How Is the Microencapsulation and Stability of Astaxanthin Powder?

Astaxanthin is one of the oxygen-containing derivatives of carotenoids and the highest level product of its synthesis. It belongs to the keto carotenoids and has about 10 times higher antioxidant activity than β-carotene in the same category. It is the most promising antioxidant pigment in nature. Existing clinical trials have shown that astaxanthin can effectively remove free radicals in the body, while promoting the production of antibodies and improving animal immunity[1]. It can not only resist inflammation and cancer[2], prevent ultraviolet radiation[3], but also prevent cardiovascular[4] and nervous system[5] diseases. It has considerable practical value and application prospects in the food, health product, and pharmaceutical industries.

Extraction and purification methods for astaxanthin have been reported in many domestic and foreign literatures, some of which are quite mature. However, astaxanthin itself has a weak polar molecular structure containing many conjugated double bonds, which leads to poor stability and water solubility, thus limiting the fields in which astaxanthin can be used in the market at this stage [6]. In order to solve the above problems, many researchers have begun to try to encapsulate astaxanthin using microencapsulation technology. This paper mainly summarizes and analyzes the main factors that affect the encapsulation effect of astaxanthin microcapsules, providing a reference for future in-depth research on astaxanthin in the field of microcapsules.

1 Microencapsulation technology

Microencapsulation technology is a new technology used to protect the contents of the capsule without affecting their original chemical properties. It mainly involves embedding and sealing solids, liquids or gases that are unstable at room temperature in a capsule wall made of a polymer, thereby isolating them from external factors such as light and oxygen.

In 1936, a US company first used paraffin as a wall material to microencapsulate cod-liver oil, and since then microencapsulation technology has come to the fore. After decades of development, methods such as spray drying, interfacial polymerization, and centrifugal porosification have also been successively applied to microencapsulation technology. As the technology becomes more and more mature, its application fields are also becoming more and more extensive, such as food, medicine, and chemicals. The common methods and characteristics of microcapsule preparation are shown in Table 1.

2 Effect of core wall material on the encapsulation efficiency of astaxanthin microcapsules

Microcapsules consist of two parts: the core material and the wall material. In the food industry, due to the safety of the product for consumption and the particularity of the use, it is usually required that the wall material must be made of a substance with high safety (i.e., tasteless, non-toxic, and not reacting with the core material, etc.) [14].

2.1 Effect of the ratio of wall material composition on the encapsulation effect of astaxanthin microcapsules

The wall materials in food microcapsules generally use natural polymer materials and semi-synthetic polymer materials that are easily soluble in water and have good sustained-release properties, such as water-soluble gums, starches, proteins, sugars, cellulose, lipids, etc. [15]. A summary of common wall materials and their characteristics is shown in Table 2.

In practical applications, it is difficult for a single wall material to achieve the ideal encapsulation state of microcapsules, so two or more wall materials are often mixed together in production to achieve the desired effect. For example, xanthan gum and guar gum can be mixed in a certain ratio to increase the viscosity of the system [20], and small molecule sugars such as trehalose or glucose can be combined with large molecule wall materials such as starch to achieve a complementary effect [21].

In studies on the microencapsulation of astaxanthin, many scholars have also tried different combinations of wall materials in order to achieve the best encapsulation effect. Shen et al. [22] mixed sodium caseinate with soluble corn fiber and whey protein in different proportions and carried out experiments on the microencapsulation of astaxanthin. The results confirmed that the astaxanthin microcapsules prepared using the two methods had excellent quality and achieved a good yield of more than 90%. In addition, Pu et al. [23] also used different ingredients of wall materials to encapsulate astaxanthin-containing oils, and selected the best wall material combination scheme, obtaining a relatively ideal yield of 84.84%. However, because a small amount of core oil remained on the surface of some capsules during the experiment, they were prone to oxidation and rancidity during storage at room temperature, affecting product quality.

2.2 Effect of the wall material compounding ratio on the microencapsulation effect of astaxanthin

The optimal wall material compounding ratio can form a stable emulsion system during the microcapsule embedding process, providing a strong guarantee for the best embedding effect. Yu et al. [24] confirmed in experiments on spray-dried microcapsules that the ratio of composite wall materials can affect the viscosity and stability of the emulsion in the microcapsule system, and there is a certain linear relationship with it. For example, when maltodextrin is combined with gelatin, soy protein, and caseinate, respectively, the stability of the emulsion system is affected by the proportion of the wall material, and the stability decreases with the increase of the ratio of maltodextrin to protein.

