Natural Astaxanthin Stability Latest Research

Astaxanthin, an important member of the carotenoid family, is not only the strongest antioxidant among natural substances [3], but also has important physiological activities such as anti-inflammatory [4], anticancer [5], prevention of cardiovascular disease [6], slowing aging [7], and improving body movement [8]. Therefore, astaxanthin has good application prospects in the markets of health products, food, medicine, cosmetics, and feed [9].

In 2010, China's Announcement No. 17 approved Haematococcus pluvialis as a new resource food, and astaxanthin derived from Haematococcus pluvialis was allowed to be added to all types of food and beverages except infant food [10]. According to market data from Global Market Insights, the global astaxanthin market is expected to reach 800 million U.S. dollars by 2024, with the North American market growing at a compound annual growth rate of over 3.5%. The Asia-Pacific region will become the main contributor to market growth (over 250 million U.S. dollars).

However, natural astaxanthin is unstable and easily degraded, which reduces its biological activity and physiological function and limits its application. Therefore, improving the stability of natural astaxanthin is one of the current research hotspots, and there have been many reports on astaxanthin delivery systems. However, this field is in its infancy, and the laws of stability changes during the extraction, processing and storage of astaxanthin are often ignored, lacking comprehensive basic data and systematic analysis. Only by comprehensively understanding the factors and essential laws that affect the stability of natural astaxanthin can the development and improvement of stabilization technology be better achieved.

This paper reviews the influence and causes of the stability of natural astaxanthin on its own structure, extraction solvent, processing and storage environmental conditions. It summarizes and compares the protective effects, technical characteristics and basic principles of stabilization of natural astaxanthin by emulsion, microcapsule, liposome and nanoencapsulation technologies. Finally, it puts forward some prospects based on the existing astaxanthin stabilization technology, which provides some reference value for the protection and delivery of astaxanthin.

1 Overview of astaxanthin

Astaxanthin, also known as Haematococcus pluvialis lutein, shrimp red pigment, shrimp yellow pigment, shrimp yellow substance and lobster shell pigment [12], is currently the substance with the strongest antioxidant activity discovered. Its antioxidant capacity is much higher than that of existing natural antioxidants such as vitamin E, β-carotene and lycopene, and it is known as “super vitamin E” [13−14].

1.1 Chemical structure of astaxanthin

The chiral carbon atoms C-3 and C-3' at both ends of the astaxanthin conjugated double bond chain exist in the form of R or S, respectively, giving rise to three stereoisomers (as shown in Figure 1 (1)), namely all-trans (3S, 3 'S), cis-trans (3S, 3'R), and trans-trans (3R, 3'R), of which the (3S, 3'S) and (3R, 3'R) isomers are mirror images (enantiomers) [15]. The multiple conjugated double bonds and unsaturated ketone groups at the ends give astaxanthin a lively electronic effect, which can attract unpaired electrons from free radicals or donate electrons to free radicals, thereby scavenging free radicals and quenching singlet oxygen physically.

Astaxanthin has multiple double bonds in the linear part of its molecule, and each double bond can be in the Z (cis) or E (trans) configuration. The all-E configuration is the most stable structure because the branched groups do not compete for spatial positions [16]. It has been found that the Z-type structure is present at positions 9, 13 and 15 in natural astaxanthin, so the possible geometric isomers of astaxanthin are all-E, (9Z), (13Z), (15Z), etc. (as shown in Figure 1 (2)). At the same time, astaxanthin has one hydroxyl group in each of its terminal cyclic structures. These free hydroxyl groups can form esters with fatty acids. One hydroxyl group forms an ester with a fatty acid, which is called a single astaxanthin ester, while two hydroxyl groups are called double esters (as shown in Figure 1 (3)). After esterification, its hydrophobicity and stability are enhanced [16−17]. It can be seen that natural astaxanthin is diverse in form, and the different molecular structures determine the differences in stability between astaxanthins.

