Formation and Stability of d-Limonene Organogel-Based

Article pubs.acs.org/JAFC

Formation and Stability of D‑Limonene Organogel-Based Nanoemulsion Prepared by a High-Pressure Homogenizer Mohamed Reda Zahi,† Pingyu Wan,‡ Hao Liang,*,† and Qipeng Yuan*,† †

State Key Laboratory of Chemical Resource Engineering, and ‡School of Science, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: D-Limonene organogel-based nanoemulsion was prepared by high-pressure homogenization technology. The organogelator type had a major role on the formation of the formulations, in which stearic acid has given nanoemulsions with the smallest droplet size. The surfactant type and concentration also had an appreciable effect on droplet formation, with Tween 80 giving a mean droplet diameter (d ≈ 112 nm) among a range of non-ionic surfactants (Tween 20, 40, 60, 80, and 85). In addition, high-pressure homogenization conditions played a key role in the nanoemulsion preparation. The stability of Dlimonene organogel-based nanoemulsion was also investigated under two different temperatures (4 and 28 °C) through 2 weeks of storage. Results showed a good stability of the formulations, which is maybe due to the incorporation of D-limonene into the organogel prior to homogenization. This study may have a valuable contribution for the design and use of organogel-based nanoemulsion as a delivery system in food. KEYWORDS: D-limonene, organogel, nanoemulsion, organogel-based nanoemulsion

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based food and beverage industries.26,27 Many researchers have reported that the incorporation of essential oils or their bioactive compounds into nanoemulsions could enhance their water solubility and improve their chemical stability, leading to the increase of their biological activity in food matrices, where microorganisms are preferably located.28,29 D-Limonene nanoemulsion was prepared by many researchers using both high- and low-energy methods. Jafari et al.17 prepared D-limonene nanoemulsion using high-pressure homogenization with a native biopolymer Angum gum. Li and Chiang 11 investigated the influence of ultrasonic emulsification conditions on both formation and stability of D-limonene nanoemulsion with a small droplet size diameter by optimizing the preparation conditions using response surface methodology (RSM). Li et al. 20 studied the process optimization and the stability of D-limonene nanoemulsion prepared by a catastrophic phase inversion method. This proves that the encapsulation of D-limonene into nanodispersions had for decades aroused the interest of researchers, which is maybe due to its wide spectrum of biological activities. Organogel-based delivery systems are relatively novel in food science.30,31 They are defined as thermoreversible semi-solid systems formed by liquid oil trapped by three-dimensional networks of structuring agent (organogelators), such as fatty alcohols, fatty acids, and monoglycerides.32 The formation of the networks may lead to the immobilization of the oil, which can improve the dispersibilty of the compound enclosed in the organogelator or the oil.33 The use of organogel as a delivery system has recently increased exponentially, which is maybe because of the ease of its preparation or its high stability.30,34

INTRODUCTION D-Limonene (4-isopropenyl-1-methylcyclohexene), a natural and functional monoterpene with a lemon-like flavor, is the main constituent of all citrus-derived essential oils, such as lemon, lime, orange, and grapefruit. It is widely used in food, cosmetics, and consumer products because of its pleasant citrus fragrance. Its transparency is preferred in commercial applications, such as beverages and food. It is tabulated in the code of federal regulation as generally regarded as safe (GRAS) for use as a flavoring agent and food preservative.1 It is endowed with a number of pharmacological and biological activities, such as antioxidant,2 antimicrobial,3−5 anticarcinogen,6 chemopreventive,7,8 and antidiabetic.9 However, the major drawback of D-limonene is its hydrophobic nature, as it is hard to achieve dispersion in water, limiting its use in liquid−solid interfaces or water-rich surfaces.10 In addition, it can be easily degraded by oxidation reactions under normal storage conditions, resulting in the loss of it lemon-like flavor.11 Djordjevic et al.12 found that Dlimonene was changed to hydroperoxides and then underwent scission reactions to form alcohols, ketones, and epoxides. In the literature, several approaches have been taken as delivery systems of D-limonene.13−16 In addition, conventional emulsions17−19 and nanoemulsions20,21 were also used to improve its hydrophobic and oxidative nature. A nanoemulsion lipid delivery system is probably a good alternative for the encapsulation of lipophilic bioactive compounds. It is defined as a colloidal dispersion comprising two immiscible liquids possessing droplet size diameters ranging from 50 to 1000 nm.22 The small droplet size offers to nanoemulsion an excellent stability against creaming and sedimentation along with a slightly turbid or translucent shape. It is endowed with a good physical stability, high optical clarity, and enhanced bioavailability,23−25 making it very convenient to incorporate lipophilic components into transparent aqueous© XXXX American Chemical Society

