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Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch Process optimization of ultrasound assisted curcumin nanoemulsions stabilized by OSA modified starch
Short Communication Process optimization of ultrasound-assisted curcumin nanoemulsions stabilized by OSA-modified starch Shabbar Abbas a , Mohanad Bashari a , Waseem Akhtar b , Wei Wei Li a , Xiaoming Zhang a, ⇑ a State Key Laboratory of Food Science & Technology, School of Food Science & Technology, Jiangnan University, Wuxi 214122, Jiangsu, China b CAS Key Laboratory for Biomedical Effects of Nanomaterials & Nanosafety, National Center for Nanoscience and Technology (NCNST), 11 Beiyitiao, Zhongguancun, Beijing 100190, China article info Article history: Received 19 September 2013 Received in revised form 25 November 2013 Accepted 17 December 2013 Available online 4 January 2014 Keywords: Ultrasonic homogenization Nanoemulsion Modified starch Curcumin Droplet diameter abstract This study reports on the process optimization of ultrasound-assisted, food-grade oil–water nanoemul- sions stabilized by modified starches. In this work, effects of major emulsification process variables including applied power in terms of power density and sonication time, and formulation parameters, that is, surfactant type and concentration, bioactive concentration and dispersed-phase volume fraction were investigated on the mean droplet diameter, polydispersity index and charge on the emulsion droplets. Emulsifying properties of octenyl succinic anhydride modified starches, that is, Purity Gum 2000, Hi-Cap 100 and Purity Gum Ultra, and the size stability of corresponding emulsion droplets during the 1 month storage period were also investigated. Results revealed that the smallest and more stable nano- emulsion droplets were obtained when coarse emulsions treated at 40% of applied power (power density: 1.36 W/mL) for 7 min, stabilized by 1.5% (w/v) Purity Gum Ultra. Optimum volume fraction of oil (med- ium chain triglycerides) and the concentration of bioactive compound (curcumin) dispersed were 0.05 and 6 mg/mL oil, respectively. These results indicated that the ultrasound-assisted emulsification could be successfully used for the preparation of starch-stabilized nanoemulsions at lower temperatures (40–45 °C) and reduced energy consumption. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Nanoemulsions or miniemulsions are thermodynamically unstable colloidal dispersions of at least two immiscible liquids with one of the liquids being dispersed as small spherical droplets, having diameter in the range of 20–200 nm, into the other liquid [1–3]. Oil–water (O/W) nanoemulsions are usually prepared by homogenizing an oil phase into an aqueous phase in the presence of water-soluble emulsifiers/stabilizers [1]. Such emulsions have found a very important role in the encapsulation of either poorly soluble or lipophilic food bioactives, i.e., polyphenols and carote- noids, and act as a vehicle to ensure the safe delivery of these active compounds to the desired site in the body [4]. Due to their small droplet size and large surface area, nanoemulsions have good stability to gravitational separation, flocculation, coalescence, and offer controlled release and/or absorption of functional ingredients, besides offering optical clarity to the product [1,5]. On the other hand, Ostwald ripening is the major destabilization mechanism in the nanoemulsions. This problem arises due to the increased sol- ubility of dispersed phase into the aqueous phase and can be tackled by introducing the dispersed phase with strong hydrophobic properties [6]. Medium chain triglycerides (MCT) are low viscosity oils with hydrophobic properties and offer im- proved bioaccessibility [7]. Nanoemulsions can be prepared either using high-energy (mechanical-based) or low-energy (chemical-based) approaches depending on the underlying principle. Mechanical methods for nanoemulsions preparation include microfluidization [8,9], high- pressure homogenization [10,11] and ultrasound homogenization [12–15]. In recent years, ultrasound-assisted emulsification pro- cess has gained popularity among food processors for the produc- tion of nanoemulsions, mainly due to its energy-efficiency, low production cost, ease of system manipulation and better control over formulation variables [16,17]. Ultrasonic emulsification in- volved the production of high intensity (low frequency) acoustic waves followed by the disruption of droplets under the influence of cavitational effects in the liquid medium. Final size and disper- sity of nanoemulsion droplets are influenced by a number of process and formulation variables [15,18–20]. Disruption of larger oil drops into nanosize droplets and their stability depend on the type and concentration of emulsifiers and stabilizers. Emulsifiers help to reduce the interfacial tension, thus, decreasing the energy required for the droplet disruption. Addi- tionally, prepared droplets are stabilized by the adsorption of emulsifiers to the freshly formed interface, concomitantly, pre- venting the droplet re-coalescence [21,22]. Commonly used emul- sifiers for the preparation of food-grade