Microstructure transitions derived from solubilization of lutein and lutein esters in food microemulsions

Abstract

Solubilization and structural transformations in nonionic food microemulsions, characterized by large continuous isotropic region, containing solubilized lutein, and lutein ester were studied. The preparations are of oil-based concentrates composed of solubilized lutein or lutein ester in reverse micelles constructed from hydrophilic surfactant, alcohol and R(+)-limonene. The reverse micelles are diluted along a 60/40 surfactant/oil dilution line with aqueous phase (containing water and glycerol) up to the opposite corner of the diagram where o/w microemulsions are formed passing through a bicontinuous phase, without any visual phase separation. The dilution goes through two structural transitions that were determined in empty (reference) and solubilized systems. The solubilization capacity of both free lutein (FL) and lutein ester (LE) is considerably higher (as expected) in the reverse micelles (no aqueous phase), and in the W/O microemulsions than in the O/W microemulsions, but the highest solubilization was obtained within the bicontinuous phase. The solubilization was found to be affected by the lipophilicity of the surfactant and is also concentration-dependent throughout all the microemulsions structures, but the most pronounced surfactant effect was shown to be in the bicontinuous region. Structural transitions w/o→bicontinuous occurred at 30 wt.% aqueous phase in ‘empty’ (reference) microemulsions as well as in microemulsions containing solubilized free and esterified lutein (at the maximum solubilization levels). However, the transitions bicontinuous→o/w occurred at higher aqueous phase contents of 50 and 60 wt.% aqueous phase for FL and LE, respectively, in comparison to a transition at 40 wt.% for the empty microemulsion. As a result, the bicontinuous region in the presence of the guest molecules becomes much broader. The results indicate that in the reverse swollen micelles the guest molecules, being practically lipophilic will be easily accommodated at the concaved water interfaces (in the w/o region), which will enable high solubilization capacities, and without significant modifications of the interface. It becomes also evident that at zero curvature interfaces (bicontinuous) and in convex interfaces (oil/water) the lutein ester (the more lipophilic molecule) penetrates into the interface and swells both the bicontinuous interface and the droplets, similarly to most guest molecules. However, free lutein is poorly accommodated at the interface and interferes with the interfacial organization and modifies the flattened curvature, affecting the transformation from bicontinuous to o/w droplets.