Maxima in Antioxidant Distributions and Efficiencies with Increasing Hydrophobicity of Gallic Acid and Its Alkyl Esters. The Pseudophase Model Interpretation of the “Cutoff Effect”

Abstract

Antioxidant (AO) efficiencies are reported to go through maxima with increasing chain length (hydrophobicity) in emulsions. The so-called "cutoff" after the maxima, indicating a decrease in efficiency, remains unexplained. This paper shows, for gallic acid (GA) and propyl, octyl, and lauryl gallates (PG, OG, and LG, respectively), that at any given volume fraction of emulsifier, the concentrations of antioxidants in the interfacial region of stripped corn oil emulsions and their efficiency order follow PG > GA > OG > LG. These results provide clear evidence that an AO's efficiency correlates with its fraction in the interfacial region. AO distributions were obtained in intact emulsions by using the pseudophase kinetic model to interpret changes in observed rate constants of the AOs with a chemical probe, and their efficiencies were measured by employing the Schaal oven test. The model provides a natural explanation for the maxima with increasing AO hydrophobicity.

Mass Transport Phenomena in Lipid Oxidation and Antioxidation

“…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.…”

Interfacial kinetics in octane based emulsions. Effects of surfactant concentration on the reaction between 16-ArN2+ and octyl and lauryl gallates

“…polarity) of the reaction site. The k I values obtained in octane based emulsions are much lower than those obtained in corn oil emulsions [22], …”

“…The general picture that arises from those studies is that the percentage of antioxidant in the interfacial region increases upon increasing the emulsifier volume fraction. However, we demonstrated recently for a series of homologous AOs with increasing alkyl chain length [22] that the percentage of AO in the interfacial region does not correlate with their hydrophobicity and an increase in the hydrophobicity may lead to a decrease in their concentration in the interfacial region. Note that an increase in the surfactant volume fraction implies an increase in the interfacial volume, and thus decreases the interfacial molarity of the antioxidant at constant ratio of oil to water, as we have shown here.…”

“…During the last years, we have exploited the reaction between a hydrophobic arenediazonium salt, 4-hexadecylbenzenediazonium, 16-ArN 2 + , tetrafluoroborate, with antioxidants (AOs) to assess their distribution in neutral and ionic emulsions [16,[22][23][24][25]. We have successfully applied our methodology to determine the partition constants and the distributions of very hydrophilic (i.e., oil insoluble) antioxidants such as phenolic acids and ascorbic acid (Vitamin C), very hydrophobic (i.e., water insoluble) antioxidants such as atocopherol, lauryl gallate or hexadecyl caffeate, and antioxidants of moderate hydrophobicity which are both oil and water soluble such as propyl gallate and catechol.…”

“…Here we have expanded our previous investigations to systems where reactants, here 16 e.g., the reaction site between the antioxidant and the lipid radicals [22,24,26]. The lipid oxidation reaction is a radical reaction that needs to be controlled and minimized in as much as possible because it degrades the foods quality by diminishing their organoleptic properties and producing undesired off-flavors and harmful products [26].…”