nanoemulsions include 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.12.017 ⇑ Corresponding author. Tel.: +86 510 85919106; fax: +86 510 85884496. E-mail address: xmzhang@jiangnan.edu.cn (X. Zhang). Ultrasonics Sonochemistry 21 (2014) 1265–1274 Contents lists available at ScienceDirect Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson small-molecule surfactants (e.g., Tweens, Spans), amphiphilic pro- teins (e.g., whey proteins), phospholipids (e.g., lecithins) and amphiphilic polysaccharides (e.g., modified starches, gums). De- spite the low cost and better efficiency of small-molecule surfac- tants, there has been increasing interest within the food industry in replacing the synthetic emulsifiers with natural alternatives so as to create products with consumer-friendly labels [23]. Conse- quently, trend of using food biopolymers (proteins, starches) for the preparation and stability of nanoemulsions is increasing [24– 26]. Although, comparatively lower concentrations of protein- based emulsifiers are needed, they are prone to denaturation and precipitation due to their sensitivity to higher processing temper- atures [24] and the pH fluctuations of medium, respectively. Octe- nyl succinic anhydride (OSA) modified starches are preferred due to their stability against high temperature and a wide range of pH and ionic strength [24,25]. For centuries, turmeric (Curcuma longa) has been extensively used as a spice, food preservative coloring material and ayurvedic medicine in India, China, Pakistan and South Eastern parts of Asia. Curcumin, the major bioactive compound of turmeric, is studied for its therapeutic effects and its potential as a functional food ingredient is recognized by several researchers [27]. Poor solubility of curcumin in aqueous media is the major issue which negatively affects the bioavailability and efficacy of this ingredient in the hu- man body. Nano-techniques, including nanoemulsions, could be a viable option to overcome these limitations [28–30]. As mentioned earlier, the production success of ultrasonic-as- sisted emulsions is dependent on the better understanding of pro- cess conditions. Purpose of the present work was to study the effects of major ultrasonic process-related parameters including ultrasonic power and sonication time, and formulation-related parameters including emulsifier and bioactive concentrations, oil volume fraction ( u ) on size, polydispersity index (PDI) and charge of the droplet. Furthermore, the optimum ranges for variables in- volved in the preparation of curcumin-loaded O/W nanoemulsions are determined. Overall goal was the preparation of food-grade curcumin-loaded nanoemulsions stabilized by OSA-starch using high-intensity ultrasonic homogenization. 2. Materials and methods 2.1. Materials Curcumin (77.90% pure, with 16.11% of demethoxycurcumin and 1.85% of bisdemethoxycurcumin) was obtained from Nanjing Zelang Medical Technology Co., Ltd. (Nanjing, Jiangsu, China) and used without further purification. MCT oil with a required HLB va- lue of $11.0 (Composition: C 8 : 57%, C 10 : 40%, C 6 : 2% and C 12 : <1%) was a product of Lonza Inc. (Allendale, NJ, USA), supplied by DIC Fine Chemical Co., Ltd. (Syn Tec Additive Ltd.), Shanghai, China. The Octenyl succinic anhydride (OSA) modified starches were gifted from National Starch and Chemicals (Shanghai, China). Two conventionally used OSA-starches, i.e., Purity Gum 2000 (PG) and Hi-Cap 100 (HC) are derived from waxy maize while Pur- ity Gum Ultra (PGU) is a newly developed OSA-starch produced using the new method with no further technical details. Doubly distilled water was used for all nanoemulsion preparations and analysis. 2.2. Methods 2.2.1. Oil phase preparation Oil phase was prepared by dispersing curcumin crystals in the heated MCT oil under continuous stirring as described by Ahmad et al. [29] and Wang et al. [30]. Briefly, MCT oil was heated and magnetically stirred at 100 °C for 5 min followed by the addition of curcumin crystals into oil and further stirred for 2 min. Heated MCT oils (curcumin-loaded) were allowed to cool down at room temperature. There was a possibility that some of the dissolved curcumin molecules might crystallize due to over-saturation when the MCT was cooled down below the solubilizing temperature [31]. In order to remove such undissolved entities, saturated oil was stored for 24 h followed by centrifugation at 14,000g,15°C for 5 min using a centrifuge (Himac CF16RXII Series, Hitachi, Rotor ra- dius: 4.4 cm). The collected supernatant (oil) was analyzed by spectrophotometer at 420 nm (Model UV-1600, Mapada Corpora- tion, P.R. China) for curcumin solubility. Wavelength of maximum absorbance (k max ) was found to be 420 nm, determined by 2802- UV/VIS spectrophotometer. Concentration of soluble curcumin was estimated by a standard calibration curve plotted from series of standard concentrations of curcumin dissolved in the MCT oil. Calculated concentrations were expressed as mg/mL of MCT oil. Oil phase (MCT) used in nanoemulsions preparation was either blank or enriched with curcumin. 