1.2 Sources of astaxanthin

Currently, astaxanthin is produced by chemical synthesis, biosynthesis, and natural extraction. Chemical synthesis is divided into total synthesis and semi-synthesis: total synthesis uses chemical raw materials as raw materials and is produced through chemical synthesis reactions; semi-synthesis uses carotenoids such as canthaxanthin, lutein, and zeaxanthin as raw materials to prepare astaxanthin [18]. This method requires multiple chemical and biocatalytic reactions, and the astaxanthin synthesized is a mixture of multiple conformations and contains by-products. The synthesis process poses significant safety risks [19].

The biosynthesis method uses yeast, algae and bacteria to produce astaxanthin. This method produces astaxanthin with a clear structure (mostly trans structures) and few by-products, but the yield is low and the culture conditions are strict. The key to achieving large-scale production is the use of cheap culture materials and the selection and breeding of high-quality, high-yield strains [20]. At present, the extraction of astaxanthin from natural resources is less expensive and can be produced on a large scale, which can alleviate the market demand for astaxanthin. Astaxanthin is mainly extracted from natural sources such as Haematococcus pluvialis, Rhodopseudomonas palustris and crustacean shells using vegetable oils[21], organic solvents[22], ionic liquids[23] and eutectic solvents[24]. Natural astaxanthin generally has advantages over synthetic astaxanthin in terms of stability, antioxidant activity, bioavailability and safety[25−27].

2 Stability of natural astaxanthin and factors affecting it

Natural astaxanthin has excellent functional properties and is of great value in the development of corresponding functional products. However, the instability of astaxanthin is the first challenge to be faced in practical applications. First, the conjugated double bond of astaxanthin makes it chemically active. Second, the difference in polarity of different solvents affects solubility and stability. Finally, astaxanthin is susceptible to degradation during processing and storage due to light, temperature, etc. Many studies have only focused on one aspect of astaxanthin stability, ignoring the influence of multiple factors. This article will comprehensively analyze the influencing factors and change laws of natural astaxanthin stability from three perspectives: the structure of astaxanthin itself, the extraction solvent, and the processing and storage environment.

2.1 The structure of astaxanthin itself

Compared with lutein, vitamin C, β-carotene, etc., the presence of conjugated double bonds, hydroxyl groups and keto groups makes astaxanthin both hydrophilic and hydrophobic, which also makes it more likely to react with free radicals and undergo structural changes [28]. On the other hand, most natural astaxanthin exists in an esterified form, containing various fatty acids, including C16:0, stearic acid (C18:0), C18:1, linoleic acid (C18:2) and γ-linolenic acid (C18:3) [29]. Studies have shown that esterified astaxanthin is more stable than free astaxanthin. For example, in a microemulsion containing DL-menthol and caprylic acid, the half-life of free astaxanthin is 13.86 days, while the half-life of astaxanthin ester is 69.31 days [17]. In addition, stability is positively correlated with the degree of esterification. Furthermore, increasing the carbon chain length and reducing the degree of unsaturation of the fatty acids are beneficial for improving the stability of astaxanthin esters. Astaxanthin docosahexaenoate diester is the most stable form of astaxanthin ester [16].

Therefore, in the production and processing of food, medicine and cosmetics, attention should be paid to distinguishing between the different structures of astaxanthin, clarifying the effect of its own structure on stability, taking targeted protective measures, effectively extending the shelf life of the product, and promoting the efficient use of astaxanthin.

2.2 Extraction solvent

The interaction between the solvent and the astaxanthin molecule has a direct effect on its stability, and different extraction conditions (temperature, time, etc.) have a significant effect on the structure of astaxanthin during the extraction process. However, many previous studies have ignored the effect of the nature of the solvent itself on astaxanthin. Astaxanthin is insoluble in water, fat-soluble, and easily soluble in organic solvents such as chloroform, acetone, benzene, etc. [22] and vegetable oil, fish oil, etc. [21]. The effect of vegetable oil extraction is poor and requires high temperatures, and astaxanthin is easily degraded [30]; although the extraction rate of organic solvents is high, the polarity of organic solvents is very strong, which is not conducive to maintaining the stability of the astaxanthin structure [31]. Therefore, the ideal extraction technology should combine the two functions of high extraction rate and astaxanthin stability.

