transparent aqueous-based products, such as some foods and beverages. The small size of the droplets in nanoemulsions also means that they typically have much better stability to gravitational separation, flocculation, and coalescence than emulsions.10−13 Finally, the bioavailability of encapsulated bioactive compounds often increases as the droplet size in emulsion-based systems decreases.14−16 Nanoemulsions can be fabricated by a number of different processing methods, which are usually categorized as either high- or low-energy methods.12 High-energy methods use specialized mechanical devices (“homogenizers”) capable of generating intense disruptive forces that breakup and intermingle the oil and aqueous phases, such as high-pressure valve homogenization, microfluidization, and sonication.17 Low-energy methods rely on control of interfacial phenomenon at the boundary between organic and aqueous phases and depend upon the nature of any surface-active molecules

INTRODUCTION Orange oil obtained from citrus peel is widely used in the food and beverage industries as a flavoring agent, because it contains volatile constituents with characteristic aroma profiles.1,2 Orange oil is a complex organic compound consisting of more than 200 components, with the main components being limonene and linalool, which are classified as terpenes.3 Orange oil, like many other flavor oils, contains a high proportion of hydrophobic molecules, and therefore, colloidal delivery systems are usually needed to encapsulate it, so that it can readily be dispersed into aqueous-based food and beverage products. The most commonly used colloidal delivery systems for this purpose are oil-in-water (O/W) microemulsions, nanoemulsions, and emulsions.4−6 Microemulsions are thermodynamically stable colloidal dispersions that consist of small lipid particles (mainly consisting of surfactant and oil) dispersed in water.7−9 On the other hand, nanoemulsions and emulsions are both thermodynamically unstable systems, but they also consist of small lipid particles dispersed in water.7 These two systems differ from each other in terms of their particle dimensions: radius of 100 nm for emulsions.10−12 Nanoemulsions have similar properties to emulsions in terms of their general compositions, structures, and thermodynamic characteristics.10−12 Nevertheless, differences in the particle size mean that nanoemulsions and emulsions do exhibit some distinctly different physicochemical © 2014 American Chemical Society

Received: Revised: Accepted: Published: 2306

January 10, 2014 February 24, 2014 February 24, 2014 February 24, 2014 dx.doi.org/10.1021/jf500160y | J. Agric. Food Chem. 2014, 62, 2306−2312

Journal of Agricultural and Food Chemistry

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present, e.g., their solubility and molecular geometry.17 A number of low-energy approaches have been developed to form nanoemulsions, including spontaneous emulsification (SE), emulsion inversion point (EPI), phase inversion temperature (PIT), and phase inversion composition (PIC) methods.17−20 These methods are currently not widely used in the food industry, and where they are used, there is a relatively poor understanding of their physicochemical basis and the major factors impacting their performance.17,21 Low-energy approaches may have advantages over high-energy approaches for certain applications: they are often more effective at producing very fine droplets; they have lower equipment and energy costs; and they are simpler to implement. On the other hand, there are also some potential drawbacks of low-energy methods, including limitations on the types of oils and surfactants that can be used to form stable nanoemulsions and the fact that relatively high surfactant/oil ratios (SORs) are typically needed to produce them.19,20,22 In the current study, we examine the potential of using an isothermal low-energy method (spontaneous emulsification) for producing transparent orange oil nanoemulsions suitable for utilization in foods and beverages. In general, this method involves titrating an organic phase (containing oil and surfactant) into an aqueous phase with continuous mixing, which leads to the spontaneous formation of ultrafine droplets because of rapid diffusion of the surfactant from the organic phase into the aqueous phase.18,21,23 This method allows for nanoemulsions to be fabricated at room temperature using simple stirring rather than expensive homogenization equipment. This approach may therefore be particularly suitable for utilization within certain sectors of the food and other industries. To the best of our knowledge, the formation of orange oil nanoemulsions by spontaneous emulsification has not yet been reported. Previous studies in spontaneous emulsification have shown that the size of the droplets generated using the spontaneous emulsification process depends upon many factors, including the type and concentration of surfactant and oils used, stirring conditions, and injection conditions.4,23−25 The purpose of the present study was to investigate the major factors influencing the formation and stability of orange oil nanoemulsions. The results of this study have important implications for the design and utilization of nanoemulsions as delivery systems in the food and other industries. In particular, they may be useful for implementation in beverage emulsions, where only low concentrations of oil and surfactant are present in the final product.26