Received: July 5, 2014 Revised: December 3, 2014 Accepted: December 5, 2014

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dx.doi.org/10.1021/jf5032108 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry

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Storage Stability. The prepared formulations were placed in amber bottles and stored at 4 or 28 °C. The stability was determined by measuring the change of the droplet sizes every 3 days for 2 weeks. Statistical Analysis. All experiments were carried out in triplicate. The data were recorded as the mean ± standard deviation (SD) for the measurements. The data were analyzed by the statistical package (SPSS, version 12.0, for Windows, SPSS, Inc., Chicago, IL).

The use of organogel as a delivery system is possibly a good choice to enhance the stability of the products. However, its use in water-rich systems may be inconvenient because of its high hydrophobicity. Emulsification of organogel may offer additional benefits and increase its application versatility. To date, the use of modern delivery systems, such as organogel-based nanoemulsions, to encapsulate a hydrophobic compound is very limited. However, Yu and Huang35 developed a novel curcumin organogel-based nanoemulsion by ultrasonication. The authors have reported that the organogel-based nanoemulsion has contributed to the improvement of the bioavailability of curcumin by enhancing its water solubility. This finding result may give evidence that these systems are very useful in terms of improving the solubility of hydrophobic compounds. The objective of the current study was to prepare a Dlimonene organogel-based nanoemulsion by high-pressure homogenization and investigate the influence of the type and concentration of organogelators and emulsifiers, homogenization pressure, and number of cycles on the formation and stability of the formulation.

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RESULTS AND DISCUSSION Effects of Organogelators on the D-Limonene Organogel-Based Nanoemulsion Formation. To investigate the impact of the organogelator type and concentration on the formation of nanoemulsions with small droplet sizes, a series of organogel-based nanoemulsions with a fixed surfactant content and homogenization conditions (10% Tween 80, 10 cycles, and 30 MPa) was prepared using two kinds of organogelators (stearic acid or monostearin) at different concentrations. As shown in Table 1, a large decrease on the mean droplet size was Table 1. Impact of the Organogelator Type and Concentration on the Mean Droplet Formation

