Chain Length Affects Antioxidant Properties of Chlorogenate Esters in Emulsion: The Cutoff Theory Behind the Polar Paradox

Abstract

Twenty years ago, Porter et al. (J. Agric. Food Chem. 1989, 37, 615 - 624) put forward the polar paradox stating among others that apolar antioxidants are more active in emulsified media than their polar homologues. However, some recent results showing that not all antioxidants behave in the manner proposed by this hypothesis led us to investigate the relationship between antioxidant property and hydrophobicity. With a complete homologous series of chlorogenic acid esters (methyl, butyl, octyl, dodecyl, hexadecyl, octadecyl, and eicosyl), we observed in emulsified medium that antioxidant capacity increases as the alkyl chain is lengthened, with a threshold for the dodecyl chain, after which further chain extension leads to a drastic decrease in antioxidant capacity. The antioxidant capacity evaluation in emulsion was possible using a newly developed conjugated autoxidizable triene (CAT) assay, which allows the assessment of both hydrophilic and lipophilic antioxidants. The nonlinear behavior was mainly explained in terms of antioxidant location since it was found from partition analysis that the dodecyl ester presented the lowest concentration in the aqueous phase and also that the quantity of emulsifier drastically changes the partition of antioxidant. In addition, this nonlinear influence was connected to the so-called cutoff effect largely observed in studies using cultured cells. Taken together, these different results allow one to make the proposal of a new scenario of the behavior of phenolic compounds in emulsified systems with special emphasis on the micellization process. Finally, in the CAT system, the polar paradox appeared to be the particular case of a far more global nonlinear effect that was observed here.

The antioxidative effect of lipophilized rutin and dihydrocaffeic acid in fish oil enriched milk

“…Fish oil without antioxidant added was supplied by Maritex Norway (subsidiary of TINE BA, Norway 11.6%. The total percentages of n-3 and n-6 PUFA in the oil were 24.7 and 2.7%, respectively.…”

“…In contrast, lipophilic antioxidants generally function better than hydrophilic antioxidants in emulsions [10]. However, recent studies have reported results that contradict the polar paradox hypothesis [11][12][13][14]. This suggests that other factors might be equally important, and more research is urgently required to improve our understanding about the relationship between the molecular structure of the antioxidants and their efficacy in different real food systems.…”

“…The current work in this area has been summarized by Shahidi and Zhong [15]. Laguerre et al [11,16] have recently reported on the antioxidative effect of lipohilized chlorogenic and rosmarinic acids. For chlorogenic acid the antioxidant capacity increased as the alkyl chain length was increased from 1 to 12 carbon atoms, whereas further increase of the chain length resulted in a drastic decrease in the antioxidant capacity.…”

“…On the basis of these results the authors suggested a so-called cut-off effect related to the length of the lipid chain attached to chlorogenic acid, which they explained as follows: When, the hydrophobicity of the lipophilized compound increases to above a certain level, the lipophilized compound is suggested to form micelles in the aqueous phase. Thereby, they will not be available as antioxidants at the interface and in turn their efficacy will be reduced [11]. For rosmarinate esters, the octyl rosmarinate improved the antioxidative effect eight times compared to rosmarinic acid.…”

“…Thus, the results lead to the conclusion that lipophilization with medium chain fatty acids is a promising way to increase the antioxidant activity [16]. However, the results obtained with chlorogenate and rosmarinate esters have also led to the suggestion that the exact location of antioxidants in the discontinuous phase, interfacial layer or oil droplets, is important for the activity of the antioxidants [11,16]. In addition, a study on lipophilized dihydrocaffeic acids and their antioxidative effect in o/w emulsions reported by Sørensen et al [17] suggested that lipophilized dihydrocaffeic acid tended to follow the newly suggested cut-off effect in relation to the alkyl chain length attached although only two chain lengths were evaluated.…”