Application of nozzleless electrostatic atomization to encapsulate soybean oil with solid substances
Introduction
Polyunsaturated fatty acids (PUFAs) are chain structures with several double bonds and components of certain liquid oils at ambient temperature (Timilsena et al., 2017). PUFAs are mainly contained in vegetable and fish oils, and we must ingest them for being unable to synthesize α-linolenic acid inside human bodies (Sapna et al., 2009, Dorni et al., 2018). PUFAs, such as omega-3 (ω-3) and omega-6 (ω-6) fatty acids, play an important role in health promotion. Recent studies have implicated PUFAs in the prevention of various systemic diseases and syndromes, including coronary heart diseases, hypertension, hypercholesterolemia, cancer (including colon, breast, and prostate cancer), inflammatory bowel diseases, diabetes, and neurodegenerative disorders (Ishido et al., 2003, Xu et al., 2013, Goyal et al., 2015, Encina et al., 2016, Azizi et al., 2018). However, PUFAs may be chemically or physically labile during food processing and storage as well as toward other food ingredients, suggesting that they are sensitive to heat, light, and oxygen because of their multiple double bonds (Albert et al., 2015). The oxidation of PUFAs results in malodors and the production of peroxides that is toxic to humans (Alamed et al., 2009). Therefore, it is indispensable to keep off the oxidation of PUFAs for preserving the quality of oils or for preventing the formation of toxic byproducts from oil (Tan et al., 2017).
In recent years, numerous techniques for oil encapsulation by powder technology have been developed (Encina et al., 2016, Tampau et al., 2017, Rasti et al., 2017, Li et al., 2017). It has been reported that a Pickering emulsion, in which solid particles instead of surfactant substances are adsorbed at the oil–water interface, forming a particle-stabilized emulsion (Lu et al., 2018). The encapsulation of oil in powder can protect it from the surrounding environment (e.g., heat, acidic or basic species, and light), thereby avoiding deterioration during storage (McClements, 2015, Wang et al., 2016). Zhu (2017) reported that functional ingredients can be encapsulated with various wall materials for controlled release in food and digestion systems. The spray-drying method (Paramita et al., 2010, Li et al., 2017, Sultana et al., 2017) and the spray freeze-drying method (Matsuno and Adachi, 1993, Hundre et al., 2015, Fioramonti et al., 2017) are frequently used for oil encapsulation. Especially, microencapsulation by using spray-drying is a technique in the food industry for its cost effectiveness (Ballesteros et al., 2017, Takashige et al., 2017). In the spray-drying process, this method may be suitable for the drying of heat-sensitive materials since the moisture content of the feed is removed quickly. This method comprises two stages of emulsification and drying (Matsuno and Adachi, 1993). First, an oil-in-water (O/W) emulsion is prepared to make dispersed oily components into wall materials such as sugars and starch with homogenizers or ultrasonication. After the obtained O/W emulsions are sprayed from a nozzle, the atomized droplets are dried inside the chamber, resulting in the encapsulation of the oil particles by the wall materials (McCarthy et al., 2016).
We developed different techniques to produce functional materials (Kadota et al., 2014). An electrostatic atomization method is expected to function as an oil-encapsulation technique with powder under the force of an electric field when used in conjunction with an aqueous solution containing a wall material in the oil phase (Kishimoto et al., 2017). In this electrostatic atomization, the application of a high voltage to the O/W interface creates a conical wave called the Taylor cone (Collins et al., 2008, Yu et al., 2016). Several droplets of the aqueous solution of wall materials are discharged from the tip of the Taylor cone, and water-in-oil (W/O) emulsions are formed. Particle formation on the interface of W/O emulsions has been carried out via a liquid–liquid interface crystallization method (Kadota et al., 2007, Deki et al., 2016, Kikuchi et al., 2017). The simultaneous preparation of emulsions and particles can be achieved in a single process via the electrostatic atomization, whereas the spray-drying method prepared the encapsulation in the two-step procedure of emulsification and drying without using nozzles. Several unresolved issues confront processes that involve a nozzle, such as the dependence of the particle size on the nozzle's diameter Use of nozzles confronts several unresolved issues like such as the dependence of the particle size on the nozzle's diameter (Ang et al., 2015) and the requirement for regular maintenance of the nozzle to avoid clogging (Sato et al., 2010).
In the present study, we propose a new powdering process of oil using nozzleless electrostatic atomization via a liquid–liquid interface. Focusing on the liquid–liquid interface crystallization method using mutual diffusion of water and oil, we devised a particle synthesis involving oil. Glycine and taurine were selected as model wall materials, because they were easily precipitated on the interface between oil and aqueous solution. In addition, the selected glycine and taurine have the amphiphilic properties and they easily absorb to the oil.
Section snippets
Materials
Glycine (98.5%), taurine (98.5%), potassium bromide (KBr) (99.0%), soybean oil, and ethanol (99.5%) were purchased from Nacalai Tesque Inc. (Kyoto, Japan). 1-Decanol was purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan). All aqueous solutions were used after filtration through a membrane filter with a 0.2 μm pore size (H020A047A; Advantec Inc., Tokyo, Japan).
Preparation of W/O emulsions using electrostatic atomization
A schematic of the experimental setup for electrostatic atomization in the liquid–liquid phase is shown in Fig. 1. The
Formation of Taylor cone for the electrostatic atomization
Fig. 2 shows the specific gravity of the oil phase with respect to the concentration of ethanol in the soybean oil. The specific gravity linearly decreased with increasing ethanol concentration. The linear change with ethanol vol. % indicates that is an ideal mixture without a volume change for mixing in the interaction between oil and ethanol. Fig. 3 shows the change in the interfacial tension of glycine-oil and taurine-oil solutions plotted against the concentration of ethanol in soybean oil.
Conclusions
In this study, encapsulation techniques were demonstrated for soybean oil using electrostatic atomization. The interfacial tension at the oil–water interface and the specific gravity of the oil phase were important factors in forming a Taylor cone. The addition of ethanol decreased the interfacial tension and the specific gravity. The decrease of interfacial tension was due to the activation of the interface by ethanol. Encapsulated soybean oil particles were prepared via nozzleless
Acknowledgments
This work was partially supported by the Japan Society for the Promotion of Science KAKENHI grant (No. 16K06837) (Tokyo, Japan) and by the Salt Science Research Foundation (No. 17A2) (Tokyo, Japan).
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