MATERIALS AND METHODS

Chemicals. D-Limonene was obtained from Florida Worldwide Citrus Products Group, Inc. (Bradenton, FL). Monostearin and stearic acid were purchased from Aladdin Reagents Co., Ltd. (Shanghai, China). Non-ionic surfactants (Tween 20, 40, 60, 80, and 85) were purchased from the Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). The Tween surfactant structures were composed of a polyoxyethylene headgroup and fatty acid tail group of various lengths, with the two moieties being connected together via a sorbitol molecule. Medium-chain triglyceride (MCT oil) was purchased from Sigma-Aldrich (Shanghai, China). Preparation of D-Limonene Organogel and OrganogelBased Nanoemulsion. D-Limonene organogel was prepared as described by Yu and Huang,35 with some modifications. Briefly, Dlimonene, MCT oil, and stearic acid or monostearin were mixed together followed by heating to ensure complete dissolution of the organogelator. The mixture was set at room temperature, and organogel was formed within a few minutes. D-Limonene organogelbased nanoemulsion preparation was carried out by a high-pressure homogenization procedure. D-Limonene organogel was used as the oil phase, and Milli-Q water containing different types of Tween surfactants was used as the aqueous phase. The organogel-based nanoemulsion formation was performed by mixing the water phase together with the oil phase. Then, the two phases were mixed by a high-blending homogenizer at 24 000 rpm for 5 min to form a coarse emulsion, followed by high-pressure homogenization to form Dlimonene organogel-based nanoemulsion. Using the above procedure, several batches were established by varying the organogelator, surfactant type and concentration, homogenization pressure, and cycles to study their effect on the formation and stability of the formulation. Particle Size Measurements. The average particle diameters and particle size distributions of the organogel-based nanoemulsions were determined using dynamic light scattering at 25 °C (Zetasizer NanoZS90, Malvern Instruments, Malvern, U.K.). The measurements were carried out at a scattering angle of 90°. Each measurement was recorded in an average of 10 scans. The samples were diluted approximately 1000-fold with Milli-Q water. The particle size of the emulsions was described by the mean particle size diameter, and the size distribution was described by the polydispersity index (PdI). Turbidity Measurements. The turbidity was determined using an ultraviolet−visible (UV−vis) spectrophotometer at 600 nm (Ultraspec 2450, Shimadzu, Ltd., Japan). Distilled water was used as a reference to the blank cells.

organogelator type

content of the organogelator in the organogel (%)

stearic acid

0 5 10 15 5 10 15

monosearin

mean droplet diametera (z average) 133.8 112.2 110.4 110.8 122.8 125.9 127.5

± ± ± ± ± ± ±

2.03b 1.17c 0.95 1.32 1.06d 1.65 0.72

Data shown are the mean ± SD. bD-Limonene nanoemulsion prepared without the addition of the organogelator. cThe mean droplet diameter of D-limonene organogel-based nanoemulsion prepared by stearic acid at different amounts. dThe mean droplet diameter of D-limonene organogel-based nanoemulsion prepared by monostearin at different amounts. a

observed, following the addition of the organogelator into the oil phase. The average diameter decreased from d = 133.8 ± 2.03 to 112.8 ± 1.17 and 122.8 ± 1.06 nm, following the addition of 5% stearic acid or monostearin, respectively. This can give evidence that the organogelator has contributed to the decrease of the droplet diameters. In addition, there was a minor change on the droplet diameters after increasing the amount of the organogelators, which means that the amount of the organogelators did not influence the formation of the Dlimonene organogel-based nanoemulsion. In comparison of the mean droplet diameters of the formulations prepared by stearic acid or monostearin, stearic acid organogel-based nanoemulsion exhibited the smallest droplet size. Possible reasons are the difference on physicochemical proprieties between this two organogelators or the low molecular weight of stearic acid compared to monostearin. On the other hand, the increase on the stearic acid amount (5, 10, and 15% for d = 112.2, 110.4, and 110.8 nm, respectively) did not contribute to a large change on the mean droplet diameters. For this reason, 5% stearic acid was used as the structuring agent of D-limonene organogel preparation for the following experiments. Effects of the Surfactant Type and Concentration on D-Limonene Organogel-Based Nanoemulsion Formation. Effect of the Surfactant Type on the Particle Size. In this part, a series of organogel-based nanoemulsions with fixed homogenization conditions (10 cycles and 30 MPa) was B



