Carotenoids are natural dyes and antioxidants widely used in food processing and in therapeutic formulations. However, their practical application is restricted by their high sensitivity to external factors such as heat, light, oxygen, metal ions and processing conditions, as well as by extremely low water solubility. Various approaches have been developed to overcome these problems. In particular, it was demonstrated that application of supramolecular complexes of “host-guest” type with water-soluble nanoparticles allows minimizing the abovementioned disadvantages. From this point of view, nanoencapsulation of carotenoids is an effective strategy to improve their stability during storage and food processing. Also, nanoencapsulation enhances bioavailability of carotenoids via modulating their release kinetics from the delivery system, influencing the solubility and absorption. In the present paper, we present the state of the art of carotenoid nanoencapsulation and summarize the data obtained during last five years on preparation, analysis and reactivity of carotenoids encapsulated into various nanoparticles. The possible mechanisms of carotenoids bioavailability enhancement by multifunctional delivery systems are also discussed.

Carotenoids are a large and diverse group of compounds that have been shown to have a wide range of potential health benefits. While some carotenoids have been extensively studied, many others have not received as much attention. Studying the physicochemical properties of carotenoids using electron paramagnetic resonance (EPR) and density functional theory (DFT) helped us understand their chemical structure and how they interact with other molecules in different environments. Ultimately, this can provide insights into their potential biological activity and how they might be used to promote health. In particular, some rare carotenoids, such as sioxanthin, siphonaxanthin and crocin, that are described here contain more functional groups than the conventional carotenoids, or have similar groups but with some situated outside of the rings, such as sapronaxanthin, myxol, deinoxanthin and sarcinaxanthin. By careful design or self-assembly, these rare carotenoids can form multiple H-bonds and coordination bonds in host molecules. The stability, oxidation potentials and antioxidant activity of the carotenoids can be improved in host molecules, and the photo-oxidation efficiency of the carotenoids can also be controlled. The photostability of the carotenoids can be increased if the carotenoids are embedded in a nonpolar environment when no bonds are formed. In addition, the application of nanosized supramolecular systems for carotenoid delivery can improve the stability and biological activity of rare carotenoids.

The antioxidant astaxanthin is known to accumulate in Haematococcus pluvialis algae under unfavorable environmental conditions for normal cell growth. The accumulated astaxanthin functions as a protective agent against oxidative stress damage, and tolerance to excessive reactive oxygen species (ROS) is greater in astaxanthin-rich cells. The detailed mechanisms of protection have remained elusive, however, our Electron Paramagnetic Resonance (EPR), optical and electrochemical studies on carotenoids suggest that astaxanthin’s efficiency as a protective agent could be related to its ability to form chelate complexes with metals and to be esterified, its inability to aggregate in the ester form, its high oxidation potential and the ability to form proton loss neutral radicals under high illumination in the presence of metal ions. The neutral radical species formed by deprotonation of the radical cations can be very effective quenchers of the excited states of chlorophyll under high irradiation.

Carotenoids are well-known antioxidants. They have the ability to quench singlet oxygen and scavenge toxic free radicals preventing or reducing damage to living cells. We have found that carotenoids exhibit scavenging ability towards free radicals that increases nearly exponentially with increasing the carotenoid oxidation potential. With the oxidation potential being an important parameter in predicting antioxidant activity, we focus here on the different factors affecting it. This paper examines how the chain length and donor/acceptor substituents of carotenoids affect their oxidation potentials but, most importantly, presents the recent progress on the effect of polarity of the environment and orientation of the carotenoids on the oxidation potential in supramolecular complexes. The oxidation potential of a carotenoid in a nonpolar environment was found to be higher than in a polar environment. Moreover, in order to increase the photostability of the carotenoids in supramolecular complexes, a nonpolar environment is desired and the formation of hydrogen bonds should be avoided.