Table 1.

Historical developments in the understanding of the physical effects of components and additives on lipid oxidation in bulk oils

References	Observations	Conclusions
Porter 36,37 and Porter et al. 38	The general rule of the polar paradox was proposed and confirmed stating that polar antioxidants (e.g., propyl gallate, tert-Butylhydroquinone (TBHQ), and Trolox C) are more effective in food systems with low surface-to-volume ratio or nonpolar lipids such as bulk vegetable oils while nonpolar antioxidants (e.g., BHA, BHT, and α-tocopherol) work better in foods with high surface-to-volume ratio or polar lipid emulsion such as o/w emulsion	The antioxidant activity is oppositely related to the polarity of antioxidants in relation to food lipids
Brimberg 26,27	Lipid hydroperoxides (LOOH) are surface-active agents that form micelles at above their critical micelle concentration (CMC). O2 is maximumly solubilized in lipids when hydroperoxide CMC is attained	Micelles formed by hydroperoxides are the site of lipid oxidation reaction
Frankel et al. 39–41	The interfacial phenomenon was proposed to explain the polar paradox. Lipophilic antioxidants (e.g., α-tocopherol and ascorbyl palmitate) were more effective in o/w emulsion system than in bulk oil because they had more affinities toward water-oil interface, while the opposite was true for hydrophilic antioxidants (Trolox, ascorbic acid, rosmarinic acid, carnosic acid, and rosemary extract), which were more oriented in air-oil interfaces in bulk oil. Mixtures of α-tocopherol and ascorbic acid were more active in bulk oils than in o/w emulsions	Interfacial phenomenon is related to the kinds of interfaces at which the antioxidants are more oriented, which may explain the polar paradox
Koga and Terao 54	In the aqueous microenvironments in bulk lipids (15:85 by mol/mol mixture of methyl linoleate and methyl laurate), phospholipid aggregates enhanced the accessibility of α-tocopherol to radicals and hence the interruption of chain initiation. The polar OH group of α-tocopherol is located not too deeply in hydrophobic region of phospholipid bilayer membrane but just near by the membrane surface	Interfacial microenvironment is the place where interactions among surfactants, antioxidants, and radicals take place
Huang et al. 60	Linoleic acid competed with Trolox for Tween 20 in the polar region of the micelles and at the o/w interface. Trolox diffused in the water phase and the mixed micelles and thus was a better antioxidant than α-tocopherol that was diffused in the oil phase	Micelle is where the oxidation and interactions of antioxidants and surfactants take place
Carlotti et al. 123	An emulsion was known to contain micellar structure. l-tryptophan was a very effective synergist with α-tocopherol because it was distributed in the micellar core or in the o/w interface	Micelle core and interface have different roles in autoxidation
Endo et al. 106,116,118	A mixture of trieicosapentaenoylglycerol and tripalmitoylglycerol (2:1, mol/mol) was most susceptible to oxidation than other ratios. The triacylglycerol (TAG) structure affected the oxidation rate of unsaturated fatty acids. TAGs with unsaturated fatty acids at sn-2 positions were more stable than those having unsaturated fatty acids at sn-1 and sn-3 positions	Physical structures, such as the position of fatty acids on TAG, have an effect on lipid oxidation
Hamilton et al. 28	Lecithin solubilizes ascorbyl palmitate and enhances its physical interactions with α-tocopherol which form reversed micelles. This versatile network had an ability to interrupt free-radical propagation by inhibiting the participation of ascorbyl radical in promoting LOOH scission	Reversed micelles are formed in w/o emulsions
Frankel and Meyer 124	The effectiveness of antioxidants in a system is influenced by several factors including the partitioning behavior of antioxidants between lipid and aqueous phase, the oxidation conditions, and the physical state of the oxidizable substrate. Surface-active substances influence the interfacial interactions between the system and antioxidant. The oil-water partition coefficients influence the distribution of relatively polar antioxidants in the lipid and aqueous phase of a food emulsion. Trolox, which is very polar, works very well in bulk oil and is more effective in o/w emulsions of linoleic acid compared to those of TAG. Unlike TAG, linoleic acid is more polar and forms micelles in aqueous system. Micelle-forming substrates enhance the activity of hydrophilic and polar antioxidant	O/w partition coefficient can explain the affinity of a compound in lipid and aqueous phase