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of organogel-based nanoemulsion prepared at 10% (w/w) with different Tween surfactants. The mean droplet diameters of the colloidal dispersions were arrayed between 112 and 158 nm with the PdI being in the range of 0.148−0.229. This can give evidence that D-limonene organogel-based nanoemulsions can be formulated using all tested surfactants, with the dispersed droplets in the nanometer scale. Among the five different surfactants tested, Tween 80 produced the smallest mean droplet size (d = 112.2 ± 1.17 nm) with a slightly turbid shape, as presented in panels A and B of Figure 2. In contrast, the largest droplets were produced in the systems prepared by Tween 40 (d = 158.5 ± 2.15 nm). The difference of the mean droplet diameters is maybe due to the hydrophilic−lipophilic balance (HLB) of the surfactants, which is a very important factor for the selection of surfactants having the ability to form

and stabilize the emulsions.36,37 Surfactants with high HLB values, such as Tween 20 and 40 (HLB = 16.7 and 15.6, with d = 137.7 and 158.5 nm, respectively) or a low HLB value, such as Tween 85 (HLB = 11.0, with d = 134.4 nm) were not able to form D-limonene organogel-based nanoemulsion with the smallest droplet sizes. However, surfactants with a medium HLB value, such as Tween 60 and 80 (HLB = 14.9 and 15.0, with d = 127.5 and 112.2 nm, respectively) could form organogel-based nanoemulsion with the smallest particle sizes. Furthermore, Tween 60 and 80 both have approximately the same HLB value of ≈15, but they have given colloidal dispersions with different droplet size diameters (d = 112.2 and 127.5 nm, respectively). This is maybe due to the molecular geometry of the surfactants, which is known to play an important role in the emulsion production and stability.38 There is thus far a prominent similarity between the geometric structures of both Tween 60 and 80 (polar headgroup and nonpolar tail), making their HLB approximately the same. However, the nonpolar tails of Tween 60 are saturated, but those of Tween 80 are unsaturated. Those saturated tails may affect the packaging of the surfactant at the oil phase interface, which may cause a disturbance in the formation of small droplets when the oil phase (organogel) and the water phases are mixed together. Moreover, many researchers have reported that the presence of unsaturation on the non-ionic surfactant structures may favor the tiny droplet formation.36,39 In the following experiments, Tween 80 was chosen as the appropriate surfactant because it contributes to the formation of an organogel-based nanoemulsion with the smallest mean droplet size diameter. Effect of the Surfactant Concentration on the Particle Size Formation. The effect of the surfactant concentration on the mean droplet size formation and distribution was also investigated. D-Limonene organogel-based nanoemulsions were prepared by Tween 80 at different concentrations with a fixed homogenization condition (30 MPa and 10 cycles). The surfactant concentrations were varied from 2.5 to 20% (w/w). As shown in Figure 3, the surfactant concentration seems to play an efficient role on the formation of the droplets. The mean droplet sizes were found to decrease by increasing the content of surfactant. For example, at 2.5% Tween 80, the mean

Figure 2. (A) Profile of the particle size distribution of D-limonene organogel-based nanoemulsion prepared with Tween 80 at a final concentration of 10% (w/w), 30 MPa, and 10 cycles and (B) its typical picture.

Figure 3. Effect of the Tween 80 concentration on the mean droplet diameter of D-limonene organogel-based nanoemulsion (data shown are the mean ± SD).

formulated using a series of non-ionic surfactants (Tween 20, 40, 60, 80, and 85). Figure 1 shows the particle size parameters

Figure 1. Effect of the surfactant type on the mean droplet diameter of D-limonene organogel-based nanoemulsion. Surfactants were Tween 20, 40, 60, 80, and 85, from the left to the right (data shown are the mean ± SD).

