Anchovy oil microcapsule powders prepared using two-step complex coacervation between gelatin and sodium hexametaphosphate followed by spray drying
Graphical abstract
Introduction
Consumers are increasingly demanding food products which offer health-promoting functions [1]. It is expected that food materials to improve health and well-being beyond their basic function of providing nutrients required for normal functioning of human body. Thus, the concept of functional foods is actively promoted by governments, industry and consumers alike [2]. Fish oil is one of such foods which contains polyunsaturated fatty acids (PUFAs) such as Eicosapentaenoic acid (EPA) and Docosahexaenoic acid (DHA) which provide health-promoting functions. It has been reported that EPA and DHA improve the brain function, slow down the ageing process and help prevent diseases such as coronary heart diseases, immune response disorders and Crohn's disease [[3], [4], [5]]. However, the application of fish oil in the food industry is limited due to its susceptibility to oxidation during production and storage. The oxidation of PUFAs in fish oil not only compromises their functional properties but also produces unpleasant flavour [6].
Microencapsulation of these fish oil in food-grade shell materials is suggested to be one of the most effective methods in improving its oxidative stability and controlled release property [7,8]. A broad range of encapsulation systems has been developed to encapsulate fish oil such as emulsions [[9], [10], [11]], liposome [12], complex coacervates [8,13] and hydrogels [14]. Microencapsulation of fish oil in multi-core microcapsules using complex coacervation technology is perhaps the most effective way of enhancing its oxidative stability while achieving high encapsulation efficiency and oil payload [6]. In our previous work, we optimised a one-step microencapsulation process using gelatin-sodium hexametaphosphate (SHMP) complex coacervates as shell material and multi-core microcapsules smaller than 100 μm in size were successfully prepared [8].These microcapsules required to be crosslinked using transglutaminase (TG), commonly named “meat glue”, prior to the spray drying, otherwise the gelatin would be redissolved in the water phase, leading to the disintegration of the microcapsules during drying. However, there are still safety concerns about using transglutaminase in food products; hence, its replacement needs to be developed urgently. Fucoidan extract (FE) from brown seaweeds, which is rich in bioactive sulfated polysaccharide and polyphenols, can be potentially utilised to crosslink gelatin by initialising oxidation reaction between polyphenols and side chain amino groups of gelatin peptides [15]. Besides, fucoidan also exhibits anti-coagulation, anti-tumour, anti-inflammatory and antioxidative activities [16] as a functional food ingredient.
In this study, a novel two-step microencapsulation method was developed in order to further enhance the oxidative stability of fish oil microcapsules based on our previous work [8]. Firstly, the secondary structure of gelatin in the inner and outer shells of the microcapsules were characterised; secondly, the formation of microcapsules through this two-step method was monitored to observe the formation of the inner and outer shells; thirdly, the characteristics of the microcapsules produced from this two-step method such as encapsulation efficiency, oxidative stability and powder morphology were evaluated and compared with the characteristics of microcapsules produced from the one-step method. Finally, the effect of crosslinker type on resisting disintegration during heating and the oxidative stability of the microcapsule powder were measured and explained.
Section snippets
Materials
Anchovy oil was provided by Lipa Pharmaceuticals Ltd. (Minto, NSW, Australia) and was stored at 4 °C before use. The anchovy oil contained 23.0% (w/w) monounsaturated fatty acids, 42.5% (w/w) polyunsaturated fatty acids 26.5% (w/w) saturated fatty acids. Type A gelatin (from porcine skin, ~300 bloom), sodium hexametaphosphate (SHMP), sodium ascorbate (SA), phosphoric acid and sodium hydroxide were purchased from Sigma-Aldrich Corporation (Sydney, NSW, Australia). Transglutaminase (TG, Activa
Secondary structure of gelatin at inner and outer shell
Circular dichroism (CD) is a sensitive and rapid method providing useful information of the secondary structure of proteins and peptides. The spectra in far UV range (250–190 nm) provide valuable information on the conformation of the polypeptide backbone. Fig. 1 presents the CD spectra of the gelatin in pure gelatin solution, aggregated droplets and free coacervates. The CD spectra of gelatin shows two typical peaks, a negative peak around 198 nm indicating a random coil conformation and a
Conclusion
Anchovy oil was encapsulated into microcapsule powder creating multi-core structure using a two-step microencapsulation method using gelatin-SHMP complex coacervates as the shell material. The inner and the outer shells of the microcapsule were created separately during the microencapsulation. The surface roughness of the microcapsules significantly decreased during the formation of the outer shell, which led to an enhanced oxidative stability. The incorporation of fucoidan extract not only
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