Studies have shown that imidazolyl ionic liquids (ILs), such as 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), have a longer half-life than acetone when extracting carotenoids, indicating that ILs are more stable than acetone when extracting carotenoids [23]. hexafluorophosphate ([BMIM][PF6]) and other ILs have a higher half-life than acetone, indicating that IL-extracted carotenoids are more stable than acetone-extracted carotenoids [23]. Previous studies have shown that hydrophobic quaternary ammonium and phosphonium ionic liquids are more soluble in astaxanthin than imidazolium ionic liquids, and that there is a good mathematical relationship between the concentration change of astaxanthin in tributylphosphonium chloride ([P4448]Cl) and the color difference parameter [32]. However, disadvantages such as the high price and poor biocompatibility of ILs limit their widespread commercial extraction of astaxanthin.

Deep eutectic solvents (DESs) are an emerging green solvent that are eutectic mixtures of a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD). Studies have shown that astaxanthin exhibits better stability in DES microemulsions than in organic solvents (ethanol, methanol, and acetone) [17]. In addition, the antioxidant activity of astaxanthin extracted with DES is higher than that extracted with organic solvents [33], and acidic DES is more conducive to the dissolution of astaxanthin [34]. Therefore, DES is a good alternative to organic solvents and ionic liquids. In summary, the choice of solvent for astaxanthin extraction should be considered comprehensively from multiple aspects such as cost, environmental protection, safety, solubility and stability.

2.3 Processing and storage environmental conditions

2.3.1 Light

Light has two effects on astaxanthin: a. the formation of cis-trans double bonds, with the electromagnetic wave spectrum shifting 2–10 nm towards the blue end; b. accelerated oxidation of astaxanthin, with the degradation and fragmentation of the chromophore, the shift of the spectrum towards the ultraviolet region, and the loss of color [35]. The astaxanthin extract was placed in conditions of no light, indoor natural light, UV light and continuous sunlight exposure. After 6 hours, the astaxanthin retention rate under sunlight exposure was only 0.57%, while the sample in the dark showed no significant change [36]. Similarly, Maohua Aihemat et al. [37] pointed out that ultraviolet light can damage the stability of astaxanthin. Therefore, astaxanthin is very sensitive to sunlight and ultraviolet light, and care should be taken to avoid light during extraction, storage and use.

2.3.2 Temperature

High temperatures have a significant damaging effect on most bioactive substances. Astaxanthin should be stored at low temperatures to slow down its degradation. Many studies have shown that the stability of astaxanthin extracts decreases with increasing temperature. For example, the absorbance of astaxanthin extracts stored at 4 °C remains unchanged, while the astaxanthin residual rate is only about 30% after being stored at 70 °C for 6 h [36]. Similarly, after storing astaxanthin oil at below 60 °C for 1 h, the loss rate of astaxanthin was less than 2%, while when the storage temperature reached above 80 °C, the loss rate exceeded 20% [38].

2.3.3 pH

The acidity and alkalinity of the environment will affect the solubility and stability of astaxanthin to varying degrees. A weakly alkaline environment has little effect on the stability of astaxanthin, but a long-term weakly acidic environment will damage its stability [39]. In addition, astaxanthin esters will undergo a saponification reaction and convert to free astaxanthin in a weakly alkaline environment [37]. Although the solubility and antioxidant activity of astaxanthin are significantly enhanced under acidic conditions, excessive acidity can affect the stability of astaxanthin [32]. Therefore, maintaining the solution in a neutral or slightly alkaline state during astaxanthin storage will help to maintain the stability of the astaxanthin structure and function.

2.3.4 Metal ions

Metal ions can promote the oxidation of astaxanthin, causing it to dissolve and fade, and even become cloudy. Song Sumei et al. [40] found that the retention rate of astaxanthin decreased significantly with the addition of Fe2+, Fe3+, and Cu2+. Moreover, the addition of Fe2+, Cu2+, and K+ caused the astaxanthin extraction solution to become cloudy [36]. Therefore, the addition of ironware and substances containing Fe2+ and Cu2+ should be avoided as much as possible during the production and transportation of astaxanthin.

