Surface-Active Properties of Lipophilic Antioxidants Tyrosol and Hydroxytyrosol Fatty Acid Esters: A Potential Explanation for the Nonlinear Hypothesis of the Antioxidant Activity in Oil-in-Water Emulsions

Abstract

Our group has recently observed a nonlinear tendency in antioxidant capacity of different hydroxytyrosol fatty acid esters in fish oil-in-water emulsions, where a maximum of antioxidant efficiency appeared for hydroxytyrosol octanoate. These results appear to disagree with the antioxidant polar paradox. Because the physical location of the antioxidants in an oil-water interface has been postulated as an important factor in explaining this behavior, we have prepared a series of tyrosol and hydroxytyrosol fatty acid esters with different chain length and studied their surface-active properties in water, because these physicochemical parameters could be directly related to the preferential placement at the interface. We have found that tyrosol and hydroxytyrosol fatty acid esters are relevant surfactants when the right hydrophilic-lipophilic balance (HLB) is attained and, in some cases, as efficient as emulsifiers commonly used in industry, such as Brij 30 or Tween 20. Moreover, a nonlinear dependency of surfactant effectiveness is observed with the increase in chain length of the lipophilic antioxidants. This tendency seems to fit quite well with the reported antioxidant activity in emulsions, and the best antioxidant of the series (hydroxytyrosol octanoate) is also a very effective surfactant. This potential explanation of the nonlinear hypothesis will help in the rational design of antioxidants used in oil-in-water emulsions.

Synthesis of lipophilic tyrosyl esters derivatives and assessment of their antimicrobial and antileishmania activities

“…This effect is probably due the surfactant effectiveness of the derivatives. Indeed, Lucas et al, [25] have observed in emulsion system that when the surfactant effectiveness is plotted against the acyl chain length for the tyrosol and hydroxytyrosol ester series, a parabolic shape with around 8-10 carbon atoms (caprylate and decanoate tyrosyl derivatives) indicating an adequate surfactant properties. Hence, these compounds could be considered as antioxidant surfactants.…”

Antioxidant activity of alkyl gallates and glycosyl alkyl gallates in fish oil in water emulsions: Relevance of their surface active properties and of the type of emulsifier

“…interphase. The decrease in mobility due to the increase in molecular size for long-chain lipophilic 320 antioxidants has also been mentioned as a possible reason for the parabolic effect observed (Fendler,surfactant properties (Lucas et al, 2010) and have proposed that the more effective surfactants would 325 locate preferentially at the oil-water interface in the emulsions inhibiting lipid oxidation more 326 efficiently. In a previous work we measured the surfactant properties in aqueous solutions of the alkyl 327 gallates 2-8 used in this study (see Table S10) (Maldonado et al, 2011).…”

Polar paradox revisited: analogous pairs of hydrophilic and lipophilic antioxidants in linoleic acid emulsion containing Cu(II)

“… The PP hypothesis simply states that, owing to different locations of polar and nonpolar antioxidants in bulk oil and water, nonpolar antioxidants should be more effective than their polar homologues in oil‐in‐water emulsions because of their better enrichment at the oil–water interface where oxidation primarily takes place. Even though this was basically confirmed in earlier cases, recent experiments carried out by varying the polarity of antioxidants via esterification with various alkyl chain lengths showed that esterified phenolic antioxidants (such as those of chlorogenic, rosmarinic and dehydrocaffeic acids, tyrosol and hydroxytyrosol fatty acids and rutin) essentially obeyed the PP hypothesis up to C 8 –C 12 chain length, beyond which antioxidant activity declined sharply (called the ‘cut‐off effect’), not in accordance with hypothetical expectations . Bulky phenolic antioxidants (such as those with long side chains), despite their hydrophobic character, may also show lower mobility because of steric hindrance, and thus decreased diffusibility towards reactive centres at the interface .…”