Khan and Shahidi 84	The synergistic interactions of tocopherols and phospholipids in borage and evening primrose TAG can be explained partly by phosphatidylcholine increasing the accessibility of α-tocopherol in the aqueous microenvironment where the induction of lipid oxidation occurs	Phospholipid synergists support antioxidants by modifying the reaction environment
Schwarz et al. 43	Antioxidants (Trolox, propyl gallate, gallic acid, methyl carnosoate, and carnosic acid) had either moderate or higher activity in bulk oil than in emulsions. The most polar antioxidants (propyl gallate and gallic acid) exhibited either prooxidant or no antioxidant activity in polar medium (i.e., o/w emulsions). Emulsifiers (Cetheareth-15, glyceryl stearate, and polyglyceryl glucose methyl distearate) form lamellar structure in bulk oil causing a higher solubilization of polar antioxidants in nonpolar medium. Antioxidant actions in bulk oil, except gallic acid which was not influenced by polysiloxan polyalcohol polyether copolymer, are enhanced by emulsifiers including α-tocopherol	The activity of antioxidants can be enhanced or reduced by emulsifiers. Mesophase structures depend on molecular structure and critical packing parameter (CPP) of the compound
Gupta et al. 87	Inverse micellar structures (∼60 Å in diameter) were formed by phospholipids in a hexane-oil mixtures containing <0.3% water. The principal domains of the phase behavior include micellar solution, two phase dispersion, and dense micellar solution. A smooth transition to dense micellar phase was observed with increased phospholipids concentration. Dynamic light scattering measurements showed that aggregate sizes were affected by the amount of phospholipids and >1.5% water, below which the available water is very limited to significantly affect core sizes	Reversed micelles are formed in w/o nanoemulsions. The size of aggregates depends on the amount of surfactants and water
Kortenska et al. 81	Polar products of lipid oxidation with oxygen containing groups (e.g., LOOH, fatty alcohols, acids, and water) tend to associate in non-polar media to form complexes and aggregates. Fatty alcohols may play a role as an initiation of formation of these aggregates and hence influence lipid oxidation rate	Polar products of lipid oxidation affect the oxidation rate by modulating the reaction environment
Kortenska et al. 68	Relatively high concentrations of polar compounds (e.g., LOOH, lipid peroxyl radical [LOO•], and BHT) form microaggregate (micelles) in the presence of fatty alcohols. This leads to an increase of the rate of termination and causes a decrease in the efficiency of BHT to protect purified sunflower oil (SFO) as LOOH decompose faster inside the polar interior of the micro aggregate	Fatty alcohols or BHT might act as surfactants and form microaggregates (micelles) in the w/o system
Velasco and Dobarganes 12	Cloudy OO was more oxidatively stable than filtered OO. Suspended and dispersed materials in cloudy olive oil (OO) play a physical stabilization role by acting as antioxidants and/or as a buffer and preventing acidity increases	Polar constituents in oils, for example, unsaponifiable materials, may play a physical role in oil solubilization
Brimberg and Kamal-Eldin 125	LOOH formed during methyl linoleate oxidation are surface-active and can form micelles. When LOOH concentration reaches CMC, lipid oxidation enters the propagation period	CMC of hydroperoxides marks the beginning of propagation period
Brimberg and Kamal-Eldin 55	The amount of oxygen solubilized in lipid is comparative to the number of micelles formed during oxidation. When lipid medium has conjugated double bonds is oxidized, no hydroperoxides are formed but instead cyclic peroxides that are not surface-active and do not form micelles, hence there is no propagation period	Organic peroxides (not hydroperoxides) are not surface active and do not atfect the oxidation rate
El-Shattory et al. 69	Reversed micelles were formed with surfactant aggregates in organic solvents, for example, LOOH, methylglucose dioleate, polyglyceryl-3-oleate, and lecithin	Reversed micelles are formed in organic system in the presence of surfactants
Kiokias and Gordon 71	The activity of norbixin as antioxidant in bulk oil is consistent with the polar paradox. Norbixin is soluble in water as aggregates and is probably oriented at the oil-water interface in the emulsion due to its massive hydrocarbon backbone but it is insoluble in oil	Norbixin is an example to supports the polar paradox