2.2.2. Curcumin loading percentage in MCT oil Different concentrations (1, 3, 6, 10 and 15 mg/mL) of curcumin were dissolved in oil and their loading percentage was calculated according to the formula: Loading percentage ¼ Amount of curcumin dissolved Total amount of curcumin added  100 ð1Þ Amount of curcumin dissolved (mg/mL MCT oil) was estimated as described previously. 2.2.3. Aqueous phase preparation Aqueous phases were prepared by dissolving varying concen- trations % (w/v) of different OSA-modified starches, i.e., PG, HC and PGU, into doubly distilled water at 50 °C. Emulsifier solution turned into clear/translucent under continuous stirring for 30 min, indicating the complete dissolution/dispersion. Appear- ance of (5% w/v) aqueous solutions of PG and HC were almost clear while 1.5% (w/v) PGU solution was found slightly turbid. Hydro- philic lipophilic balance (HLB) value of commonly used OSA-mod- ified starches is 10–13. Hydrophilic emulsifiers may cause turbidity in the aqueous media with the decrease in their HLB va- lue (onset of lipophilic character); our observations indicated that the HLB value of PGU was slightly lower than that of other two starches. 2.2.4. Critical micelle concentrations (CMC), interfacial tension Surface tension of modified starches was measured to study the micelle formation in the aqueous solution at varying starch concentrations (0.0025–0.1 g/100 mL). CMC of starches were calculated through surface tension values. Interfacial tensions of OSA-modified starches at different concentrations in aqueous solution were determined against MCT oil. Results were determined by Wilhelmy Plate method on a digital DataPhysics Ò Tensiometer (Model: DCAT21, Germany) at 20 °C. The system temperature was maintained by circulating refrigerated/heating water bath (Julabo, Germany). 2.2.5. Viscosity determination A digital rotational viscometer (Brookfield, Model DV-II + PRO, Brookfield Engineering Laboratories Inc., MA, USA) was used to measure the apparent viscosities of aqueous and dispersed phases at 25 and 45 °C. Measurements were performed using spindle V- 6.5 LV with the speed adjusted at 100 rpm. Viscosity values ( g ) were reported in m Pa s and all measurements were made in 1266 S. Abbas et al. / Ultrasonics Sonochemistry 21 (2014) 1265–1274 duplicate, and average results were given. Disperse-to-continuous phase viscosity ratios ( g d / g c ) of PG, HC and PGU at varying emulsi- fier concentrations were calculated to determine the compatibility of two phases for emulsion preparation. Additionally, g d / g c values of all three starches were calculated at 25 and 45 °C and compared to assess whether g d / g c value was affected by the ultrasonic homogenizing temperature (45 °C). 2.2.6. Coarse emulsion preparation Coarse emulsion (O/W) was prepared by homogenizing the oil and aqueous phases using a high speed blender (Ultra-Turrax T25 IKA Works Inc., Wilmington, NC, USA) at 14,000 rpm for 2 min at room temperature. Total volume of each coarse-emulsi- fied sample was 100 mL. 2.2.7. Nanoemulsion preparation Coarse emulsion samples, 50 mL each, were subjected to high- intensity sonication at the operating frequency of 20 kHz using 1200 W ultrasonic processor (JY98-IIIDN, 20 kHz, volume process- ing capacity: 50–1000 mL, Ningbo Scientz Biotechnology Co., Ning- bo, China) equipped with 20 mm diameter probe. Temperature variations in the sample during sonication were monitored with a digital thermometer attached to a thermocouple. Applied power ranged of 10%, 20%, 30%, 40%, 50%, 60% and 70% (120, 240, 360, 480, 600, 720 and 840 W, respectively) of the maximal equipment power (1200 W) while sonication time varied from 1 to 13 min. Work time and the rest time for sonication were set at 5s and 7s, respectively, in order to avoid the overheating. Cold water circulat- ing through the containers jacket helped to maintain the samples temperature at 40–45 °C. The absolute power dissipated into the sample at a certain ap- plied power was estimated according to the formula presented by Tiwari et al. [32]. Briefly, energy dissipated, in terms of power (P), was determined through the calorimetric method by following relation: P ¼ mCp D T D t t¼0 ð2Þ Here, ‘‘m’’ is the mass (kg), Cp is the specific heat (kJ/kg/°C) of coarse emulsion and D T/ D t is the change in temperature over time (°C/s), of the sample. Energy dissipated into the sample is prefera- bly expressed in the terms of energy density or power density [33,34]. Power density (W/mL) for different applied powers (% of maximal power) was calculated by dividing the absolute power dissipated (P), determined in the Eq. (2), with the total volume (V) of the sample to be sonicated (mL), as shown in Eq. (3): Power density ¼ P=V: ð3Þ 2.2.8. Mean droplet diameter (MDD) and polydispersity index (PDI) MDD and PDI of the emulsion droplets were determined in trip- licate, using Zetasizer Nano ZS Ò (Malvern Instruments, UK) equipped with dynamic light scattering (DLS) technology. Samples were diluted (1:200) prior to the size measurement studies in order to ensure the free Brownian motion of the droplets. Samples were equilibrated at 25 °C for 1 min. The PDI was a dimensionless measure of the width of size distribution calculated from the Cumulant analysis of each sample’s correlation function. 