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spontaneous emulsification was performed by titration of an organic phase (containing different amounts of orange oil, MCT, and nonionic surfactant) into an aqueous phase (5 mM citrate buffer at pH 3.5) while continuously stirring (500 rpm) the system with a magnetic stirrer at ambient temperature (≈25 °C). Unless otherwise stated, the experiments were carried out using standardized conditions: a total oil phase (orange oil + MCT) content of 10 wt %, surfactant content of 20 wt %, and aqueous phase content of 70 wt %. In these samples, the oil (10 g) and surfactant (20 g) were first mixed together and then the mixture was titrated into 70 g of aqueous phase at a rate of 2 mL/min. In some experiments, systems with different surfactant/emulsion ratios (SERs) were prepared by varying the amount of surfactant in the overall system. For example, a system with SER = 10% contains 10 g of surfactant in 100 g total (surfactant + oil + buffer). Particle Size Measurements. The particle size distributions and mean particle diameters (Z averages) of nanoemulsions were measured using a dynamic light scattering instrument (Zetasizer Nano ZS, Malvern Instruments, Malvern, U.K.). This instrument determines the particle size from intensity−time fluctuations of a laser beam (633 nm) scattered from a sample at an angle of 173°. Each individual measurement was an average of 13 runs. To avoid multiple scattering effects, samples were diluted before the particle size measurements using citrate buffer solution (5 mM, pH 3.5). Nanoemulsion Thermal Stability Measurements. The thermal stability of the nanoemulsions was determined by measuring their turbidity (600 nm) during heating and cooling using a temperaturescanning spectrophotometer (Evolution Array, Thermo Scientific). The turbidity was measured as the temperature was increased from 20 to 90 °C at a rate of 1 °C/min and then decreased back to 20 °C at the same rate. A citrate buffer solution (5 mM, pH 3.5) was used as a blank. Statistical Analysis. All experiments were carried out 2 or 3 times using freshly prepared samples, and the results are reported as the calculated mean and standard deviation of these measurements. Means were subjected to Duncan’s test, and a p value of 2000 nm) and phase separation occurred quickly. On the other hand, at SERs = 5, 10, 15, 17.5, and 20 wt %, the mean particle diameter decreased to 139, 67, 56, 34, and 25, respectively (Figure 2). In addition, the systems became optically transparent at the highest surfactant levels. Thus, it was possible to make clear colloidal delivery systems that contained very small droplets at high SERs. This phenomenon is in agreement with earlier studies21 and can be attributed to a number of physicochemical mechanisms. 2308

dx.doi.org/10.1021/jf500160y | J. Agric. Food Chem. 2014, 62, 2306−2312


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Figure 4. Effect of the surfactant type and oil-phase composition on the mean particle diameter of emulsions produced by spontaneous emulsification. The oil-phase composition (varying ratio of orange oil/ MCT; total oil = 10 wt %) was indicated in the legend. The surfactant was 20 wt % in system. The results indicate that Tween 40 can load the most amount of orange oil (6%), producing transparent nanoemulsions with very small particle sizes (d ≈ 25 nm), while Tween 60 can load the least amount of orange oil (4%), and Tween 60 is in the middle (5% orange oil). Other types of surfactants cannot produce transparent orange oil nanoemulsions, regardless of the oil phase.

