Mechanisms of Antioxidant Interactions Inform 2013

8/13/2019 Mechanisms of Antioxidant Interactions Inform 2013

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Whereas -tocopherol and ascorbic acid work synergistically inbiological membranes, this is not always the case in food systemswhere lipids can be in the form of tr iacylglycerols and ex ist asemulsion droplets. In addition, the range of antioxidant that could


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interact synergistically is limited because most natural antioxi-dants have higher reduction potentials than -tocopherol. ere-fore, in foods it is not well understood if natural antioxidants canregenerate - tocopherol or if - tocopherol is actually a second-ary antioxidant that regenerates other antioxidants to producesynergistic effects. In addition, it is not well understood howphysical locations of antioxidants in O/W emulsions influenceantioxidant regeneration.

To address these issues, rosmarinic acid and its estersprovide an excellent tool for analyzing the influence how physi-cal locations can impact interactions with -tocopherol becauserosmarinic acid can be esterified with fay acids to alter polarityand thus physical location in O/W emulsions. Rosmarinic acidand its esters have higher oxidation peak potentials (approxi-mately 1115 mV) than -tocopherol (635 mV). Moreover, 90%of rosmarinic acid (R0) partitions in the aqueous phase, whereasbutyl (R4), dodecyl (R12) and eicosyl (R20) rosmarinate estersas well as -tocopherol are highly associated with the emulsiondroplets (approximately 29% in the aqueous phase). A concep-tual model is shown in Figure 1 of how these differences in physi-

cal location could impact interactions between -tocopherol androsmarinic acid and its alkyl esters used O/W emulsions.

In the absence of-tocopherol, rosmarinic acid and its alkyl estersexhibited nonlinear antioxidant activity, the so-called cut-off effectas shown in Figure 2 (page 540). e combination of -tocopherol(T) with R0 showed strong synergistic activity (synergistic index= 5.25) as calculated from the induction period of hexanal forma-tion in O/W emulsions. R4 and R12 showed slight synergisticeffects (synergistic index = 1.92.0). However, an antagonistic

effect was observed with combinations of -tocopherol with R20(synergistic index = 0.3). Also, the synergistic interactions between-tocopherol and the alkyl esters of rosmarinic acid were pH depen-dent. Under acidic conditions (pH 35), synergistic interactionbetween -tocopherol and rosmarinic acid decreased to an additiveeffect. For example, the synergistic index of T + R0 at pH 3 was 1.2.is highlights the problem that predicting synergistic interactionsbetween antioxidants in food systems is difficult and may involvemore than just electron-transfer mechanisms.

Front-face fluorescence quenching and electron parametric reso-nance techniques are two techniques that can be used to analyzedirect physical interactions and direct electron transfer amongthe antioxidants. Figure 3 (page 540) shows that rosmarinic acid(mostly dispersed in aqueous phase) highly interacted with and/or regenerated-tocopherol at the interface of emulsion dropletsmore than with the other rosmarinate alkyl esters. In addition,the regeneration efficiency of rosmarinic acid decreased withdecreasing pH, which could explain why synergistic activity alsodecreased under acidic conditions.


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Another way to study antioxidant interactions is to look atthe kinetics of their decay during storage of O/W emulsions. Inthis method, if one antioxidant is regenerating another then onewould expect to see one of the antioxidants decompose fasterthan the other. Interestingly, a significant reduction of the oxi-dation rate of -tocopherol was observed in the presence ofrosmarinic acid, but this did not seem to be only due to regener-ation of -tocopherol, as rosmarinic acid degradation resulted inthe formation of ca eic acid (a part of the rosmarinic acid mol-ecule). Since caffeic acid also has antioxidant activity in O/Wemulsions, it is possible that the formation of this second anti-oxidant is responsible for the observed synergistic activity. s,synergistic antioxidant interaction cannot be explained solelyby electron-transfer mechanism; the complexity of several othermechanisms, antioxidants, and their oxidation products must beconsidered as well. Overall, this study shows the importance ofunderstanding antioxidant interactions in complex food systemsso that beer predictions can be made on which antioxidant com-bination could result in synergistic inhibition of lipid oxidation.


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However, LC-PUFA are vulnerable to oxidation during oilextraction, food processing, and storage. is generates oxidationproducts that negatively affect the flavor, color, nutrient value, andfunctionality of the fortified foods and food components. In addi-tion, some lipid oxidation products are detrimental to the healthof consumers. us, the benefits of LC-PUFA can be realized onlywhen no oxidation occurs before consumption.

e incorporation of antioxidants is the most successful way toachieve that goal and has been broadly applied in the food industry.But the traditional natural antioxidants that are applied to preventcommon food oil oxidation are inefficient in inhibiting LC-PUFAoxidation. e obstacles impeding the oxidative stability of foods

having LC-PUFA are the higher susceptibility of LC-PUFA tooxidation and the relatively lower efficacy of antioxidants. Hence,special strategies should be developed to maximize the oxidativestability of oil or food product containing LC-PUFA.

Our previous study suggested the existence of nanoreactorsin bulk heterogeneous vegetable oil that form by self-assemblyof endogenous surface-active minor compounds and trace water(~200 ppm). We also found that those reactors could potentiallyaffect the activity of antioxidants of different polarity in bulk veg-etable oil by altering their locations (Inform21:577578, 2010).

Mechanisms of Antioxidant Interactions Inform 2013

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