2.2.9. f-potential The surface charge of the nanoemulsion droplets was deter- mined by measuring the electrophoretic mobility at 25 °C. Samples were diluted 200-fold in water before measurement and values of f-potential were expressed in mV. 2.2.10. Measurement of foaming The foam formation during homogenization is a well docu- mented issue related to the application of modified starches as emulsifiers. Foam formation during coarse emulsification (for 2 min) was calculated for 5% (w/v) PG and HC, and 1.5% (w/v) of PGU. Sample volume before coarse homogenization and 5 min after homogenization was recorded using a graduated cylinder. Foaming extent ( e ) in the coarse emulsion was calculated by using the following relation: e ¼ h L ð4Þ where, h = volume before coarse homogenization (mL) and L = vol- ume after coarse homogenization (mL). Foaming index values were ranged between 0 and 1; e values of coarse emulsions closer to 1 represented the least foaming, and vice versa. 2.2.11. Microscopy of emulsions Confocal laser scanning microscopy (CLSM) was employed for the comparative study of conventional emulsion and ultrasound- assisted nanoemulsion prepared under optimized conditions. Briefly, 2 l L of Nile Red fluorescent dye was added into 200 l Lof emulsion samples and mixed by gently shaking the mixture for 2 min in order to evenly disperse the dye, and to stain the oil drop- lets. About 5 l L of the stained samples of emulsions were placed on the slide, and coverslip was applied. Samples were analyzed with a Zeiss LSM 710 confocal microscope (Leica, Heidelberg, Ger- many) at the magnification of 40 and 63Â. Nile red dye was ex- cited with the 543 nm continuous-wave argon ion laser (Ar-ML Laser). The images were obtained via LSM 710 ZEN software. 2.2.12. Experimental parameters and statistical analysis The primary parameters which may affect the droplet size, size distribution and surface charge, i.e., sonication time and power density, emulsifier type and concentration, bioactive concentration (curcumin) in oil and oil volume fraction ( u ) were investigated at different levels (see Table 1). Data were analyzed and given as mean ± standard deviation. One-way ANOVA was used to compare results while <0.05 of the p values were considered statistically sig- nificant. All statistical analysis was performed using SPSS, version 19 (SPSS Inc., Chicago, USA). 3. Results and discussion 3.1. The effect of power density Mixing of emulsion components and the breakdown of larger oil drops into nanosize droplets is governed by the extent of dis- ruptive forces or energy delivered to the liquid sample. As final size and distribution of the nanoemulsions droplets are influenced by the coarse emulsion preparation [35], in the first step of this study, coarse emulsion was prepared prior to the sonication in order to increase the efficiency of the process (Table 1). In the second step, coarse emulsion was subjected to sonication. High-intensity ultra- sound produces shear forces which are required for the disruption of droplets. Additionally, successful stabilization of newly formed droplets is governed by the type and concentration of surfactant applied to droplets. Surfactants tend to decrease the interfacial tension as well as retard the rate of droplet coalescence, thereby offering the stability to droplets. It is very critical to determine the optimum power for ultrasound-assisted industrial scale pro- cesses in order to minimize the energy loss and production cost [33]. For applied powers of 10%, 20%, 30%, 40%, 50%, 60% and 70% of the maximum power, their corresponding power densities for the S. Abbas et al. / Ultrasonics Sonochemistry 21 (2014) 1265–1274 1267 MDD of PG, HC and PGU-stabilized emulsions were determined (see Table 2). Blank, MCT-based emulsion (0.05 oil volume frac- tion) was prepared by treating the sample for 2 min. As shown in Fig. 1, an increase in the applied power from 10% to 40% (power density increased from 0.82 to 1.36 W/mL for PG, 0.84 to 1.40 W/ mL for HC and 0.87 to 1.45 W/mL for PGU) of maximum power re- sulted in the significant decrease (p 6 0.05) of MDD at a fixed time for PG, HC and PGU. Above 40% applied power, the decrease in the MDD for all starch-stabilized emulsions was insignificant. Though, increase in the applied power above 40% slightly decreased the MDD of PG-stabilized emulsions, it was unnecessary and uneco- nomical to further increase the power as it may consume extra en- ergy. Similar strategy (use of minimum power) was suggested by Hielscher [36] to obtain the required outcome. As power density was needed to be set at an appropriate level to achieve the droplets of desired diameter, therefore, applied power was fixed at 40% of the maximum power for further experiments. 