Figure 3. Effect of the surfactant type on the mean particle diameter and physical stability of emulsions produced by spontaneous emulsification. Oil phase = 4% orange oil + 6% MCT. Surfactant was 20 wt % in the system.

formation of orange oil emulsions, because Tween 40, 60, and 80 (with HLB numbers 15.6, 14.9, and 15.0, respectively) were able to form clear nanoemulsions that were stable for separation (Figure 3). There are a number of possible reasons why surfactants with intermediate HLB values may favor the formation of ultrafine orange oil nanoemulsions. The spontaneous formation of ultrafine droplets at the oil−water boundary may require a very low interfacial tension to stimulate interfacial turbulence. Surfactants with intermediate HLB numbers might be expected to form monolayers with low interfacial tensions. If the HLB number is too low, then the surfactant will remain in the oil phase rather than moving into the water phase. If the HLB number is too high, then the interfacial tension and optimum curvature of the surfactant monolayer may be too high to form very fine oil droplets. Effect of the Different Surfactant Types and Oil-Phase Compositions on the Particle Size. Having established that Tween 40, 60, and 80 could all form transparent orange oil nanoemulsions, we further investigated the effect of oil-phase composition on the size of the droplets produced using only these surfactants. Our aim was to establish which surfactant was most effective at producing small droplets and high orange oil loading capacities. In these experiments, the total surfactant concentration (20 wt %) and oil-phase concentration (10 wt %) were held constant, while the mass ratio of orange oil/MCT in the oil phase was varied. Our results indicated that surfactant type had a major influence on the formation of nanoemulsions with high loading capacities (Figure 4): Tween 40, 60, and 80 were capable of incorporating 6, 4, and 5% orange oil while still maintaining ultrafine droplets, i.e., d < 30 nm. At higher orange oil levels, there was an appreciable increase in the mean particle diameter. There are a number of possible reasons for the observed increase in the particle size at higher orange oil levels, which are associated with either emulsion formation or emulsion stability. During emulsion formation, there may be an optimum surfactant and oil-phase composition that leads to an ultralow interfacial tension and spontaneous droplet

formation at the oil−water boundary. After emulsion formation, droplet growth may occur because of Ostwald ripening or coalescence processes, which depend upon surfactant type and the composition of the oil phase. In particular, Ostwald ripening has been shown to occur rapidly in oil-in-water emulsions containing flavor oils because of their relatively high water solubility.28,29 The addition of water-insoluble oils (such as MCT or long-chain triglyceride) to flavor oils has been shown to inhibit this form of droplet growth, which has been attributed to an entropy of mixing effect that opposed Ostwald ripening.29 Thus, there may be a minimum amount of MCT that has to be present in the emulsions to prevent rapid droplet growth from occurring after emulsion formation. Thermal Stability of Selected Orange Oil Nanoemulsions. Emulsified flavor oils may be subject to thermal treatments (such as pasteurization) during food processing. We therefore examined the thermal stability of selected orange oil nanoemulsions (10% oil phase and 20% surfactant) by measuring the change in sample turbidity during a controlled heating−cooling cycle. Nanoemulsions with three different oilphase compositions were examined: 4 wt % orange oil + 6 wt % MCT, 5 wt % orange oil + 5 wt % MCT, and, 6 wt % orange oil + 4 wt % MCT. The nanoemulsions were heated from 20 to 90 °C and then cooled to 20 °C at a heating/cooling rate of 1 °C/ min. The turbidity of all of the samples before and after heating is summarized in Figure 5. The full turbidity−temperature profiles during heating and cooling of selected samples are shown in Figure 6. Tween 40. The nanoemulsions with three different oil compositions were all initially transparent (τ ≈ 0 cm−1), but they all became highly turbid (τ > 0.6 cm−1) after the heating− cooling cycle, which is indicative of temperature-induced droplet growth (Figure 5). A representative turbidity−temperature profile for the 5% orange oil + 5% MCT system is shown 2309



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Figure 5. Turbidity of representative orange oil emulsions/nanoemulsions before and after heating. The samples were heated from 20 to 90 °C at a rate of 1 °C/min and then cooled to 20 °C at the same rate. In the figure, “before” means the turbidity (at 600 nm) of samples before the heat−cooling circle and “after” means the turbidity after the heat−cooling treatment.

