Astaxanthin is a xanthophyll carotenoid discovered in multifarious marine animals and microorganisms [1]. It is commonly used as a nutritional supplement and has been related to several antioxidant and anticancer activities, preventing neurodegenerative disorders, cardiovascular diseases, and diabetes, and also stimulating immunization [2,3]. However, as with other carotenoids, astaxanthin has highly unsaturated bonds in its molecular structure which is prone to be damaged (oxidation) after exposed to poor environmental conditions (pH extremes, oxygen, light, and high temperature) [4]. The destruction of astaxanthin's chemical structure leads to loss of biological activity and fading [5]. In addition, poor water-solubility of astaxanthin leads to low bioavailability and makes it difficult to be incorporated into aqueous-based foods [6].

Researchers have developed several delivery systems to improve the water solubility, chemical stability, and bioavailability of astaxanthin. However, the encapsulation efficiency, chemical stability and bioavailability of astaxanthin in previous studies cannot meet the current product requirements, making it difficult for them to be widely used in food. For example, Zhang et al. prepared gliadin nanoparticles stabilized Pickering emulsions to deliver astaxanthin and found that the bioaccessibility of astaxanthin was just increased to 22.37 ± 0.62 % [7]. The encapsulation efficiency of astaxanthin in microcapsules prepared by complex coacervation with alginate and gelatin as wall materials was only 85 % [8]. Our previous research showed that the chemical stability of astaxanthin-loaded microcapsules was insufficient, and only when microcapsules were applied into effervescent tablets can the retention rate of astaxanthin reached 80 % for 4 weeks of storage at room temperature [5]. Therefore, it is necessary to further develop delivery systems with high stability, encapsulation efficiency, and bioavailability to enhance the delivery of astaxanthin.

Microcapsules are small spheres composed of wall (the shell coating) and core (the bioactives inside) materials [9]. Natural, semi-synthetic or synthetic polymers were commonly applied as the wall materials for the manufacturing of microcapsules [10]. Owing to the wrapping and blocking effect of the shell, microcapsules have been reported to prevent the core ingredients from environmental influences (such as light, oxygen and temperature) and prolong the shell-life of the bioactives [11]. Moreover, microcapsules have huge surface areas, which lead to improved solubility and tremendously increased biological activity [12]. The controlled release of the encapsulated compounds can also be achieved by controlling the size of the microcapsule particles, the composition and thickness of the wall materials [13]. Therefore, microcapsule has always been a promising carrier for delivering the bioactives [14]. Numerous techniques have been exploited to fabricate microcapsules, for instance, the secondary emulsion technology, layer-by-layer (LBL) assembly and complex coacervation methods [15]. Among these methods, LBL assembly has drawn extensive attention in the food field, due to its unique, straightforward and multipurpose characteristics for microcapsule production [16]. LBL assembly method is usually conducted by the sequentially deposition of complementary polyelectrolytes or polymeric compounds onto colloidal particles through electrostatic, hydrogen bonding, van der Waals, or hydrophobic interactions [17], which leads to the formation of multilayer-coated oil droplets. Assembling multilayer coatings may form a dense biopolymer network around the oil droplets, which can improve the stability of bioactive substances to environmental stresses and modulate the digestion of the lipid droplets [18]. Furthermore, the layer number, type, and sequence of polymers can be controlled to modulate the delivery of encapsulated substances [19].

A variety of proteins, polysaccharides and small molecule ionic surfactants have been applied to assemble multilayer microcapsules [19]. Among them, polysaccharides (for example, chitosan, alginate, xanthan gum, and carrageenan) are attractive, because they can provide strong steric and electrostatic repulsion [19]. Chitosan is the only positively charged polysaccharide in nature with extensive physiological activities [20]. Alginate is a linear anionic polysaccharide substantially applied in drug delivery, wound healing, gene therapy, tissue engineering and environmental protection due to its various favorable characteristics, such as great biocompatibility, high safety, biodegradability, and affordable price [21]. Zhang et al. [22] displayed that chitosan and alginate can stabilize carotenoids-loaded emulsions by the electrostatic deposition technique. Tang et al. [23] improved the physiochemical stability of Pickering emulsion stabilized by chitosan and sodium alginate coating and found that the proper level of the two (1:1–1:2) could increase the stability of Pickering emulsions. Consequently, chitosan and alginate are excellent ingredients for layer by layer self-assembly delivery systems. Although there have been some studies on multilayer emulsions stabilized with chitosan and alginate by LBL assembly method, their combined application in multilayer microcapsules is less explored. The drying process, and the concentration and ratio of chitosan and alginate may affect the formation, stability, and performance of microcapsules.

The innovation of this study is to apply chitosan and alginate, two natural polymers, to stabilize lecithin based emulsions by LBL assembly method, and to prepare three-layer microcapsules by freeze drying to deliver astaxanthin. Surfactants containing soybean lecithin was utilized to stabilize astaxanthin-loaded oil-in-water emulsions. Chitosan and sodium alginate were further coating by the electrostatic deposition technique to obtain tertiary emulsions. Finally, a filling matrix, maltodextrin was incorporated to fabricate astaxanthin-loaded microcapsules by lyophilization method. This research will optimize and characterize the properties of lecithin/chitosan/alginate stabilized microcapsules applied in food.