Decker et al. 72	Differences in the effectiveness of the antioxidants in oil systems are mainly due to their physical location in the system, namely the antioxidant paradox. Polar (hydrophilic) antioxidants are more effective in bulk oil because they can accumulate at the air-oil interface or in reversed micelles within the oil, where lipid oxidation occurs. On the other hand, nonpolar (lipophilic) antioxidants are more effective in o/w emulsions because they accumulate in the oil droplets and/or may accumulate at the oil-water interface, where interactions between LOOH at the droplet surface and pro-oxidants (e.g., transition metals) take place	Antioxidant effectiveness depends on how and where they are partitioned in the system. In bulk oil, lipid oxidation occurs at the air-oil interface as well as in the reversed micelle (oil-water) interface
Calligaris and Nicoli 126	Salts with the antichaotropic anionic species were able to form weak bonds may form a “hydrophilic” structure around them and inhibit the solubility of other substances with lower polarities. Thus, these salts may enhance the activity of certain antioxidants	Hydrophobic structure formed by the salts might salt-out amphiphilic molecules and affect lipid oxidation
Becker et al. 44	Antioxidant activity in bulk oil was related to the polarity of the antioxidants, within the order: quercetin >α-tocopherol ≫ astaxanthin = rutin. Rutin was an exception in that it is relatively hydrophilic but had the lowest activity in bulk oil. This indicated that it is not only the polarity that govern the effectiveness of antioxidants. Poor solubility of rutin in bulk oil or degradation of its glycoside at high temperature also influenced its effects	Hydrophilicity (or lipophilicity) do not always correlate with the antioxidant effectiveness in bulk oil
Chaiyasit et al. 14	Edible oils contain polar lipids (e.g., monoacylglycerol (MAG), diacylglycerol (DAG), free fatty acid (FFA), phospholipids, sterols, cholesterols, phenolic compounds, aldehydes, and ketones), which have amphiphilic nature. Components with especially low HLB can self-assemble due to hydrophobic interactions and form association colloids, including lamellar structures and reversed micelles. These surface active molecules partition at the o/w interface and induce the concentration of antioxidants at the surface of colloids, thus increasing interactions between antioxidants and/or prooxidants with metal at the interface or water core	The term association colloids, include geometric forms such as lamellar structures and reversed micelles, which are formed by surfactants was proposed
Chaiyasit et al. 33	Edible oils contain surface-active compounds and water that can form physical structures such as reversed micelles. Both phosphatidylcholine and oleic acid were suggested to be located at the o/w interface by 5-dodecanoylaminofluorescein probe measurement, and phosphatidylcholine was found to increase the accessibility of α-tocopherol to radicals while oleic acid acted as prooxidants	More examples on the effects of surface-active compounds and reversed micelles on lipid oxidation were presented
Kasaikina et al. 70	LOOH do not form classical micelles but form associates (1–500 nm in size) alongside water, surfactants, alcohols, acids, ketones, and other oxidation products. LOOH is amphiphilic and concentrates on the boundary of micelle and water. In a natural olefin (limonene), cationic surfactant promotes oxidation, whereas anionic and nonionic surfactants did not have any influence	Associates rather than micelles were suggested. Charges of surfactants affect the role of the surfactants as antioxidant or prooxidant
Koprivnjak et al. 73	Bipolar molecules such as lecithin form reversed micelle where their polar groups are pointed toward the interior and their nonpolar tails are directed toward the exterior (oil). Lecithin ability to increase oxidative stability was due to its bipolar character and its ability to entrap hydrophilic antioxidants to concentrate on the micellar interface	On the role of phospholipids as stabilizers of reversed micelles