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droplet size was large (d = 269.7 ± 2.02 nm). In contrast, the amount of surfactant was increased, the droplet size diameters decreased (d = 163.3 ± 4.5, 112.4 ± 1.17, 97.44 ± 2.85, and 95.6 ± 4.03 nm at 5, 10, 15, and 20%, respectively). Our results are in agreements with a previous study reported by Yuan et al.,40 who found that the droplet sizes of β-carotene nanomulsions prepared by the high-pressure homogenization method decreased by increasing the amount of surfactant. These results can be explained by a greater amount of Tween 80 being able to completely dissolve the dispersed phase (Dlimonene organogel) into the dispersing phase (water). On the other hand, Tween 80 can also form a layer between the two phases, leading to the formation of a minimum interfacial tension, hence promoting the formation of a tiny droplet size with a narrow distribution. It is already known the use of a high amount of surfactants on the nanoemulsion preparation is costly and may have an undesirable taste in commercial formulations. Thus, an intermediate amount of surfactant is maybe more suitable for the preparation of stable nanoemulsions. In this regard, we found it necessary to use 10% Tween 80 in the following experiments. Effects of the Homogenization Pressure and Number of Cycles on the Proprieties of D-Limonene Organogel-Based Nanoemulsion. It is well-known that the homogenization pressure and number of cycles significantly influence the mean droplet size formation and particle distributions.41 In this part, the influence of the homogenization pressure and number of cycles on the average droplet diameter of D-limonene organogel-based nanoemulsion was investigated (Figure 4).

nanoemulsion can be formed using a high-pressure homogenizer method, and this technique is efficient for preparing organogel-based nanemulsions with small droplet sizes. The average particle size of the organogel-based nanoemulsion continued to decrease by increasing the number of cycles, but the decrease was modest in a certain stage. Organogel-based nanoemulsion prepared at 30 MPa showed the lowest average of the particle size, as it was decreased from 136.3 ± 0.45 after 1 cycle to 103.2 ± 1.84 after 20 cycles. It should be noted that the average particle sizes did not mainly change after about 10 cycles. Therefore, we found it necessary to use this value as a processing condition for our experiments. Storage Stability of D-Limonene Organogel-Based Nanoemulsion. For most commercial applications, it is very important that nanoemulsion-based delivery systems remain physically stable during their shelf life (i.e., there is little change in their particle size diameter during storage). The change on the turbidity of D-limonene organogel-based nanoemulsion was investigated in this part during a storage time of 2 weeks at 4 or 28 °C. As shown in Figure 5, the turbidity of D-limonene organogel-based nanoemulsion increased rapidly throughout the first 6 days and the increase was slight for the remaining storage time.

Figure 5. Storage turbidity of D-limonene organogel-based nanoemulsion at 4 and 28 °C (data shown are the mean ± SD).

The droplet diameter and size distribution are important parameters affecting nanoemulsion stability.44 The change of the nanoemulsion droplet sizes is due to numerous physiochemical processes, which include coalescence, flocculation, gravitational separation, and Ostwald ripening.28 We therefore investigated the influence of the storage time at 4 and 28 °C on the stability of D-limonene organogel-based nanoemulsion for 2 weeks of storage. There was a slight increase in the mean droplet diameters for all samples stored at 4 and 28 °C, respectively (Figure 6). This increase is maybe due to the movement of the dispersed droplet throughout the dispersing phase, which increased the opportunities of droplet collisions.45 In comparison to conventional emulsions, nanoemulsions are endowed with good stability against phase separation, creaming, and sedimentation because of their small droplet sizes. However, many researchers have reported that Ostwald ripening and/or coalescence are the main mechanisms causing the instability of nanoemulsions.46 Ostwald ripening

Figure 4. Effect of the high-pressure homogenization pressure (5, 15, and 30 MPa) and number of cycles (1, 3, 5, 10, 15, and 20) on the mean droplet diameter of D-limonene organogel-based nanoemulsion (data shown are the mean ± SD).