in Figure 6a. This system was initially transparent. During heating, the turbidity remained close to zero from 20 to 63 °C, increased to a maximum around 77 °C, and then decreased to close to zero again around 80 °C. We postulate that the increase in turbidity observed around 75 °C was due to droplet coalescence as the system approached the PIT. The decrease in turbidity at higher temperatures is attributed to the formation of a clear microemulsion around the PIT. Upon cooling, the turbidity increased steeply around 74 °C and the system remained opaque at lower temperatures, which was attributed to rapid droplet coalescence at temperatures just below the PIT. Tween 60. Before heating, nanoemulsions containing 4 wt % orange oil were transparent (τ ≈ 0 cm−1), whereas those containing 5 or 6 wt % orange oil were opaque (τ > 3 cm−1) (Figure 5). Conversely, the nanoemulsions containing 4 wt % orange oil became opaque after heating, whereas those containing 5 or 6 wt % orange oil were much lower. A representative turbidity−temperature profile for the 5% orange oil + 5% MCT system is shown in Figure 6b. This system initially appeared optically opaque (τ > 3 cm−1). During heating, the turbidity remained high from 20 to 75 °C, then decreased steeply, had a value close to zero from 79 to 85 °C, and then increased steeply around 86 °C. We postulate that the decrease in turbidity observed around 75 °C was due to the formation of a clear microemulsion phase around the PIT, while the steep increase at 86 °C was due to the formation of a water-in-oil (W/O) emulsion containing relatively large water droplets at temperatures well above the PIT. Upon cooling, there was a steep decrease in turbidity around 83 °C until the system became transparent from around 77 to 66 °C, which was attributed to the formation of a clear microemulsion around the PIT. Upon further cooling, the turbidity of the system increased somewhat, which was attributed to some droplet coalescence at temperatures just below the PIT.

Figure 6. Turbidity−temperature profiles during heating and cooling of selected orange oil emulsion/nanoemulsion samples. Oil compositions and surfactants are shown in the legend. (c) Nanoemulsion containing 20% Tween 80, 5% orange oil, and 5% MCT was temperature-reversible (i.e., remaining transparent after heating and cooling). 2310



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Tween 80. Prior to heating, all of the nanoemulsions had relatively low turbidities, but after heating, the systems containing 4 wt % orange oil became opaque, whereas those containing 5 or 6 wt % orange oil were clear (Figure 5). A representative turbidity−temperature profile for the 5% orange oil + 5% MCT system is shown in Figure 6c. This system initially had a relatively low turbidity. During heating, the turbidity remained close to zero from 20 to 60 °C, increased to a slight maximum around 66 °C, decreased close to zero from 72 to 80 °C, and then increased steeply around 81 °C until the system became opaque (τ > 3 cm−1). We postulate that the increase in turbidity observed around 66 °C was due to droplet coalescence as the system approached the PIT. We attribute the low turbidity from 72 to 80 °C to the formation of a microemulsion around the PIT and the large increase in turbidity above 81 °C to phase inversion to a W/O emulsion containing large water droplets that scatter light strongly. Upon cooling, the turbidity of the system decreased rapidly below 78 °C, which was attributed to the formation of a clear microemulsion again around the PIT and a nanoemulsion at lower temperatures. These results clearly show that the temperature dependence of orange oil nanoemulsions is complex and depends upon the nature of the surfactant and oil phase used to fabricate them. In some cases, clear nanoemulsions formed at ambient temperatures by spontaneous emulsification remain stable after thermal treatment, whereas in other cases, they do not. In other cases, unstable opaque emulsions formed at ambient temperatures by spontaneous emulsification revert to clear nanoemulsions after thermal treatment, whereas in other cases, they remain unstable. We postulate that the most important factor determining droplet growth after heating is their coalescence stability near the PIT. Some systems underwent rapid droplet growth during cooling, whereas others did not. Storage Stability of the Spontaneously Emulsified Orange Oil Nanoemulsions. For most commercial applications, it is important that nanoemulsion-based delivery systems remain physically stable throughout their shelf life; i.e., there is little change in their particle size during storage. We therefore examined the influence of storage time on the stability of a representative nanoemulsion: 5% orange oil + 5% MCT and 20 wt % Tween 80. This system was selected because it was the only one that formed transparent nanoemulsions at ambient temperature after spontaneous emulsification and was also stable to heating and cooling (see previous section). There was an appreciable increase in the mean droplet diameter from around 25 to 46 nm during storage for 40 days at ambient temperature (Figure 7), which suggests that some droplet growth occurred because of either coalescence or Ostwald ripening. Nevertheless, when the nanoemulsion was diluted 10 times in aqueous solution (5 mM citrate buffer at pH 3.5), the rate of droplet growth was appreciably reduced; i.e., the diameter only increased around 25−32 nm after 40 days of storage at ambient temperature (Figure 7). It should also be noted that, after a certain time, the droplet diameter remained relatively stable; e.g., after 19, 26, and 40 days, the particle size of the diluted nanoemulsions was 31, 32, and 32 nm (p > 0.05). Practically, the concentration of essential oil required in a final food product is usually very low, and therefore, nanoemulsions will be highly diluted after formation. Consequently, they should remain relatively stable in commercial applications. The fact that the stabilities of the nanoemulsions improved after dilution suggests that the instability of the undiluted