3.2. The effect of sonication time In the next set of experiments, blank emulsions were prepared by varying sonication times, as given in Table 1, at fixed applied power of 40% or power density (1.36, 1.40 and 1.45 W/mL for PG, HC and PGU, respectively). Other parameters, including, MCT vol- ume fraction and surfactant concentration were unchanged. Effect Table 1 Process parameters studied and their levels. Experimental parameter Levels (1) Power applied 10%, 20%, 30%, 40%, 50%, 60% and 70% of maximum power (1200 W) (2) Sonication time 1, 3, 5, 7, 9, 11 and 13 min (3) Emulsifier type & concentration A. Purity Gum 2000, Hi-Cap 100 1, 3, 5, 7, 9, 11 and 13 min B. Purity Gum Ultra 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 4% and 5% (w/v) aqueous solution (4) Curcumin concentration 1.5, 3, 5, 10 and 15 mg/mL of MCT oil (5) MCT volume fraction ( u ) 0.02, 0.05, 0.08, 0.11 and 0.14 10 20 30 40 50 60 70 160 180 200 220 240 MDD (nm) Applied power (% of maximum power) 5% PG 5% HC 1% PGU Fig. 1. MDD at different applied powers for 2 min sonication time. Composition: Blank O/W nanoemulsion (50 mL sample), oil u : 0.05, Modified starches: 1–5% (w/ v) as emulsifier. Aqueous phase viscosity of 5% (w/v) PG, 5% (w/v) HC and 1% (w/v) PGU solutions at 25 °C were 2.19, 1.38 and 2.37 m Pa s, respectively. 03691215 120 140 160 180 200 220 240 260 MDD (nm) Ultrasonic treatment time (min) 5% PG 5% HC 1% PGU Fig. 2. MDD at different sonication times at fixed power density of 1.36, 1.40 and 1.45 W/mL for PG, HC and PGU, respectively. Composition: blank O/W nanoemul- sion (50 mL sample), oil u : 0.05, Modified starches: 1–5% (w/v) as emulsifier. Aqueous phase viscosity of 5% (w/v) PG, 5% (w/v) HC and 1% (w/v) PGU solution at 25 °C were 2.19, 1.38 and 2.37 m Pa s, respectively. 0.00 0.02 0.04 0.06 0.08 0.10 40 45 50 55 60 65 70 75 (mN/m) c (g/100 cm 3 ) PG HC PGU γ 0.0 0.5 1.0 1.5 2.0 2.5 3.0 8 10 12 14 16 18 Interfacial Tension (mN/m) Emulsifier concentration in aqueous phase (w/v %) PG HC PGU (a) (b) Fig. 3. Determination of the (a) CMC values for OSA starches, (b) interfacial tension of OSA starches against MCT. 1268 S. Abbas et al. / Ultrasonics Sonochemistry 21 (2014) 1265–1274 of different sonication times, that is, 1, 3, 5, 7, 9, 11 and 13 min on MDD is shown in Fig. 2. There was a significant decrease in the MDD (p 6 0.05) with an increased in sonication time from 1 to 7 min for all three surfactants. Similar trend was noticed by Ken- tish et al. [18] for 15 vol.% flax seed O/W emulsions prepared at 200 W power. Total energy input ‘‘E’’ (expressed in joules or kilojo- ules) delivered to the sample depends on the input power (P) and the total sonication time (t), as E = P  t. For different sonication times, i.e., 1, 3, 5, 7, 9, 11 and 13 min at a fixed power density, their corresponding energy input values were determined as 4.09, 12.27, 20.45, 28.63, 36.81, 44.99 and 53.17 kJ, respectively, for PG, 4.20, 12.6, 21.0, 29.40, 37.80, 46.20 and 54.60 kJ, respectively, for HC, and 4.37, 13.10, 21.84, 30.57, 39.31, 48.04 and 56.78 kJ, respec- tively, for PGU. Our results indicated that decrease of MDD with the increase of total energy dissipated into the system was the function of time at a constant power density. Sonication time of 7 min was found optimum as further increase in time had little ef- fect on the MDD reduction. Additional sonication time had a little impact on the MDD reduction of PG, HC and PGU-stabilized emul- sions. Besides, application of prolonged ultrasonic treatment is dis- couraged as it may deteriorate the bioactive compounds present in the formulation. Consequently, sonication time of 7 min was se- lected for further experiments. 3.3. CMC, interfacial tension of starches The interfacial properties of emulsifiers are also found to be critical in the preparation and stability of emulsions [37]. The CMC values of PG, HC and PGU were 0.038, 0.05 and 0.0025 g/ 100 mL, respectively, as shown in Fig. 3a. Results for PG and HC were comparable to the results presented in the previous study, by Wang et al. [38]. Interestingly, CMC value of PGU was much smaller compared to that of other starches. Interfacial tensions of OSA-modified starches at different con- centrations were measured against MCT oil phase, as shown in Fig. 3b. All three emulsifiers tended to decrease the interfacial ten- sion when their concentration increased; indicating that the emul- sifiers were adsorbed to the oil–water interface. Although, decrease in the interfacial tension was almost similar at lower con- centrations (61% w/v) for all three starches, PGU provided compar- atively lower interfacial tension than PG and HC at higher concentrations. At 2.5% concentration, interfacial tension for PGU decreased to 8.713 ± 0.009 mN/m, suggesting its emulsifying po- tential for emulsion preparations. 3.4. The effect of emulsifier concentration 3.4.1. Viscosity Viscosity of aqueous solutions of OSA-modified starches is di- rectly related to their dissolved concentration. Additionally, ratio 0 2 4 6 8 10121416 0 2 4 6 8 10 12 14 16 18 20 22 24 η d / η c Emulsifier concentration in aqueous phase (w/v %) PG - 25°C PG - 45°C HC - 25°C HC - 45°C PGU - 25°C PGU - 45°C 0246810 140 160 180 200 220 240 260 MDD (nm) Emulsifier concentration in aqueous phase (w/v %) PG HC PGU (a) (b) Fig. 4. Effect of emulsifier concentration on (a) g d / g c , i.e., viscosity ratio of dispersed to continuous phases, (b) MDD. Composition: blank O/W nanoemulsion (50 mL sample); oil u : 0.05, prepared at 40% of applied power and 7 min sonication time. (a) (b) 5% PG 5% HC 1.5% PGU 0 20 40 60 80 100 120 140 160 180 200 Blank PDI Loaded PDI MDD (nm) Emulsifier Type 0.05 