Short CommunicationProcess optimization of ultrasound-assisted curcumin nanoemulsions stabilized by OSA-modified starch
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], [2], [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 carotenoids, 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 solubility 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 improved 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], [13], [14], [15]. In recent years, ultrasound-assisted emulsification process has gained popularity among food processors for the production 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 involved 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 dispersity of nanoemulsion droplets are influenced by a number of process and formulation variables [15], [18], [19], [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. Additionally, prepared droplets are stabilized by the adsorption of emulsifiers to the freshly formed interface, concomitantly, preventing the droplet re-coalescence [21], [22]. Commonly used emulsifiers for the preparation of food-grade nanoemulsions include small-molecule surfactants (e.g., Tweens, Spans), amphiphilic proteins (e.g., whey proteins), phospholipids (e.g., lecithins) and amphiphilic polysaccharides (e.g., modified starches, gums). Despite the low cost and better efficiency of small-molecule surfactants, 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]. Consequently, trend of using food biopolymers (proteins, starches) for the preparation and stability of nanoemulsions is increasing [24], [25], [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 temperatures [24] and the pH fluctuations of medium, respectively. Octenyl 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 human body. Nano-techniques, including nanoemulsions, could be a viable option to overcome these limitations [28], [29], [30].
As mentioned earlier, the production success of ultrasonic-assisted emulsions is dependent on the better understanding of process 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 (φ) on size, polydispersity index (PDI) and charge of the droplet. Furthermore, the optimum ranges for variables involved 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.
Section snippets
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 value of ∼11.0 (Composition: C8: 57%, C10: 40%, C6: 2% and C12: <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
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 disruptive 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
Conclusions
Ultrasound-assisted nanoemulsions were prepared and stabilized successfully by OSA-starches. Although, nanoemulsions were produced at all levels, optimum process and formulation parameters values were identified for the preparation of emulsion with smallest size droplets at lowest possible delivered power and sonication 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
Acknowledgment
This study was supported by the National Key Technology R&D Program of China (2011BAD23B04) and (2013AA102204).
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