When we incorporate astaxanthin as a functional ingredient into foods, it is necessary to consider its low physicochemical stability when exposed to external agents such as light, extreme pHs, oxygen, and heat.

Therefore, industrial processing will negatively affect the potential health effect of food-containing astaxanthin. Other factors, such as the interaction with other compounds in the food matrix, will limit the bio-availability of astaxanthin.

The stability and bioavailability of astaxanthin could then be improved by loading it in micro/nanoparticles made with edible matrices before being incorporated into functional foods. In this research field, different lipid-based, organic, or inorganic-based and hybrid micro and nanoparticles, more or less complex, have been designed to improve the stability and bio-availability of astaxanthin.

Among them, the coacervates and the lipid-based micro/nanoparticles are the systems that have attracted the most attention in food industries, mainly due to the possibility of being scaled up.

Despite the great potential of astaxanthin carriers in the sector of functional foods, there is scarce information on delivery systems with the ability, in vivo, to protect astaxanthin against adverse environments and target at the intended biological site where it can be released in a controlled manner and at the desired time.

Encapsulated astaxanthin in lipid-based micro/nanoparticles improves the bio-accessibility and bio-availability of astaxanthin. These particles are composed of one or more lipid bilayers. They can protect astaxanthin against gastrointestinal digestion and stimulate the secretion of bile acids and pancreatic lipases to facilitate lipid digestion and astaxanthin release.

The free astaxanthin could then be solubilized into mixed micelles and absorbed by passive diffusion or specific epithelial transporters in the intestinal epithelium.

The composition and the physicochemical and colloidal properties of the lipid-based particles will influence the bio-accessibility and bio-availability of astaxanthin and the presence of specific components in foods, such as soluble dietary fiber, which compromise astaxanthin absorption.

Liposomes seem to be less resistant to degradation than nanostructured lipid carriers in the presence of bile acids and pancreatic lipases.

Consequently, the stability and its half-life could be enhanced by various mechanisms, such as saturated lipids or the simultaneous use of phytosterols and phospholipids. Surfactant-based delivery systems (niosomes) have also been proposed for their resistivity in acid media and against enzymatic hydrolysis.

However, it is important to note that liposomes and niosomes show low stability when dispersed in a liquid media; thus, incorporation in liquid foods would not be recommended.

Other materials have also been used to protect astaxanthin against gastrointestinal digestion. Among them, alginate stands up, as it is insoluble in gastric acidic pH, which prevents degradation, but allows dissolution at intestinal pH where the encapsulated compounds are released.

Scientists encapsulated astaxanthin oil in alginate/gelatin microcapsules by coacervation and evaluated the stability of the microcapsules and the astaxanthin release under in vitro digestion.

They observed resistance of the microcapsules under gastric digestion and a burst release of astaxanthin within the first four hours of intestinal digestion (more than 60%) because of the rapid degradation of the microcapsules under mild alkaline conditions.

The scientists reported an increment of bio-availability in mice, showing the highest plasma concentration of free astaxanthin after 2.5 h of gastric administration. They compared the potential bioavailability of astaxanthin oleoresin with that of esterified astaxanthin encapsulated in whey protein/gum Arabic coacervates. Both were subjected to simulated gastrointestinal digestion.

The amount of free astaxanthin at the end was significantly higher when astaxanthin was protected, ascribed to the presence of whey protein and gum Arabic, which facilitated the activity of pancreatic lipases by the generation of lipid-water interfaces.

A further in vivo experiment with mice gavaged with the microcapsules confirmed an increment of concentration of free astaxanthin in plasma. The maximum plasma astaxanthin concentration was reached nine hours after administering the microcapsules.

The differences between both studies are ascribed to differences in the controlled release rate of astaxanthin in the digestive tract, suggesting that the microcapsules made with whey protein/gum Arabic show more mechanical strength under intestinal conditions than those prepared with alginate/gelatin, which was more susceptible to degradation or erosion.

The microcapsules were degraded by mechanical forces, resulting from gastrointestinal peristalsis.

Thus, these works lead us to think that the selection of the blends used to prepare the coacervates is important to modulate the intestinal release of astaxanthin, being essential not only to a resistance to the environment in the intestine but also to the mucoadhesive properties that will delay gastrointestinal transit, and hence permit the retention of the microcapsules until degradation. Additionally, the particle size is important in the transit rate since it is slower for the smallest microcapsules.

The role of some delivery systems is not only limited to the protection of astaxanthin against gastrointestinal digestion and later release in the intestine. Some astaxanthin loaded-carriers, lower than 500 nm, could maintain their integrity and be absorbed through Peyer’s patches or by endocytosis, thus enhancing astaxanthin’s bio-availability.

The composition, size, charge, and surface of the nanoparticles used to load astaxanthin play an important role. Recently, Complex chitosan-casein-oxidized dextran nanoparticles have been developed to enhance the oral availability of astaxanthin. These particles (120 nm and yield of 70%) improved the dispersibility of astaxanthin and were stable in simulated gastrointestinal fluids. Chitosan-based nanoparticles to load astaxanthin presents the advantage that chitosan is generally recognized as safe (GRAS), biodegradable, exhibits low toxicity, and can improve the transport of bioactive molecules through the tight epithelial junctions because of its affinity to the cell membrane.

However, nanoparticles only made with chitosan will not protect astaxanthin during gastric digestion, as it is degraded under low pH values. Hence, to use chitosan, it is necessary to blend it with nonionic polymers to improve the physicochemical stability of the particles.

Nanoliposomes below 200 nm size have also been suggested as suitable astaxanthin carriers across the intestinal barrier because of their lipophilic nature and ability to adhere to membranes and penetrate cells. The amount of phosphatidylcholine significantly affects the cell uptake efficiency, and high doses in the liposome (70%) were recommended instead of lower doses (23%), which did not show a significant uptake in Caco-2 monolayers.