Mechanical characterization of fish oil microcapsules by a micromanipulation technique
Introduction
Fish oil is beneficial to human health as it contains large amounts of omega-3 polyunsaturated fatty acids (ω-3 PUFAs) (Bakry et al., 2016). The demand for fish oil as a dietary supplementary has been high in recent years because modern food technology has made it possible to fortify various foods on the market with fish oil, such as infant formula, yogurt, spaghetti, ice-cream, bread, sauces, and instant concentrates, and has made it indistinguishable from non-fortified foods (Kolanowski & Laufenberg, 2006). Fortification methods include the direct addition of bulk fish oil, emulsion, or microcapsules. Especially the addition of microcapsules can effectively improve the oxidative stability of fish oil and mask its unpleasant fishy flavor in fortified products (Jamshidi et al., 2020; Kolanowski et al., 2007; Wang et al., 2011). Fish oil-loaded microcapsules convert liquid oil into solid powder through microencapsulation processes, thereby extending its shelf life and facilitating the development of value-added products (Celli et al., 2015). The commonly used microencapsulation processes in industrial-scale production are spray drying and complex coacervation (Bakry et al., 2016). Spray drying is a cheap process and the equipment (spray dryer) required is easily available (Gharsallaoui et al., 2007). Complex coacervation is a simple, low-cost, and solvent-free physicochemical process involving liquid-liquid phase separation phenomenon that occurs when mixing oppositely charged polyelectrolyte solutions (Xiao et al., 2014). There are many studies on fish oil microcapsules produced by the above two methods, mainly focusing on their physicochemical and structural properties, such as particle size, morphology, flowability, oxidative stability, and thermal stability (Kagami et al., 2003; Polavarapu et al., 2011; Zhang et al., 2015). We aim to develop a final dosage form in tablet containing fish oil microcapsules, which can be produced by first mixing fish oil microcapsules with excipients and then compaction since tablet is the most widely used dosage form acceptable to consumers for pharmaceutical and nutraceutical products, e.g. drugs, probiotics and vitamins. For such application, it is crucial to maintain the mechanical integrity of the microcapsules during handling, transport, mixing and compaction. However, little is known about the mechanical properties of fish oil microcapsules or even food-grade microcapsules although there have been studies on the mechanical properties of microcapsules with a shell of synthetic polymers including melamine formaldehyde and urea formaldehyde (Hu et al., 2009; Sun & Zhang, 2002).
Generally, the mechanical properties of microcapsules can be determined by measuring their deformation under a mechanical load at both population and individual levels (Zhang et al., 2009). At the population level, a number of microcapsules are measured simultaneously and hence the mechanical properties obtained are average values across the batch. In contrast, measuring the mechanical properties of individual microcapsules can provide more detailed information on the variation among microcapsules, which is more advantageous for their formulation and process optimization (Gray et al., 2016). Although it is relatively difficult, a novel micromanipulation technique has been well established and demonstrated to be effective to characterize the mechanical behavior of individual microcapsules, and it is capable of applying a greater range of forces (micro Newtons to Newtons) to individual microcapsules than other techniques such as atomic force microscopy (Mercadé-Prieto & Zhang, 2012; Sun & Zhang, 2001; Zhang et al., 1991).
In this study, three different encapsulation methods of spray drying, complex coacervation, and double encapsulation (coacervation followed by spray coating) were used to prepare five kinds of fish oil microcapsules and their mechanical properties were evaluated using the micromanipulation technique. Combinations of gelatin and gum Arabic or maltodextrin were chosen as wall (shell) materials because they are commonly and commercially used for the industrial production of food-grade microcapsules. Based on this, the purpose of this study was to understand the mechanical properties of fish oil microcapsules and to find out the effects of encapsulation method and wall material combination on their mechanical properties. Therefore, both fundamental physical properties (including morphology, structure, and particle size and distribution) and mechanical properties (including rupture force, nominal rupture stress and strain, and Young's modulus (Gray et al., 2016)), encapsulation efficiency, and loading capacity of microcapsules were investigated. These experimental results not only contribute to the better understanding of oil-loaded food-grade microcapsules from the perspective of mechanical properties but also provide guidance for their further processing to produce tablets for potential industrial applications.
Section snippets
Material
Fish oil (FO) from menhaden, gelatin (GE type B, from bovine skin, 225 bloom), gum Arabic (GA from Acacia tree), and maltodextrin (DE16.5–19.5) were purchased from Sigma-Aldrich Chemical Reagent Co., Ltd. All chemicals and reagents used in this study were of analytical grade.
Preparation of fish oil microcapsules
The following three different encapsulation methods are based on our published paper with some modifications (Yu et al., 2017).
Morphology of fish oil microcapsules
The obtained fish oil microcapsules were divided into 2 groups based on their structures: matrix structure with multiple cores ((GE-GA)SD and (GE-MD)SD) and core-shell structure ((GE-GA)CC+SD), (GE-GA)CC+SC(GE-GA) and (GE-GA)CC+SC(GE-MD)). SEM images of external and internal morphology of microcapsules are presented in Fig. 3, which shows that all the microcapsules varied in size and were intact without cracks. Most microcapsules appeared teeth or concavities on the surface due to the rapid
Conclusions
Microcapsules to be incorporated into tablet must have sufficiently high mechanical strength in order to maintain their structural integrity during the compaction. For this purpose, five kinds of fish oil microcapsules were prepared using three different encapsulation methods with GE and GA or MD as shell materials. The microcapsules produced using direct spray drying ((GE-GA)SD and (GE-MD)SD) had a matrix structure whilst microcapsules made by coacervation ((GE-GA)CC+SD) and microcapsules with
CRediT authorship contribution statement
Fanqianhui Yu: Conceptualization, Methodology, Data curation, Writing – original draft. Changhu Xue: Writing – review & editing, Supervision. Zhibing Zhang: Methodology, Writing – review & editing, Supervision.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgment
This work was supported by China Scholarship Council [Grant number 201806330047].
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