Therefore, studies have used whey protein, gum arabic and maltodextrin as wall materials, and have combined them to encapsulate astaxanthin, and have explored the changes in the yield and efficiency of astaxanthin microcapsules when different combinations of wall materials are used at different gradient ratios. It was finally determined that the best encapsulation effect is achieved when gum arabic and whey protein are used in a ratio of 1:3 [25].

At present, there are many reports in the literature on the types of wall materials for astaxanthin microcapsules. However, due to the different properties of the selected wall materials, the experimental ratios of the wall material combinations also vary slightly with the composition. Therefore, screening the appropriate type and ratio of wall material is of great significance for the microencapsulation of astaxanthin.

2.3 Effect of core wall material on the encapsulation of astaxanthin

In microencapsulation experiments, the mixing ratio of core material to wall material can determine the formation of the microcapsule shell and affect product quality. It is therefore often used as one of the experimental screening conditions. Hu Tingting et al. [26] screened five gradients of the core-wall material ratio in the experiment of astaxanthin microcapsule encapsulation, and found that during the gradual increase of astaxanthin content, the measured encapsulation rate and yield of microcapsules showed an overall increasing and then decreasing trend. Some studies have suggested that the reason for this phenomenon is that when spray drying, the core material content in the microcapsules is low, and the viscosity of the system is high, causing the outer wall of the microcapsules to form slowly and to accumulate too thickly. The resulting product has a low encapsulation rate and poor quality [27].

In addition, Laohasongkram et al. [28] confirmed that if the concentration of the core material in the system is supersaturated, it may cause the phenomenon of difficulty in coating the core material due to insufficient wall material content, which affects the thickness and density of the capsule wall. A decrease in thickness is likely to cause cracking or rupture, while low density is likely to cause the core material to pass through the relatively loose capsule wall structure and reach the outside of the wall. Both results can greatly reduce the embedding effect.

3 Effect of spray drying on the encapsulation of astaxanthin microcapsules

In general, the microencapsulation process can be roughly divided into the preparation of an emulsion containing the core wall material and the film-forming treatment of the microcapsules. In spray drying, the quality of the film-forming microcapsules mainly depends on the size of the homogenization atomization pressure and the temperature of the inlet and outlet air.

3.1 Effect of homogenization pressure and number of homogenization cycles on the encapsulation efficiency of astaxanthin 

The homogenization pressure affects the atomization effect, which determines the reaction area of the microcapsules. Therefore, some studies have found that there is a positive correlation between the homogenization pressure and the encapsulation efficiency of microcapsules within a certain range in spray drying experiments [29]. Huang Wenzhe et al. [30] also confirmed in an experiment on the encapsulation of astaxanthin that the microencapsulation effect of astaxanthin gradually reached its optimum as the homogenization pressure increased, and the highest yield and efficiency were obtained at 50 MPa, 98.08 % and 30.6 % respectively. The main reason for this is that during high-pressure homogenization, as the homogenization pressure increases, the atomized emulsion droplets can be further refined, the reaction area is correspondingly increased, and the encapsulation is more uniform. At the same time, the refinement of the emulsion also facilitates the rapid evaporation of water in the capsule during drying, preventing the occurrence of wall sticking [31].

An increase in the number of high-pressure homogenizations can improve the stability of the emulsion, but it can also lead to an increase in the temperature of the system, causing the astaxanthin dispersed in the emulsion to degrade due to heat, which affects the quality of the microcapsules [27].

3.2 Effect of inlet and outlet air temperature on the encapsulation efficiency of astaxanthin microcapsules

During the spray drying process, the temperature of the inlet and outlet air often has a certain effect on the retention rate of the core material and the formation of the microcapsule shell. Raposo et al. [32] confirmed in their study on the microencapsulation of astaxanthin that, for the same outlet air temperature, if the inlet air temperature is lowered, it will cause the microcapsule system to retain water and cause wall sticking, which will affect the compactness of the shell. However, if the inlet air temperature is too high, it will not only accelerate molecular movement in the system and accelerate the degradation of astaxanthin, but may also cause cracks or small pits on the surface of the microcapsule wall, resulting in poor encapsulation.

In addition, a moderate increase in the temperature of the outgoing air helps evaporate the water inside the microcapsules, which accelerates the formation of the microcapsules and improves the retention rate of the core material. Huang Lixin et al. [33] believe that if the temperature of the outgoing air is too low, the droplets after atomization are likely to form shells prematurely due to the high temperature, resulting in the presence of water inside the microcapsule particles. During the deceleration drying stage, steam is likely to accumulate, causing the capsule walls to expand and crack or the water content to be too high, which affects product quality. On the other hand, if the air temperature is too high, the product is prone to degradation due to prolonged heating, and the microcapsule particles cannot form shells in time after high-temperature treatment, resulting in a sticky wall phenomenon and affecting product quality. Therefore, selecting the appropriate inlet and outlet air temperature can cause the wall material to transform into a glass state as soon as possible, thereby reducing the loss of the core material and achieving the best encapsulation effect [34].

