Natural astaxanthin powder is known to be the first choice of antioxidant by virtue of its powerful antioxidant ability. However, since most carotenoids are highly unsaturated molecules that are easily degraded, this paper discusses the stability of astaxanthin in terms of the factors affecting stability and ways to improve the stability of astaxanthin.
How Many Factors Affect Astaxanthin Stability?
• Light
Because most carotenoids are highly unsaturated molecules. Therefore astaxanthin is easily degraded under conditions of sufficient light and oxygen. Studies have shown that astaxanthin from Erythrocystis rainbowensis sources is poorly photostabilized. Light significantly affects astaxanthin stability (C00) especially sunlight. The absorbance decreased substantially with the increase of light exposure time. Sunlight irradiation of about 5H, astaxanthin absorbance decreased to close to 0. Indoor natural light conditions. 24h after the determination of astaxanthin residual rate of 75% avoiding light preservation, astaxanthin absorbance is basically unchanged. Examination of the changes in the stability and antioxidant properties of astaxanthin under light and heating conditions revealed that the reduced all-trans astaxanthin was converted to its cis-isomer under light and heating conditions. And it was mainly converted to 1,3-cis-isomer, corresponding to the conversion rate of 50% and 100%, respectively.
It was found by UV irradiation test that the changes in the content of each isomer of natural astaxanthin powder were similar to the results of light test. It indicates that the effect of light on astaxanthin is mainly caused by the UV light in it. Therefore, astaxanthin is prone to degradation as well as conversion between isomers under light and heating. The handling and preservation of astaxanthin should be operated as far as possible from light (Zhang Liyao et al., 2018).
• Solution alkali concentration
The alkali concentration in the solution also has a large impact on the stability of astaxanthin. As the alkali concentration increases, the free astaxanthin content shows a significant decreasing trend. This indicates that free astaxanthin is not stable to alkali, the greater the alkali concentration, the more serious free astaxanthin degradation. Therefore, many factors should be considered when saponification reaction of astaxanthin using different alkali concentrations. For example:
(1) Dynamic equilibrium of alkali on the degradation of the saponification product, i.e. free astaxanthin
(2) Control of the conditions of the saponification reaction
(3) Selection of suitable alkali concentration
(4) It is suitable temperature and time
(5)It is possible to completely saponify astaxanthin esters into free astaxanthin
This reduces the degradation of free astaxanthin by the alkali concentration.
What Are The Methods To Improve Stability?
• Add stabilizer
Under the condition of avoiding light, the astaxanthin content in the pigment extract with and without the addition of reducing agent did not change much. Especially add reducing agent group astaxanthin absorbance change is not obvious. However, the absorbance of astaxanthin in the non-reducer group decreased sharply with the prolongation of placement time. And add reductant group astaxanthin absorbance change is very small. It can be seen that the reducing agent has a better protective effect on astaxanthin (Huang Shui-Ying, 2008).
Since natural astaxanthin powder as a functional lipid. It can be easily incorporated into different water-based food formulations in the form of nanodispersions. So some studies have prepared astaxanthin nano-dispersion systems using different stabilizer systems such as polysorbate 20 (PS20), sodium caseinate (SC) and gum arabic (GA). And the three stabilizers were analyzed. The results showed that the degradation of astaxanthin followed a first-order kinetic law. In most cases, the stability of astaxanthin in the stabilized nano-dispersion system of composite is better than that of single stabilizer (Amarjanand Tan, 2013).
• Formation of complexes
In order to prevent astaxanthin from thermal or oxidative degradation during production and storage, some researchers have used chitosan encapsulated homogenized cells to protect astaxanthin. Firstly, the algal powder of Rhodococcus rainieri was made to form beads. Then it was treated with 5 layers of chitosan coating. It formed chitosan-algae capsules with an average diameter of 0.43 cm and a total film thickness of about 100 um The antioxidant activity of encapsulated Rhodococcus rainbowensis was reduced by about 3%. The stability results under different storage conditions showed that although encapsulation resulted in a 3% loss of antioxidant activity. However, in terms of long-term stability of R. rainieri biomass, microspheres and capsules, the chitosan encapsulated overlay protects R. rainieri cells from oxidative stress (itikaiwanetal, 2007).
Similarly, in order to overcome the susceptibility of astaxanthin bulk powder to external light, heat, metal ions and other factors and to improve the stability of astaxanthin. Nanoencapsulation technology was also introduced. Lecithin and chitosan were used as the materials of nanocarriers, and the amphiphilic lecithin/chitosan was used to form monodispersed nanoemulsions in aqueous solution by probe sonication. The average particle size of the nanoemulsions was taken as the response value, and the response surface method was applied to optimize the experimental design, and the optimal preparation conditions were finally obtained as the ratio of lecithin/chitosan was 20: 1 (w/w), the ultrasonication time was 16.0 min, and the systematic dosage of emulsifier Tween 80 was 1.5% (w/v), and the average particle size of the lecithin/chitosan nanoemulsions was 121.34 nm (number of samples measured n= 3). The surface zeta potential of the nano-emulsion was + 39.6 mV as determined by nano-particle size analyzer, which indicated that the prepared nano-emulsion was a more ideal carrier.
The stability test showed that astaxanthin was less prone to isomeric degradation and more stable than free astaxanthin after encapsulation by nanoemulsion. The total reducing power test and antioxidant test showed that astaxanthin could maintain the total reducing power and antioxidant activity of natural astaxanthin powder effectively for a long time after embedding. Therefore, astaxanthin nanoemulsions should be stored in low-temperature phosphate buffer solution (pH 7.4) (Zhang, 2013).
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