0.10 0.15 0.20 0.25 0.30 PDI 5% PG 5% HC 1.5% PGU -50 -40 -30 -20 -10 0 10 Z-Potential (mV) Emulsifier Type Blank Loaded Fig. 5. Effect of emulsifier type on (a) MDD and PDI (b) f-potential of curcumin- loaded and blank nanoemulsions, prepared at power density of 1.36, 1.40 and 1.36 W/mL for PG, HC and PGU, respectively, and 7 min sonication time. Compo- sition: 5% PG, 5% HC and 1.5% w/v PGU, oil u : 0.05, curcumin concentration: 10 mg/ mL MCT, and water. S. Abbas et al. / Ultrasonics Sonochemistry 21 (2014) 1265–1274 1269 of dispersed ( g d ) to continuous phase viscosity ( g c ), i.e., g d / g c of the system is very important for the stability of emulsions [21,39]. For g d / g c , 0.5–5 is considered an optimal range for the efficient breakdown of droplets in the turbulent shear conditions [40]. In our case, viscosity ratio ( g d / g c ) of starch solutions was determined at 25 and 45 °C, as shown in Fig. 4a. MCT oil was used as a dispersed phase while continuous phase was consisted of OSA-modified starch solutions of varying concentrations (1– 15% w/v). As optimal range value of g d / g c can be achieved by fine tuning of the phase viscosities, either by increasing g c or decreasing the g d , decrease in g d / g c for PGU was found to be lowest due to the increased g c value, as g d value (for MCT oil) was kept constant throughout the study. Results indicated that g d / g c for PGU was within optimal range, i.e., 4.66, 2.54 and 0.84 for 3%, 5% and 10% concentration, respectively. On the other hand, optimal range of g d / g c for PG and HC were achieved at fairly high concentrations (at P15%). Finally, it was noted that the g d / g c value for PGU was almost unaffected when the temperature was increased from 25 to 45 °C. On the other hand, g d / g c value of PG increased at 45 °C for all concentrations, though, this increment was much clearer at lower concentrations. In the case of HC, no clear trend was found between g d / g c and the temperature increase. 3.4.2. MDD, PDI and f-potential Different concentrations of three OSA starch emulsifiers were used for the preparation of blank MCT (0.05 oil volume fraction) nanoemulsions under standardized conditions of an applied power (40%) and sonication time (7 min). For all three emulsifiers, increasing their concentration resulted in significant decrease (p 6 0.05) in MDD during the first phase, as shown in Fig. 4b. This could be due the fact that larger surface area of oil droplets can be covered when sufficient concentration of emulsifier is available during homogenization, thus, providing stability to newly-formed droplets [22]. A 5% (w/v) concentration of PG and HC was found to be the most suitable to produce smaller droplets (MDD $ 148 nm). Surprisingly, a small amount (1.5% w/v) of PGU at 1.36 W/mL power density was found enough to get smaller droplets (MDD $ 141 nm). This could be due to number of factors including the speed at which emulsifier adsorbed to the oil–water interface and their ability to reduce the interfacial tension [22], thus, consolidating our results presented in the previous sections. It is well established that stabilizers influence the PDI of an emulsion. In the first phase, an increase of emulsifier concentration resulted in the decrease of droplet PDI for all OSA starches. In the second phase, increase of PG and HC concentration up to 5% had al- most no effect on the PDI due to the fact that droplet surfaces were MDD ( nm ) 14 0 15 0 16 0 17 0 18 0 19 0 20 0 21 0 22 0 () 0 0 0 0 0 0 0 0 0 0 5% 5 % 1.5 1 S PG % H C % P Stor C P G U torage U 2 ge tim 2 ime (w (weekeks) 3 4 (a) (b) Fig. 6. (a) Effect of storage at 25 °C on MDD, (b) comparative foaming phenomena of PG, HC and PGU for coarse homogenization. Parameters are similar to that indicated in the Fig. 5. 1270 S. Abbas et al. / Ultrasonics Sonochemistry 21 (2014) 1265–1274 saturated. In the final phase, further increase of concentration re- sulted in the sudden increase in PDI. This behavior could be due to the insufficient energy input at increased continuous phase vis- cosity, although, emulsifier was present in excess [22]. Further- more, such conditions may lead to aggregation of emulsifier molecules instead of their uniform distribution on the oil–water interface [26]. Narrowest distribution (0.132) was found for 1.5% w/v concentration of PGU. f-potential is considered an important parameter to assess the emulsions stability as it governs the degree of repulsion between similarly charged dispersed droplets. f-potentials of droplets for all emulsifiers were negative which can be related to the presence of negatively charged (carboxylic) groups on the modified starch molecules [41]. The f-potential ranged from À29 to À30 mV and À26 to À30 mV for PG and HC, respectively. Surprisingly, negative f-potential for PGU, i.e., À42 to À43 mV was much higher as Table 2 Energy dissipated and power input (power density) supplied to samples. Sample Applied power (% of 1200 W) a Sonication time (min) Energy dissipated (kJ) Input power (W) Power density (W/mL) PG 5% HC 5% PGU 1% PG 5% HC 5% PGU 1% PG 5% HC 5% PGU 1% 1 10 2 4.91 5.04 5.24 40.92 42.0 43.67 0.82 0.84 0.87 2 20 2 6.54 6.72 6.99 54.2 56.0 58.25 1.09 1.12 1.16 3 30 2 7.36 7.14 7.42 61.33 59.5 61.83 1.23 1.19 1.24 4 40 2 8.18 8.40 8.73 68.17 70.0 72.75 1.36 1.40 1.45 5 50 2 9.81 9.24 10.04 81.75 77.0 83.67 1.64 1.54 1.67 6 60 2 11.86 10.93 11.35 98.83 91.08 94.58 1.98 1.82 1.89 7 70 2 12.27 12.26 12.67 102.25 102.17 105.58 2.04 2.04 2.11 8 40 7 – – 28.55 b – – 67.98 – – 1.36 a Maximum equipment power: 1200 W. b PGU concentration: 1.5% (w/v). 