2.3.5 Oxygen

Oxygen can cause auto-oxidation, photo-oxidation and chemical oxidation of astaxanthin. When astaxanthin is exposed to air at room temperature of 25 °C and stored in the dark for 30 days, the retention rate of free astaxanthin is only 20%, while that of microencapsulated astaxanthin can reach 80% [41]. This may be because oxygen in the air reacts with astaxanthin in an oxidative reaction, causing astaxanthin to decompose. Some studies have attempted to improve the stability of astaxanthin by adding antioxidants, but it was found that the addition of the antioxidant 2,6-di-tert-butyl-4-cresol (BHT) does not improve the stability of astaxanthin, and the two antioxidants VC and Na2SO3 actually reduce astaxanthin stability [36]. This may be because the antioxidant properties of astaxanthin are much higher than those of VC and Na2SO3, and astaxanthin oxidizes itself to protect VC and Na2SO3 from oxidation.

3 Stabilization technology for natural astaxanthin

Although natural astaxanthin has strong antioxidant properties, its highly unsaturated structure means that it tends to chemically degrade when exposed to high temperatures, light, etc., which can cause it to fade and its biological activity to decline, limiting its application in the food, pharmaceutical and cosmetic industries. In order to improve the utilization rate of astaxanthin in various applications, different stabilization techniques have been studied, including emulsion encapsulation, microencapsulation, liposome and nano-level encapsulation. Therefore, the following will describe the process of embedding astaxanthin using the above techniques and the stability of the astaxanthin after embedding, while comparing the stabilization effects and advantages and disadvantages of different stabilization techniques.

3.1 Emulsion delivery system

The emulsion system for delivering astaxanthin is to dissolve astaxanthin in an organic phase, then fully disperse the organic phase in an aqueous phase containing an emulsifier, and form a colloidal system under the action of certain external forces (such as stirring, homogenization, ultrasound, etc.) [42]. In addition to traditional emulsions, nanoemulsions, microemulsions, Pickering emulsions and multi-layer emulsions have gradually emerged in recent years. The rapid development of astaxanthin stabilization technology has been promoted by the updating of emulsion preparation technology, the iteration of ingredients and the diversification of functions (as shown in Table 1).

3.1.1 Traditional emulsions

Traditional emulsions, also known as conventional emulsions or giant emulsions, refer to coarse dispersion systems with droplet radii between 300 nm and 100 μm, which tend to break up over time. In the past, the combination of protein and polysaccharide emulsifiers has had a good stabilizing effect, but it tends to degrade the substances embedded in it under ultraviolet or heat treatment [43]. Recent studies have found that a casein-caffeic acid-glucose-stabilized emulsion is beneficial for protecting the internal astaxanthin against adverse environments due to the presence of polyphenols (caffeic acid) [44]. However, traditional emulsions are inherently unstable, and how to further maintain the stability of the emulsion itself has always been a challenge in this field.

3.1.2 Nanoemulsions

Nanoemulsions generally consist of water, oil and a surfactant. They can achieve a small particle size (50–200 nm) and are kinetically stable through high-pressure homogenization. Compared to traditional emulsions, they can better improve the stability and bioavailability of active substances [45]. The selection of emulsifiers and the use of complex emulsifiers are the key to preparing nanoemulsions with excellent properties.

A nanoemulsion of astaxanthin prepared with soy lecithin as the emulsifier and stored under the same conditions as free astaxanthin for one week had an astaxanthin retention rate of 85.34%, which was much higher than the 54.92% of the latter [46]. In addition, mixtures of small molecule emulsifiers, proteins and polysaccharides have been shown to greatly improve the properties of the prepared emulsions [47]. For example, the degradation rate of astaxanthin was only 20% after 8 weeks of storage at 25 °C when astaxanthin nanoemulsions were prepared using a complex emulsifier (polysorbate 20, sodium caseinate, and gum arabic) [48]. However, high-pressure homogenization is likely to cause changes in the structure of sensitive compounds in the system, reducing their biological activity and making them thermodynamically unstable.

3.1.3 Microemulsions

Compared with nanoemulsions, microemulsions have a smaller particle size (between 10 and 100 nm) and are transparent. They can form spontaneously under the action of surfactants and are thermodynamically stable systems [49]. Microemulsions have good properties, including excellent stability, low viscosity and strong solubilizing ability of lipophilic compounds. They are a kind of astaxanthin extraction solvent that takes both solubility and stability into account. In recent years, ionic liquid-based microemulsions [50] and eutectic solvent-based microemulsions [17] have shown good results in the extraction and stabilization of astaxanthin. Compared with organic solvents, microemulsions can improve the solubility of astaxanthin, and free astaxanthin and astaxanthin esters in eutectic solvent-based microemulsions exhibit better storage stability than in organic solvents [17].