Laguerre et al. 17	Not all nonpolar antioxidants behave as antioxidant in polar medium; the antioxidant capacity of homologous series of chlorogenic acid esters in o/w emulsions increased as the alkyl chain length increased until dodecyl chain. Further chain extension caused a drastic drop of antioxidant capacity (a cut-off effect)	The Polar Paradox is not linear. As the alkyl chain length increase, the hydrophilicity and the antioxidant activity in o/w emulsions increase to a certain extent, but further increase reduces the antioxidant activity (a cut-off effect)
Belhaj et al. 103	The size of nanoemulsions was influenced by the pressure, oil composition, and the surface-active properties of surfactants. Changes of α-tocopherol antioxidative effect in bulk oil was more significant than that in emulsions	The importance of nanoemulsions in lipid oxidation was proposed
Bendini et al. 127	When virgin OO was subjected to temperature close to 0°C, changes in the physical state happened leading to destabilization of the microdroplets of water and the concentration of polar phenolic compounds and finally the lost of antioxidant activity	At lower temperatures (close to 0°C), destabilized microdroplets in bulk oils may accelerate the rate of lipid oxidation
Chen et al. 7	When the phospholipid concentration exceeds their CMC, reversed micelles were formed. Dioleoylphosphatidylcholine and water formed spherical association colloids in SBO, and they were prooxidative because more (small) non-scattering association colloids were formed. 1,2-dibutyryl-sn-glycero-3-phoshocholineformed cylindrical structures and had no impact on oxidation rates	As amount of surfactant increased, CMC was affected and so the formation of reversed micelle. The kinds of physical structures affect oxidation differently. Spherical shapes of association colloids were prooxidants, while cylindrical shapes had no impact on oxidation rates
Gramza-Michalowska and Stachowiak 128	Astaxanthin causes no protection of bulk oils, which indicates that antioxidant activity was correlated with its polarity. Astaxanthin is hydrophobic, it is located in the oil not at the air-oil interface protecting o/w emulsions but not bulk oils and liposome	Lipophilic compounds do not affect the oxidation in bulk oils
Kasaikina et al. 86	Primary amphiphilic products of the oxidation of LOOH and lipids, and cationic surfactants form mixed micelles, which accelerated the decomposition of LOOH and other polar components (e.g., metal-containing compounds, inhibitors etc.)	Mixed micelles with different geometric forms were detected in w/o emulsions that enhance the decomposition of LOOH
Medina et al. 104	The effectiveness of antioxidants relies on its chemical reactivity (as radical scavenger or metal chelator), its interaction with other food components, their concentration and physical location in homogeneous or heterogeneous system. For instance, resveratrol had a low activity in inhibiting lipid autoxidation in w/o emulsions and bulk oil because it has a low incorporation in the droplet interface and its poor solubilty in water, thus probably located far away from the air-oil interface	On the importance of physical effects of antioxidants
Chen et al. 7	Amphiphilic surface active compounds, which exist after oil refining (such as MAG, DAG, phospholipids, sterol, and FFA), interact with water to form association colloids (in the forms of reversed micelles, microemulsions, lamella, and cylindrical aggregates). Increasing water concentration had very little impact on the IP of lipid oxidation (by hexanal) at 55°C. MAG formed ordered lamellar structures in hazelnut oil. Association colloids impact on lipid oxidation depends on the additives ability to form the colloids and how the additives are partitioned in the micelles	Different surfactants form different kinds of mesophase structures that affect lipid oxidation. Water concentration had a limited effect on oxidation at 55°C
Chen et al. 20	Lipid oxidation is not only influenced by the traditional chemical factors, such as lipid compositions, transition metals; but also by the existence of physical structures. Phospholipids formed microstructures known as association colloids within soybean oil (SBO). Reversed micelle of dioleoylphosphatidylcholine shorthened the IP of SBO at 55°C	Physical structures are important affectors of lipid oxidation