03691215 100 110 120 130 140 150 160 170 180 MDD (nm) PDI ZP (mV) Curcumin concentration added (mg/ml) 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 MDD (nm) ZP PDI -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 0.02 0.04 0.06 0.08 0.10 0.12 0.14 100 110 120 130 140 150 160 170 180 MDD (nm) PDI ZP (mV) Oil volume fraction 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 MDD (nm) ZP PDI -40 -30 -20 -10 0 10 (a) (b) Fig. 7. MDD, PDI and f-potential of PGU(1.5% w/v)-stabilized nanoemulsion droplets, prepared at power density of 1.36 W/mL and 7 min sonication time, as affected by (a) curcumin load at 0.05 oil u , (b) varying oil u at 6 mg/mL curcumin load. S. Abbas et al. / Ultrasonics Sonochemistry 21 (2014) 1265–1274 1271 compared to values reported in the previous studies [42,43]. Our results suggested that PGU offered better stability to MCT-based blank O/W emulsions, and droplet charge was almost unaffected by the variation in the emulsifiers concentration. 3.5. Effect of emulsifier-type on MDD, PDI and ZP of loaded nanoemulsions Effect of emulsifiers-type on the particle size and PDI of curcu- min-loaded nanoemulsions is shown in Fig. 5a. In the first step, the loading capacity of the MCT oil for curcumin was determined by dissolving series of concentrations. From our preliminary work (re- sults not shown), it was noted that the maximum curcumin load- ing without significantly increasing the MDD and PDI was 10 mg/ mL of added curcumin (oil loading capacity: 93.1 ± 0.07%). There- fore, the actual concentration of curcumin in the oil used in this set of experiments was 9.3 mg/mL. Results revealed that MDD of blank OSA-stabilized emulsions were found almost similar for all three emulsifiers while PDI for PGU-stabilized emulsion (0.121) was comparatively better than that of conventional OSA starches, thus, indicating the potential of PGU as an emulsion stabilizer. In the case of loaded emulsions prepared from two conventional emulsifiers, MDD and PDI were similar while PGU performed bet- ter with lowest MDD (146.0 ± 1.56 nm) and PDI (0.15) at 3.3 times lesser concentration than that of conventional emulsifiers. Addi- tionally, higher negative charge (À39.4 mV) was found on the PGU-stabilized curcumin-loaded nano-droplets as compared to PG (À26.15 mV) or HC (À29 mV) stabilized nanoemulsions (Fig. 5b). 3.6. Size stability of loaded nanoemulsions and foam formation of coarse emulsions One month storage at 25 °C for all three types of OSA-starch- stabilized nanoemulsions showed that PGU performed better in terms of MDD (164.1 nm) as compared to conventional OSA starches (182 and 216 nm for HC and PG, respectively) (Fig. 6a). Although, lipid drops of nanoemulsions were still stable, dark col- ored sediments were found at the bottom of all three samples. Sim- ilar results were found for nanoemulsion-based delivery systems of polymethoxyflavone, which is poorly water-soluble bioactive compound [44,45]. In our case, possible reason for sedimentation over prolong storage could be the nucleation and formation of cur- cumin crystals under the influence of supersaturation as high tem- perature (100 °C) was used to dissolve the curcumin into oil. Furthermore, coarse emulsification process for PG and HC was challenging due to foaming issue (Fig. 6b). Results showed that among all three modified starches, PGU-stabilized emulsions were least affected by the foaming problem ( e = 0.95) followed by HC ( e = 0.80) and PG ( e = 0.76). Due to these favorable characteristics, PGU was selected as an emulsifier/stabilizer in the further studies. 3.7. Effect of curcumin load and u on the MDD, PDI and f-potential PGU-stabilized emulsions were prepared from curcumin-loaded MCT oil with different concentrations (see Table 1) of curcumin at standardized conditions (power density: 1.36 W/mL), sonication time: 7 min, 1.5% (w/v) PGU) to study the effect of bioactive con- centration on the MDD, PDI and f-potential of the emulsion, as Fig. 8. Confocal laser scanning microscopy (CLSM) of conventional emulsions and ultrasound-assisted nanoemulsion. 