3.1.4 Pickering emulsions

Conventional emulsions stabilized by surfactants (e.g. polysaccharides and proteins) are usually thermodynamically unstable and will break down over time through flocculation, coagulation and Ostwald ripening. Pickering emulsions, on the other hand, enhance their own stability through colloidal particles [51]. Common colloidal particles are protein-based particles (e.g. lupin protein particles [52]) or polysaccharide-protein particles (e.g. alcohol-soluble protein and sodium alginate [53]). At the same time, the astaxanthin carried by Pickering emulsions is more resistant to heat, high temperatures or metal ions than free astaxanthin [54].

3.1.5 Multilayer emulsions

“Multilayer emulsion” is an emerging technology for encapsulating astaxanthin. It consists of many biopolymer layers (or emulsifiers) surrounding lipid droplets, which are deposited on top of each other through attractive electrostatic interactions [55]. Studies have shown that the degradation rate of astaxanthin in chitosan-pectin multilayer emulsions is 3 to 4 times slower than that in traditional emulsions during storage [56]. However, the multi-layer emulsion technology also faces challenges, first of all, designing a reasonable system composition, and secondly, optimizing the many factors that affect stability (such as the type of biopolymer, droplet concentration, ionic strength, etc.).

Whether it is a conventional emulsion or a nanoemulsion, microemulsion, Pickering emulsion or multilayer emulsion, which have gradually emerged in recent years, their inherent instability greatly limits their application as encapsulation and delivery systems for bioactive substances such as astaxanthin. At present, research in this field mainly focuses on improving the stability of the emulsion itself. In contrast, the stability of microemulsions, Pickering emulsions and multilayer emulsions is significantly improved because they contain amphiphilic substances. However, there is a lack of research on further improving the extraction rate, encapsulation effect and storage stability of astaxanthin, and theoretical research on the composition of the emulsion needs to be strengthened.

3.2 Microencapsulation delivery system

3.2.1 Basic methods

Encapsulating astaxanthin in a wall material matrix (liquid/solid, homogeneous/heterogeneous material, etc.) can protect astaxanthin from external interference [61]. Common methods include spray drying [62], freeze drying [63] and complex coacervation [64]. Table 2 lists the process parameters, encapsulation efficiency and stability of these astaxanthin microencapsulation techniques. Spray drying is fast, simple and economical, but drying at too high a temperature can damage the core material [62]. In contrast, the low-temperature frozen state of the freeze-drying method can effectively protect the internal astaxanthin, but it is time-consuming and has high operating costs [63]. Although the coacervation method does not require organic solvents or high temperatures and is suitable for use in the food industry, the encapsulation rate of this method is generally low [65]. Therefore, it is important to understand the principles, operating conditions, process parameters, advantages and disadvantages of each method in order to prepare astaxanthin microcapsules with good properties.

3.2.2 Common wall materials

The composition and selection of the wall material are crucial to the properties of the microcapsule and are also conditions for obtaining highly efficient and superior-performing microcapsule products. An ideal wall material should have the following advantages: high concentration and low viscosity (good fluidity at high concentrations), superior emulsifying properties, easy drying and desolvation, and low cost [66−67]. Common wall materials include carbohydrates (sucrose, maltodextrin, corn fiber), hydrophilic gums (gum arabic and cashew gum), proteins (whey protein and gelatin) and oils and fats (sucrose fatty acid esters, lecithin).