Figure 7. Increase in the mean particle diameter of selected nanoemulsions during 40 days of storage at room temperature. Nanoemulsions were prepared using 10 wt % oil (5% orange oil + 5% MCT), 20 wt % surfactant (Tween 80), and 70 wt % water (pH 3.5 citrate buffer solution) at a stirring speed of 500 rpm at ambient temperature (≈25 °C). This nanoemulsion was selected because it was transparent with very small sizes (d ≈ 25 nm) and temperaturereversible (remaining to be transparent after heating−cooling circles). The nanoemulsion was either undiluted or diluted 10 times by citrate buffer. The results indicated that the size of the nanoemulsion increased over storage, but this effect was much reduced if the nanoemulsion was diluted.

emulsion may have been due to the relatively high amount of surfactant present (SER = 20 wt %). At relatively high surfactant levels, there may have been appreciable amounts of free surfactant micelles present that could transfer oil molecules between droplets through Ostwald ripening or promote droplet aggregation through depletion,30,31 which is in accordance with results that we reported previously on essential oil nanoemulsions.20 In this paper, we have described the influence of surfactant and oil composition on the fabrication and stability of orange oil nanoemulsions formed by spontaneous emulsification. We found that oil-phase composition (orange oil/MCT ratio), surfactant type, and surfactant concentration all had an appreciable impact on the particle size and nanoemulsion stability. Clear nanoemulsions could be formed under certain system compositions: 20% Tween 40, 60 or 80 and 4−6% orange oil with 6−4% MCT). Surfactant type and oil-phase composition also affected the thermal stability of the nanoemulsions. Only nanoemulsions containing 20% Tween 80, 5% orange oil, and 5% MCT remained clear after heating and cooling. The droplet size of these nanoemulsions increased over time during storage at ambient temperature. Nevertheless, the rate of droplet growth could be retarded by dilution of the nanoemulsions in aqueous solutions prior to storage. These nanoemulsion preparation methods described in this study could be implemented by the food and beverage industries. Under the optimized conditions established in this study, physically stable and transparent orange nanoemulsions could be formed by simple mixing. The fabrication of nanoemulsions by spontaneous emulsification has a number of potential advantages: simplicity of implementation, no expensive equipment required, and lower energy and operating costs. Additionally, in comparison to high-energy methods (e.g., 2311



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high-pressure homogenization), spontaneous emulsification can produce nanoemulsions with smaller droplet sizes, making them particularly suitable for application in clear beverages. The major disadvantage of this method is that high amounts of synthetic surfactant are required, but this may not be a problem where the oil concentration required in a final product is relatively low, e.g., beverage emulsions.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 413-545-1019. Fax: 413-545-1262. E-mail: [email protected]. Funding

This material is based on work supported by United States Department of Agriculture, National Research Initiative (NRI) and Agriculture and Food Research Initiative (AFRI) Grants. The author Yuhua Chang thanks the National Natural Science Foundation of China (31101242) for support. Notes

The authors declare no competing financial interest.

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