An et al. 129	Antioxidative and prooxidative properties are determined by internal factors (i.e., the oxidation substrates, structural organization and the microenvironment for the bioactive compound) and external factors (i.e., heat, pressure, and exposure to light). Hydrophobic alkyl chain increased water insolubility of 7-n-alkoxydaidzeins: daidzein, 7-n-butyloxy-daidzein, 7-n-octyloxy-daidzein, 7-n-dodecyloxy-daidzein, and 7-n-hexadecyloxy-daidzein. Daidzein increased membrane fluidity, but 7-n-butyloxy-daidzein until 7-n-hexadecyloxy-daidzein decreased fluidity. The compounds were suggested to present in the central domain of the liposome bilayer in the order 7-n-dodecyloxy-daidzein > 7-n-octyloxy-daidzein > 7-n-butyloxy-daidzein > 7-n-hexadecyloxy-daidzein > daidzein, leaving 7-n-dodecyloxy-daidzein as the most effective antioxidant, as monitored by fluoresence spectroscopy using a fluorescence probe	Changes in the hydrophilicity of an antioxidant affect its inhibitory activity of lipid oxidation
Shahidi and Zhong 53	In this review, the polar paradox was re-examined. The distribution of polar antioxidants at the oil-air interface was questioned because air is much less polar than oil. Antioxidants action was influenced by various micro- or nanoenvironments (such as lamellar and reversed micelles) which are formed by water, amphiphilic compounds, and oxidation products (e.g., LOOH, aldehydes, and ketones) alter the physical location of antioxidants. The association colloids are the site of lipid oxidation in bulk oil. A cutoff effect was observed, a non-linear phenomenon occurred wherein antioxidant activity increases as the alkyl chain lengthens until a threshold is achieved, then further increased of chain length caused a drastic collapse on activity. Molecular size also influenced antioxidant effectiveness and causing a cutoff effect, antioxidants with bulky structures (e.g., phenolic derivatives with long alkyl chains) have steric hindrance thus lower mobility than those of smaller size, therefore lower diffusibility toward reactive centers	A cut-off effect was found for hydrophilic antioxidant in nonpolar medium
Sorensen et al. 61	W/o emulsion resemble bulk oil, of which water is located in micelles and aqueous phase is surrounded by emulsifier. The efficacy of antioxidants in emulsions of water in omega-3 lipids follow polar paradox hypothesis, but not for the o/w emulsion. In the case of w/o, at pH7, ascorbic acid had negative charges and repulsive forces existed between the interface and ascorbic acid, thus it was located away from the interface. The polar paradox was insufficient to explain antioxidant effects in multiphase systems such as emulsions, as there are interactions between iron, emulsifiers, and antioxidants	W/o emulsions resemble bulk oils in their response to the polarity of compounds
Sun et al. 23	Polar antioxidants with higher affinity were known to concentrate on oil/air or oil/water interface of the reversed micelle. Thus the antioxidant polar paradox does not always prevail, as some research found different results. Thus the influencing factors of antioxidant activity in reversed micelle were not solely based on antioxidant polarity	Polar paradox is affected by other factors contributing to non-linearity in the effect of antioxidants in lipid oxidation
Sun-Waterhouse et al. 74	Caffeic acid and p-coumaric acid are hydrophilic. They tend to partition into the water phase, locate outside of the oil droplets and chelate metal ions which exist in the oils. Both antioxidants stabilized oil against autoxidation but facilitated the hydrolysis of TAG in the oils	Antioxidants may cause other adverse effects, for example, hydrolysis of TAG
Chen et al. 34	Soybean oil is found in seeds inside micro-sized oil bodies, which consist of a central neutral lipid core (94–98% w/w) and is surrounded by phospholipids monolayer (0.5–2% w/w) and a coat of strong amphiphilic oleosin (0.5–3.5% w/w). These soybean oil bodies had a better physicochemical stability than emulsified soybean oil. Heat treatment (up to 55°C) did not affect the LOOH and hexanal content of oil body suspensions (2% wt at pH 3).	Natural organization protects unsaturated fatty acids. Water exists as nano-scale droplets in w/o emulsions
Rukmini et al. 130	W/o microemulsion exist in bulk oil with nano-scale droplets of water inside. Formulation and stabilization of water-in-virgin coconut oil were prepared with food grade nonionic surfactants (Span 80, Span 20, and Tween 20). Cosurfactants may not be suitable for foods because of the toxicity and irritation induced by short- and medium-chain alcohols. Nonionic surfacts permitted stabilization of such w/o emulsion without the use of cosurfactants, but phase separation was observed when the microemulsion was heated at 70°C or higher	Nonionic surfactants offer an alternative solution as it stabilizes w/o emulsions and do not contribute to oxidation