In practice, several wall materials are often mixed and used together, such as a combination of proteins and carbohydrates, or a combination of proteins and hydrophilic gums. The type and ratio of the wall material combination are key factors in the formation of a stable system during the microencapsulation process, but they need to be combined reasonably according to the application requirements.

a. Combination of carbohydrates with each other and with proteins or hydrophilic gums. Although carbohydrates have low viscosity and are very soluble, they often need to be combined with proteins or gums to achieve high compactness due to their high porosity and low emulsifying ability [68−69]. For example, astaxanthin microcapsules prepared with a 1:1 ratio of zein and oligochitosan (OCH) as a wall material not only have a high encapsulation rate (94.34% ± 0.64%), but also can withstand ultraviolet light, with an astaxanthin retention rate of 82.4%, which is much higher than the 60% of free astaxanthin [69]. In addition, the addition of an emulsifier can significantly improve the stability and encapsulation efficiency of astaxanthin [41].

b. Protein and hydrophilic gum blending. Although proteins have good emulsifying properties, protein particles tend to aggregate and are easily hydrolyzed by proteases. However, hydrophilic gums can improve the surface activity and viscosity of proteins and enhance the stability of the wall material. For example, microcapsules prepared by embedding astaxanthin esters with whey protein and gum arabic as wall materials were found to have good resistance to strong acid (pH 4) environments [64].

c. Blending lipids and carbohydrates. Studies have shown that astaxanthin embedded in a wall material composed of β-cyclodextrin and sucrose fatty acid ester (in a ratio of 1:1) is more stable at different temperatures than free astaxanthin [63]. The possible reason is that lipid substances such as sucrose fatty acid ester can promote the crystallization of β-cyclodextrin, forming a dense network structure on the molecular surface to stabilize the astaxanthin inside.

Although the microencapsulation of astaxanthin can achieve good stabilization and encapsulation efficiency through the combination of several wall materials, the interaction between the wall materials and the microscopic molecular structure is still unclear. Further research at the molecular level is needed to provide a theoretical basis for the precise design of microcapsules for encapsulating astaxanthin.

3.3 Liposome delivery system

Liposomes are ultra-microscopic spherical porous particles formed by self-aggregation of concentric phospholipid bilayers dispersed in an aqueous phase. They have a vesicle structure with hydrophilic inner and outer layers and a hydrophobic middle layer [76]. It can not only encapsulate polar substances in the water core, but also non-polar substances in the non-polar region formed by the phospholipid. The common preparation methods of liposomes include solvent injection [77], reverse evaporation [78], thin film dispersion [76], thin film sonication [79], etc.

As shown in Table 3, astaxanthin liposomes prepared from phosphatidylcholine as a raw material have an encapsulation rate of 97.68% and exhibit good storage stability [80]. However, conventional liposomes have defects such as being prone to oxidation and aggregation. Therefore, surface modification of liposomes is a factor in improving stability and encapsulation efficiency. Various polysaccharides (e.g., chitosan [81]) and proteins (e.g., lactoferrin) have been used as surface modifiers. Wu et al. [82] showed that the encapsulation of astaxanthin in liposomes increased the retention rate by 10% compared to free astaxanthin. Modified liposomes such as phosphatidylcholine galactose and phosphatidylcholine neocarboxymannan also had higher astaxanthin encapsulation efficiency and antioxidant activity than the original phosphatidylcholine liposomes. liposomes have higher encapsulation efficiency and antioxidant activity than the original phosphatidylcholine liposomes. The large number of hydroxyl groups on the polar head of the modified phospholipids helps to form hydrogen bonds on the membrane surface to improve stability.

In addition to single liposomes, the preparation of complex liposomes has also been a research hotspot in recent years. The double-layer vesicle structure of liposomes can embed astaxanthin and bacteriocin in the lipid layer and aqueous layer, respectively, without affecting each other. It is a substance with both antioxidant and preservative effects [78]. The excipients and equipment required for the preparation of liposomes are relatively expensive, and high-dose liposomes may be highly toxic. At present, there is a lack of research on the safety evaluation of liposome-stabilized astaxanthin.

3.4 Nanometer-scale delivery systems

In addition to nanoliposomes and nanomicelles, there are also encapsulation technologies for astaxanthin such as nanoparticles and nanosuspensions.

