An Investigation of the Versatile Antioxidant Mechanisms of Action of Rosmarinate Alkyl Esters in Oil-in-Water Emulsions

Abstract

The antioxidant polar paradox postulates that nonpolar antioxidants are more effective in oil-in-water emulsions than polar antioxidants. However, this trend is often not observed with antioxidants esterified with acyl chains to vary their polarity. In this study, the nonpolar eicosyl rosmarinate (20 carbons, R20) was less effective at inhibiting lipid oxidation in oil-in-water emulsions than esters with shorter fatty acyl chains such as butyl (R4), octyl (R8), and dodecyl (R12) esters. Interestingly, in the presence of surfactant micelles, the antioxidant activity of R20 was significantly increased while the antioxidant activity of R4 and R12 was slightly decreased. The presence of surfactant micelles increased the concentration of R20 at the interface of the surfactant micelles and/or emulsion droplets as determined by partitioning studies, front-face fluorescence properties, and the ability of R20 to interact with the interfacial probe, 4-hexadecylbenzenediazonium. A possible explanation for why the antioxidant activity of R20 was so dramatically increased by surfactant micelles is that a portion of the nonpolar R20 localizes in the emulsion droplet core and the surfactant micelles are able to increase the interfacial concentrations of R20 and thus its ability to scavenge free radicals produced from the decomposition of interfacial lipid hydroperoxides.

Mass Transport Phenomena in Lipid Oxidation and Antioxidation

“…In particular, this is the case in oil-in-water emulsions when an excess of surfactant is added (more than is needed to saturate the emulsion droplet surface). Using rosmarinate alkyl esters in a stripped soybean oil-in-water emulsion, it has been shown that eicosyl rosmarinate (20 carbon atoms) is mainly located in the oil droplet, whereas butyl and dodecyl rosmarinates are predominantly concentrated at the interface (Panya et al 2012). As expected, a cut-off effect was observed: butyl and dodecyl rosmarinates were much better antioxidants than was eicosyl rosmarinate ester.…”

“…Twenty-eight formulations were associated with an estimated proportion of unadsorbed emulsifiers higher than 90%; 20 resulted in an unadsorbed proportion of 70-90%, and 13 in an unadsorbed proportion of 50-70%. There is abundant experimental evidence from studies using oil-in-water emulsions and microemulsions that unadsorbed surfactants present in the aqueous phase at concentrations higher than their critical micelle concentration (CMC) serve as vehicles for transferring antioxidants (Kiralan et al 2014;Laguerre et al 2009Laguerre et al , 2010Panya et al 2012;Richards et al 2002), hydroperoxides (Nuchi et al 2002), and iron (Cho et al 2002) from the oily phase to the aqueous one and/or the interface.…”

“…Using a broad range of homologous series of antioxidants in dispersed lipid models and cultured cells, it has been demonstrated that the antioxidant activity increases progressively with increasing chain length up to a critical point, beyond which the activity of the compounds decreases (Figure 4). Since its first mention (Laguerre et al 2009), this cut-off effect for antioxidants has been the subject of extensive research in lipid dispersions (Alemán et al 2015;Costa et al 2015;Laguerre et al 2010;Lee et al 2013;Losada-Barreiro et al 2013;Medina et al 2009;Panya et al 2012;Sørensen et al 2012Sørensen et al , 2014Sørensen et al , 2015 and cellular models , Laguerre et al 2011, Munoz-Marin et al 2013) as well as of review articles (Laguerre et al 2013a(Laguerre et al ,b, 2015Shahidi & Zhong 2011;Zhao et al 2015). Table 1 gives some examples of the experimental confirmations of the occurrence of a cut-off effect.…”

“…It was hypothesized that the excess surfactant allowed long-chain esters to move from the oily core to interfacial regions (droplet surfaces or micelles), where they are more effective at counteracting lipid oxidation (Panya et al 2012). Within the current location-based paradigm, this suggested, at least in emulsions, that this nonlinear effect with a cut-off for long-chain antioxidants was due to the location of the latter in the lipid interior, where they would be too far from the interface.…”

Managing flavour changes during storage

“…Although this has proven to be a simplification (Panya et al, 2012), it does illustrate the important effect of product (micro)structure on the oxidation process and the efficiency of antioxidants used in different systems.…”