4 Effect of storage conditions on the stability of astaxanthin microcapsules

Microencapsulation technology can significantly improve the stability of substances, and its application is of great significance for extending the storage time of products.

Previous studies have shown that pigment microcapsules have better stability than their monomers. For example, Han Ning et al. [35] compared β-carotene crystals with their microcapsules in an experiment and verified the stability of the two under different storage conditions (temperature, oxygen, light, humidity). The results showed that the retention rate of β-carotene microcapsules was higher than that of its crystals under different conditions, indicating that microencapsulation can improve the degradation of β-carotene in different environments.

Astaxanthin is similar in nature to β-carotene. Since the molecules of the microcapsule wall material cover the surface of the astaxanthin particles, the influence of the external environment on it is to some extent avoided. Hu Tingting [36] placed astaxanthin microcapsules and astaxanthin crystals under different light, temperature and oxygen conditions for a 28-day storage experiment and measured their retention rates. The results showed that although both underwent degradation in the same environment, the retention rate of the former was above 70%, which was much higher than that of the latter. Therefore, encapsulating astaxanthin with microcapsule technology can significantly slow down the degradation of astaxanthin, which to a large extent solves the problem of astaxanthin not being able to be effectively developed due to its own nature, and has played a very important role in promoting its development in various fields.

5 Current status of the application of astaxanthin microcapsules in the food industry

With the deepening of research on microcapsules, more and more active substances have achieved multi-field applications through microencapsulation technology, which not only meets market demand but also enriches product variety. Astaxanthin, as an emerging antioxidant, has gradually attracted widespread attention in the food industry for its use in microencapsulated products.

5.1 Astaxanthin microcapsules and high-end health products

Astaxanthin microcapsule products have been researched abroad for a relatively long time and are also relatively widely used. At present, most astaxanthin microcapsule products on the market are nutritional supplements, and their product positioning focuses on anti-oxidation, delaying aging, lowering blood sugar, boosting immunity, and protecting the retina. For example, Eulara's beauty capsules, the anti-thrombotic capsules from the American company Aquasearch, and the Japanese company Fancl's “Astaxanthin 30 Days” immune-boosting nutritional supplement all contain astaxanthin.

In addition to tablet and capsule health products, health drinks made with astaxanthin microcapsules have also gradually entered the market in recent years. Many countries have already tried using astaxanthin microcapsules in fermented liquid dairy products, unfermented liquid dairy products, fermented soy products and fruit drinks for adults, which not only provides health benefits but also enriches the variety of astaxanthin products.

5.2 Astaxanthin microcapsules and food additives

Astaxanthin microcapsules can not only be used as nutritional supplements in health products, but also as food additives such as colorants and antioxidants to improve the sensory properties of products or to maintain the original nutritional content of the food without damage. Bjerkeng et al. [37] confirmed in 1995 that the superior antioxidant properties of astaxanthin can protect the color and shelf life of trout fillets. In Japan, studies have also been conducted on the use of microcapsules containing astaxanthin oil to preserve vegetables, seaweed and fruit. The results have shown that astaxanthin microcapsules have a significant effect on food preservation [38].

In addition, natural astaxanthin also has good coloring properties. Some studies have found that when astaxanthin microcapsules are used as a food coloring agent, the color development also varies from light to dark with increasing dosage, showing different effects. Today, many different types of food use this characteristic of astaxanthin to color their products, such as margarine, ice cream, yogurt, fruit juice, candy, cakes, noodles, condiments, etc., which not only have a good coloring effect but also a significant shelf-life effect [38].

At present, there are many studies on the use of microencapsulation technology to improve the solubility and stability of pigments in China, such as capsanthin, lycopene, zeaxanthin, etc. Some microencapsulated products of pigments have been put on the market and widely recognized. Although systematic research on the microencapsulation of astaxanthin has begun to be valued in recent years, due to technical and market constraints, the application of astaxanthin in many fields is still blank, so there is huge potential for development. As the excellent properties of astaxanthin become better known, and combined with China's traditional concept of “medicines and foods of the same origin,” the development of functional foods and cosmetics using astaxanthin microcapsules will have very broad development potential and an ideal application prospect.

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