3.4.1 Nanoparticles

Nanoparticles are usually assembled from natural polymers such as proteins, polysaccharides and synthetic polymers [39]. They are an ideal carrier with special physical properties (e.g. uniformity, strong permeability, etc.) that can be used to encapsulate active substances, reduce external influences and achieve targeted release in response to specific stimuli [84–85]. The choice of nanoparticle carrier can have a different effect on the stabilization of astaxanthin. For example, the water solubility, stability and bioactivity of astaxanthin are significantly enhanced when encapsulated in polymeric nanoparticles prepared from polysaccharide-protein (alginate and chitosan) [86–87]. As shown in Table 4, nanoparticle-encapsulated astaxanthin has been shown to improve its stability. However, the potential toxicity of nanoparticles can have an impact on human health and the environment [88].

3.4.2 Nanodispersions

Nanodispersions are colloidal systems formed by the stable dispersion of nanoparticles in a dispersing medium [89]. Astaxanthin in nanodispersions is stabilized by emulsifiers, and the key to the design is to optimize the type and amount of emulsifier [90]. For example, the combination of gelatin and other active substances can improve the stability. Among them, the nanodispersion of gelatin and sodium caseinate as emulsifiers showed the lowest astaxanthin degradation rate [90]. The reason may be that sodium caseinate has functional groups such as cysteine residues and disulfide bonds in its structure, which can scavenge free radicals and prevent lipid oxidation [91]. An appropriate combination of emulsifiers can improve the emulsion dispersion performance and stabilize astaxanthin by forming molecular complexes at the interface [92–93] (as shown in Table 4).

3.5 Comparison of astaxanthin stabilization techniques

3.5.1 Stabilization effect

Although there is an increasing amount of research on the stabilization of natural astaxanthin, there is a lack of comparative studies between different methods. Comparing Tables 1 to 4, based on the principles of different stabilization techniques and the storage effects of astaxanthin, it can be concluded that the inherent thermodynamic stability of microemulsions and the use of Pickering emulsions with colloidal particles instead of traditional emulsifiers is better than that of traditional emulsions (the degradation rate of astaxanthin is generally less than 20%); astaxanthin encapsulated in microcapsules is more stable than emulsion systems with poor self-stability due to the protective effect of the wall material, and the retention rate of astaxanthin can reach 85%. Astaxanthin in liposomes, nanoparticles and nanodispersions can also protect astaxanthin, but it is related to factors such as raw materials and process parameters. Therefore, the most suitable stabilization method should be selected based on a comprehensive consideration of all factors.

3.5.2 Problems with each technology

Although the existing astaxanthin stabilization technologies have improved the stability of astaxanthin to varying degrees, they also have their own problems that need to be solved. The emulsion system itself has poor stability, so a high content of emulsifier is used, which not only increases production costs, but also makes it more difficult to transport the emulsion [58]. Microencapsulation technology usually requires the help of spray drying to produce a small particle size, which is a complex process with high equipment investment and high production energy consumption [45]. The excipients required for liposomes and the cost of equipment are relatively high, and high-dose liposomes may be highly toxic [76]. The preparation of nano-dispersions with good performance faces the dilemma of large particle sizes, complex preparation processes, expensive raw materials and storage difficulties, and the difficulty of achieving large-scale production [90].

4 Conclusion and outlook

Natural astaxanthin has extremely high biological activity and medicinal value, and has broad application prospects in the fields of food, medicine and cosmetics. However, the instability of the properties and functions of natural astaxanthin due to its own structure, extraction process and storage environment limits the exertion of its biological functions. The construction of various astaxanthin delivery systems such as emulsions, microcapsules, liposomes, nanoparticles and nanodispersions can help improve the stability of natural astaxanthin and exhibit different technical characteristics.

At present, the development of astaxanthin delivery systems such as emulsions, microcapsules, liposomes and nanoparticles is progressing at different speeds. However, overall, the current astaxanthin stabilization technology is still in the preliminary research stage, and there are still many scientific problems to be solved. Therefore, the following points should be noted in the future: a. Strengthen basic research, combine molecular simulation and other technologies to design the compounding of emulsifiers or wall materials from the molecular level, optimize the structure of the stabilization system, and improve the encapsulation and stabilization effects; b. Seeking greener and smarter systems, such as the use of eutectic solvents, new surfactants, and responsive emulsions; c. Focusing on the correlation and continuity between the astaxanthin extraction system, the homeostasis system, and the application delivery system; d. Accelerating the establishment of safety evaluation methods and systems for astaxanthin homeostasis systems.

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