1272 S. Abbas et al. / Ultrasonics Sonochemistry 21 (2014) 1265–1274 shown in Fig. 7a. Volume fraction of oil was kept constant at 0.05 for all formulations. Based on the results (Fig. 7a), MDD (143– 148.6 nm), PDI (0.13–0.16) and f-potential (À39.4 to À42 mV) were almost unaffected at all curcumin concentrations. However, sediments were observed at the container bottom at higher curcu- min concentrations. Sedimentation occurred within hours to few days depending upon the concentration of dispersed/dissolved cur- cumin. It was observed that curcumin concentration of 66 mg/mL MCT was successfully dissolved and incorporated into the nano- emulsion without the sediment growth during 1 month storage period. Consequently, a nanoemulsion formulation consisting low- er curcumin concentration, that is, 5.6 ± 0.213 mg/mL MCT (6 mg/ mL MCT of added curcumin) was preferred for further studies. To study the effect of oil volume fraction ( u ) on MDD, PDI and f- potential of the emulsion droplets, emulsions were prepared using five different oil levels (Table 1). MCT used in this set of experi- ments was loaded with about 5.6 ± 0.213 mg/mL MCT of curcumin. Increase of u above 0.05 resulted in gradual increase of MDD (Fig. 7b). Similar results were obtained by Guo and Mu [46] in the preparation of corn oil–water emulsions stabilized by sweet potato proteins. This could be, either due to the decreased coverage of OSA-modified starch at higher u , or increased viscosity of emul- sion [47], as it requires higher energy input for droplet disruption, thus, triggering droplet aggregation and coalescence. As u in- creased from 0.05 to 0.11, MDD increased from 145.4 ± 0.85 to 178.0 ± 1.06 nm. Furthermore, minimum PDI value (0.11 ± 0.02) was recorded at 0.02 u while negative f-potential was ranged from À39.0 to À35.15 mV when u increased from 0.02 to 0.14. 3.7.1. Microscopic images of conventional emulsions and nanoemulsions The Nile Red fluorescent probe dyed oil droplets of PGU-stabi- lized emulsions were observed by confocal laser scanning micros- copy (CLSM), as shown in Fig. 8. The oil droplets present in the coarse emulsion were micron size with a spherical shape and poly-dispersed size distribution. Under the influence of high-inten- sity ultrasonic homogenization, curcumin-enriched oil droplets of micron size went through the process of size (MDD) reduction due to cavitational forces. Though, it was challenging to observe the structure and further details of nanosize emulsion droplets by using the CLSM, this technique was used for the comparative imagery of micro and nanosize emulsions. Cryogenic Transmission Electron Microscopy (Cryo-TEM) could be a better choice for the detailed study of starch-stabilized nanoemulsions [48]. 4. Conclusions Ultrasound-assisted nanoemulsions were prepared and stabi- lized successfully by OSA-starches. Although, nanoemulsions were produced at all levels, optimum process and formulation parame- ters values were identified for the preparation of emulsion with smallest size droplets at lowest possible delivered power and son- ication time. Furthermore, minimum emulsifier concentrations and maximum loading % of curcumin in MCT were found for the formation of stable emulsions. It was noted that 40% of applied power (power density: 1.45 W/mL) and 7 min sonication time was optimum. Among three OSA-starch based emulsifiers used, PGU performed the best at 1.5% (w/v) concentration (power density: 1.36 W/mL) with smallest droplet diameter (140.25 ± 2.77 nm) while higher concentrations of PG and HC were needed to achieve optimal results. PDI was below 0.2 for all PGU concentrations while negative charge on PGU-stabilized blank emulsion droplets was higher in all samples (À41.7 to À43.0) than that of PG and HC-stabilized droplets. PGU-stabilized emulsions stored for 4 weeks at room temperature were found to be the most stable (MDD increased from 146.0 to 164.1 ± 0.85 nm) with nar- rowest size distribution (0.09 ± 0.01). Curcumin loading increase had no effect on MDD, PDI and charge. When higher concentrations of curcumin, that is, more than 6 mg/mL dissolved/dispersed in MCT, sediments appeared within few hours to few days of prepa- ration, when stored at room temperature. Curcumin concentration of 6 mg/mL (actual concentration: 5.6 ± 0.213) and 0.05 volume fraction of curcumin-loaded MCT oil was found optimum for the preparation of 1.5% (w/v) PGU-stabilized emulsion of smallest MDD, i.e., 145.4 ± 0.85 nm, having 0.15 PDI and 39.4 ± 1.84 mV f- potential. CLSM images confirmed that the ultrasonic homogeniza- tion successfully broke down the micro size oil droplets into nano- size. Optimized nanoemulsion could be used as a template for the fabrication of multilayered food-grade nanoemulsions or nanoparticles. Acknowledgment This study was supported by the National Key Technology R&D Program of China (2011BAD23B04) and (2013AA102204). References [1] D.J. McClements, Emulsion design to improve the delivery of functional lipophilic components, Annu. Rev. Food Sci. 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Technol. 1 (2000) 127–134 . [48] C. Preetz, A. Rübe, I. Reiche, G. Hause, K. Mäder, Preparation and characterization of biocompatible oil-loaded polyelectrolyte nanocapsules, Nanomed-Nanotechnol. 4 (2008) 106–114 . 1274 S. Abbas et al. / Ultrasonics Sonochemistry 21 (2014) 1265–1274 . Short Communication Process optimization of ultrasound-assisted curcumin nanoemulsions stabilized by OSA-modified starch Shabbar Abbas a , Mohanad Bashari a , Waseem. homogenization Nanoemulsion Modified starch Curcumin Droplet diameter abstract This study reports on the process optimization of ultrasound-assisted, food-grade oil–water nanoemul- sions stabilized by modified starches the detailed study of starch -stabilized nanoemulsions [48]. 4. Conclusions Ultrasound-assisted nanoemulsions were prepared and stabi- lized successfully by OSA-starches. Although, nanoemulsions were produced