The mean droplet diameter was decreased by increasing the homogenization pressure and number of cycles. These results are in agreement with previous studies reported by many researchers.40,42,43 A large decrease of the mean droplet diameter was observed when the coarse emulsions were passed through the high-pressure homogenizer at all pressures studied. For example, the average droplet size decreased from 1340 ± 340 nm before homogenization to 216.9 ± 4.6, 160 ± 1.61, and 136.3 ± 0.45 nm after passing 1 cycle through the high-pressure homogenizer system working at 5, 15, and 30 MPa, respectively. These results showed that organogel-based D



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conditions seem to have a good benefit on both nanoemulsion formation and storage stability by decreasing the polarity and increasing the interfacial tension, thereby acting as Ostwald ripening and/or coalescence. In the current study, D-limonene organogel-based nanoemulsion was successfully prepared by high-pressure homogenization technology. The type and concentration of organogelators and emulsifiers, homogenization pressure, and number of cycles on the formation and stability of the nanoemulsions were investigated. It was found that stearic acid was the most suitable structuring agent for small droplet formation. In addition, among the non-ionic emulsifiers tested, Tween 80 at 10% (w/w) was appropriate for the formation of D-limonene organogel-based nanoemulsion with the smallest mean droplet size. The optimum conditions of high-pressure homogenization emulsification of the nanoemulsion were 30 MPa and 10 cycles. The prepared formulation showed good stability in terms of Ostwald ripening and/or coalescence through the storage period at the two different temperatures tested. In summary, our results may have a valuable implication for the design and use of organogel-based nanoemulsion as a delivery system in food and other industries.

Figure 6. Storage stability of D-limonene organogel-based nanoemulsion at 4 and 28 °C (data shown are the mean ± SD).

can occur because of the difference in the chemical potential of the oil phase between the droplets.47 In this phenomenon, larger droplets grow at the expense of smaller droplets, which is caused by the molecular diffusion through the dispersing phase. In other words, the small droplets become smaller and the larger droplets become bigger. D-Limonene has a slight solubility in water around 13.8 g/L.48,49 This can increase the susceptibility to the Ostwald ripening process of the nanoemulsion made from it. As demonstrated previously, the Ostwald ripening can be inhibited by incorporating a highly water-insoluble oil into a relatively water-insoluble oil prior to nanoemulsion formation,50,51 because of the entropy of the mixing effect. Li and Chiang11 prepared D-limonene nanoemulsion and reported that the main destabilization mechanism of the system was Ostwald ripening. In addition, the droplet growth of the nanoemulsion was large through the storage period of 98 days. In another study, Li et al.20 reported that Ostwald ripening was also the major destabilization mechanism for D-limonene nanoemulsion. Furthermore, they have mentioned that the growth of the particle size increased at a relatively higher rate to 128 nm at 4 °C and 67 nm at 28 °C during a storage period of 2 weeks, whereas, in our study, the increase of the particle size was 10 nm at 28 °C and 6 nm at 4 °C during a storage period of 2 weeks. In comparison to their results, D-limonene organogel-based nanoemulsion showed an ameliorated stability with a slight increase on the droplet size diameter at the same storage conditions. This improvement is maybe due to the incorporation of D-limonene into the organogel prior to nanoemulsion preparation, which contributed to the decrease of the D-limonene polarity, thereby increasing the stability of the nanoemulsion. Coalescence is the process in which two or more droplets merge together when they colloid.52,53 A higher amount of essential oils may lead to the droplet coalescence acceleration, which is caused by the low interfacial tension and the relatively high polarity of this kind of oil.54 In addition, the polarity of the oil may play an efficient role in the rate of droplet coalescence (i.e., nanoemulsions containing more polar oils occur more rapidly than the nonpolar oils), and this phenomenon is maybe due to the influence of the oil phase proprieties on the optimum curvature.55,56 The incorporation of D-limonene on the organogel (MCT oil and monostearin) and the preparation

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-10-6443-1557. Fax: +86-10-6443-7610. Email: [email protected]. *Telephone: +86-10-6443-7610. Fax: +86-10-6443-7610. Email: [email protected]. Funding

This project was supported by the National High Technology Research and Development Program of China (863 Program, Grants 2012AA021403 and 2014AA021705) and the Beijing Higher Education Young Elite Teacher Project (YETP0520). Notes

The authors declare no competing financial interest.

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